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I want to build this supply for tube preamp / SS amp, but I can't buy transformer with additional 6.3V secondary for heater, so I thought that I could make that supply using two separate transformers, one for +/- voltage for pre/amp, and another one for heater supply, like this:

(http://i65.tinypic.com/2hog0uc.jpg)

I have two questions for experienced builders:
1. Is it OK to use common ground, like this?
2. Is it better to put transformer primaries in parallel, as shown on picture, or I should put them in series? I presume parallel is better.

Quote from: rankot on November 15, 2017, 02:48:06 AM
1. Is it OK to use common ground, like this?

Yes. Ideally you should treat them separately before finally connecting the heater ground to the other ground, at one point.

Quote
2. Is it better to put transformer primaries in parallel, or in series? I presume parallel is better.

Yes, parallel is the only way this would work.

Thanks a lot, Merlin!  8)

Diy-tubes.com sells some transformers that would probably work for you if you want to go a little less roll-your-own.  I just bought some stuff from them to make my very first tube supply, so looks like I'm high voltage adventuring along with you.  :icon_biggrin:

(Safety first, of course.  :icon_wink:)

You also may try to use unrectified 6.3V for filament supply, connecting each side of it with 100R resistor to a point created by a resistive voltage divider from screen grid supply (if you use a pentode on output stage) or from preamp plate supply with a 22μF cap, to avoid hum or any other noise..
(I can't post a schematic for the moment but if you find a AX84 power supply diagram it will be all clear..)

You actually "off-set" ground reference point to a DC one, avoiding hum caused by filament "e-field" emission due to unrectified supply..

Just for educational/fun purpose..  :icon_wink:

I know all that, but I live in Serbia and it is not practical due to shipping time to order transformers from abroad, not to mention shipping costs :)

However, I am asking all this because I have built this preamp already and it works fine when using it's own, separate PS with step up from 9V to 50V for B+, but when I connect it to amplifiers PS (B+ directly to +50V rail, and heater to the same rail as shown here), I experience huge hum. I can't use it's separate PS inside main amp (no place to put that inside), so I need to use amp's PS.

So I presumed that the problem is with heater, it draws too much (and dropping resistor also waste too much power) and I wanted to provide separate power for it. And it is already connected to common ground, so I asked if it is OK, because if it is not, I can also fix it, but it means that I have to desolder tube socket and solder heater supply to appropriate pins using separate cable.

This is how it is connected right now:

(http://i66.tinypic.com/e80hg9.jpg)

Quote from: rankot on November 15, 2017, 07:46:20 AM
I know all that, but I live in Serbia and it is not practical due to shipping time to order transformers from abroad, not to mention shipping costs :)

Got it.  :icon_wink:  I'm in the US, so I took a deep breath before clicking to see what the shipping costs from Russia were, and I feel your concerns.  I ended up paying $15USD on a $30USD order.

I'm mentally adding Serbia to my list of countries that I am aware of at least one forum member designating as their home.  To me, this long list is one of the very best things about this community.  I have to admit that I misread it as Siberia at first though.   :icon_lol:

You are definitely not the only one person confusing Serbia and Siberia :)

It's nice meeting people from all around the globe here, cause music is the ultimate common language. I understood that for the first time while watching first videos of "Playing for change". It really touched me!

Quote from: antonis on November 15, 2017, 05:54:52 AM
You also may try to use unrectified 6.3V for filament supply, connecting each side of it with 100R resistor to a point created by a resistive voltage divider from screen grid supply (if you use a pentode on output stage) or from preamp plate supply with a 22μF cap, to avoid hum or any other noise..
(I can't post a schematic for the moment but if you find a AX84 power supply diagram it will be all clear..)

You actually "off-set" ground reference point to a DC one, avoiding hum caused by filament "e-field" emission due to unrectified supply..

Just for educational/fun purpose..  :icon_wink:

There's one: http://ax84.rru.com/media/ax84_m116.gif

But I am not in the mood to use this, because I am not sure if using that kind of common ground will affect solid state amp behind my preamp :(

I'll try to use the 1st schematic I posted, with separate transformer and rectifier.

Well, I have posted this video if one has will to watch it - and maybe help find the solution for the problem.



I have noticed that turning volume potentiometer affects the hum, and this can be shown on video. This is the schematic I used to build this (heater supply not shown, since it is already posted):

(http://i67.tinypic.com/11j65qh.jpg)

I did two experiments: when I plug another preamp into override input (this preamp still running, but disconnected from amp), there is a hum, but lower in intensity. I have also tried to desolder heater supply and then there is no hum at all, and of course not audio through this preamp :)

Irrelavent here, but C2 is unnecessary.

Indeed it is, but I usually put one high voltage capacitor there to block DC when working with high B+, so I don't have to use expensive high voltage caps for the tone stack. I forgot that this one works on 50V only :)

QuoteThis is how it is connected right now:

I'm assuming you have it like this right now.

You need to connect the ground side of the heater to the junction where all of C1, C2, C3, C4 join together.

It is normal for the a volume control at the end of the circuit to increase the hum.  The hum is a fixed level within the circuit and you are just controlling the volume of it like you would the level of the guitar.

Thanks Rob, but what puzzles me is the fact that hum is almost equally strong on both sides of volume pot, while there is a certain decrease at approx. 10 o'clock.

This is how my PCB looks like:
(http://i64.tinypic.com/if6i2p.jpg)

Red GND is the place where I brought ground from junction of C1, C2, C3 and C4. Shall I add another wire (short and thick) from that place to heater ground?

Quote from: rankot on November 15, 2017, 09:57:58 AM

Quote from: antonis on November 15, 2017, 05:54:52 AM
You also may try to use unrectified 6.3V for filament supply, connecting each side of it with 100R resistor to a point created by a resistive voltage divider from screen grid supply (if you use a pentode on output stage) or from preamp plate supply with a 22μF cap, to avoid hum or any other noise..
(I can't post a schematic for the moment but if you find a AX84 power supply diagram it will be all clear..)

You actually "off-set" ground reference point to a DC one, avoiding hum caused by filament "e-field" emission due to unrectified supply..

Just for educational/fun purpose..  :icon_wink:

There's one: http://ax84.rru.com/media/ax84_m116.gif

That's the "wrong" one..
(most common "humming" fault in many guitar tube amps..)

I'm talking about the following way of connecting filament supply "GND"..
(you can see the difference between filament grounding through 100R resistors in your scheme and "offseting" AC ground to about 50VDC in the following scheme..)
(https://i.imgur.com/oeKE5y5.png)

QuoteThanks Rob, but what puzzles me is the fact that hum is almost equally strong on both sides of volume pot, while there is a certain decrease at approx. 10 o'clock.

Things like that are often a sign of multiple ground loop issues
It's probably cause by:
- the fact you have three ground points on your PCB
- the pcb connects to the power amplifier in your amp, and the preamp and the power amp share the same power supply.   
- Your filament current is also flowing through the same grounds as you signal.
- Possibly even more ground connections through sockets.

There's ground connections all over the place. Ideally you want to use a star ground connection.

http://www.geofex.com/Article_Folders/stargnd/stargnd.htm

You have to solve the hum with the volume pot at minimum first because that problem exists even without the preamp signals.

You might you need to complete separate the filament supply on your PCB (by cutting the grounds for those away from the ground plane)  then running them back to a star point at the junction of the big caps.    There's also a right way to connect to the caps:  draw a loop from the transformer through the rectifier diodes then through the big caps through some ground tracks then back to the transformer.    Any ground wires in that loop are considered noisy.  You need to connect to a ground point just outside of that loop.

Try to look how the amp handled the grounds and connections between the preamp an power amp on the existing preamp.

Here's a good picture to follow:
https://media.tumblr.com/tumblr_m8scigy0fL1qascvc.png

Also note how the start point isn't right at the junction of the filter caps.

OK, Antonis, this is much clearer, but I can't use it here. I am upgrading existing solid state amp with tube preamp, so I must rely on it's existing PS.

What puzzles me is the fact that this preamp worked fine, no hum at all, with it's own PS, which was made of 9V/2A DC as the main PS, which later powered heater through 7805 (lifted to 6.3V with two diodes) and B+ was generated with 555 based boost circuit. And it was even built on a breadboard, just plugged to amplifier's direct input (original preamp overridden)! The ground was common for signal, power and heater, and also connected with amplifier's ground through the guitar cable. And it worked???

Now when I assembled it on PCB and connected to amplifier's PS, hum is horrific! I presume that + side of amp's PS is overloaded and that creates instability and oscillation, because when I measure voltage, +rail is much lower than -rail in absolute value, when I connect this preamp. They are practically the same when preamp is not connected. It seems that there's not enough power on positive rail to run all this.

I will try to make a separate, rectified heater supply, as shown on the first picture. It will share the ground with the others (so I don't have to alter PCB too much), and if there would be hum again, I will modify the PCB so to have separate ground for heater and everything else.

Regarding ground points (what Rob mentioned) - I actually use only one (marked red), and GP near input is for guitar jack connector, so it's not connected to the rest of the circuit in any other way than through the 'red' ground. No loop. I don't use ground point at output, but I had to make it because connectors I have are double, I don't have single.

This is how it is connected right now:

(http://i67.tinypic.com/2wqhif4.jpg)

Wires on the left are original ones - red is positive rail, black is negative rail and blue is ground. This is the star point of the amp. Wires on the right are mine (OK, I know I connected colours incorrectly, but they go to right places on my PCB), and they go to the connector on PCB. Yellow wire goes from this star point to red marked ground point on preamp PCB.

All the weird behaviour you are seeing is quite normal when you have grounding issues.   Separate supplies naturally decouple everything and avoid many issues.

QuoteI will try to make a separate, rectified heater supply, as shown on the first picture. It will share the ground with the others (so I don't have to alter PCB too much), and if there would be hum again, I will modify the PCB so to have separate ground for heater and everything else.

That's a good place to start.

QuoteWires on the left are original ones - red is positive rail, black is negative rail and blue is ground. This is the star point of the amp. Wires on the right are mine (OK, I know I connected colours incorrectly, but they go to right places on my PCB), and they go to the connector on PCB. Yellow wire goes from this star point to red marked ground point on preamp PCB.

OK, it looks like you are mimicking what was there before.

What about how the preamp connected to the power amp? How is the grounding done?  Try to copy that as well.

To get a little further, I tend not to trust ground lifted voltage regulators..
(although it worked well on your prototype breadboard..)

OK, I will use LM317 to precisely set 6.3V :)

Only after verifing no grounding issues...  :icon_wink:

Looks to me like hum picked up on the cable because the circuit is unshielded and your volume pot is waaay too high resistance. Use something in the range of 5k to 20k (and use a correspondingly larger output cap, like 10uF or something).

I'll try that too.

I have tried to disconnect heater from main amp's PS and to connect it to it's own PS. Now rail balance is almost perfect, but I still have the hum (and it changes with volume pot, with maximum on max, lowest hum at 10-12 o'clock).

I will try to connect to ground at different place and see what happens. Old preamp was connected at different point, so I will try to connect there and see if there is change. I will also try to use shielded wire for signal.

:icon_frown:

I have also tried to remove IC1 and see what happens if I connect C6 to amp. It works of course, but there is the same hum. And I have also noticed that if I connect amp's input to input jack (thus bypassing preamp at all), there is almost the same hum, but not so strong (because it is not amplified by preamp). Less hum if I don't plug guitar in. So it must be picking up some noise from input jack to preamp input, I will try to use shielded cable here.

I tried to remove tube itself and disconnect heater PS. And the hum is not present when I connect premap out to amp in, but if I connect input jack to amp in, bypassing preamp completely, the same old hum returns.

:icon_mad:

QuoteI have tried to disconnect heater from main amp's PS and to connect it to it's own PS. Now rail balance is almost perfect, but I still have the hum (and it changes with volume pot, with maximum on max, lowest hum at 10-12 o'clock).

Situations like this there is usually more than one problem and they cancel out.  This is why I think you should focus on the hum when the pot is at zero.  This case depends on ground issues (and power routing) and not the circuit. It doesn't matter what you do to the circuit that one won't go away until it is fixed.  And if you don't fix it first you can arrive at false conclusions (due to complicated interactions.)

QuoteI tried to remove tube itself and disconnect heater PS. And the hum is not present when I connect premap out to amp in,

Can you run all wires for the heaters back to the +rail and the ground point at the caps? 

Quotebut if I connect input jack to amp in, bypassing preamp completely, the same old hum returns.

OK this is a very common issue.  I'm assuming the input sock connects to chassis ground?   What other ground connections are there to the chassis?  If you have more than one it is much more difficult to arrive at a solution.
[edit: what about the speaker output jacks?]

Another issue, not related to the grounding, I noticed your tubes a fed from the 50V rail.   You might need to add an RC filter to remove ripple from the 50V rail.   Maybe 1k and 100uF to start.

Quote from: Rob Strand on November 17, 2017, 04:26:54 PM
Another issue, not related to the grounding, I noticed your tubes a fed from the 50V rail.   You might need to add an RC filter to remove ripple from the 50V rail.   Maybe 1k and 100uF to start.

Oh, I thought it is already filtered enough through main amps PS, so I will try this too. But first things first - to solve grounding problem. :(

I am trying to figure out differences between old SS preamp and my new hybrid, regarding connections.

There are two differences:

• Original one's input jack is connected to chassis, while my new preamp used jack just hanging outside, because I am still experimenting with it.
• Original is grounded through the point where input jacks connect to PCB, while my amp is grounded to the star point between main PS capacitors.

Original (not exactly original, because that's another construction by myself, only SS) preamp gets noisy (but very little) only when volume is at 70% or more.

So I will try to ground this new one exactly as the old one was grounded, and report what happens.

Quote from: rankot on November 18, 2017, 05:17:09 AM
I am trying to figure out differences between old SS preamp and my new hybrid, regarding connections.

There are two differences:

• Original one's input jack is connected to chassis, while my new preamp used jack just hanging outside, because I am still experimenting with it.
• Original is grounded through the point where input jacks connect to PCB, while my amp is grounded to the star point between main PS capacitors.

Original (not exactly original, because that's another construction by myself, only SS) preamp gets noisy (but very little) only when volume is at 70% or more.

So I will try to ground this new one exactly as the old one was grounded, and report what happens.

That could definitely be part of the poblem. Have a read through Merlin's article on grounding (http://www.valvewizard.co.uk/Grounding.html).

Grounded input sockets can be a pain.

Maybe like this,

(http://www.ozvalveamps.org/generic/groundpathawker.gif)

You put a break between the low level signals and high current signals.
This way you have two star points with a bridge between them.  One for quiet stuff and one for noisy.
The wire bridge must not carry significant current or ripple.  The bridge wire can be the chassis.
Generally you don't want power supplied across the bridge at all but in some cases you can have
the preamp power running across it.   (Also it's not good if the low signal side looses is 0V when if you disconnect the input socket from the chassis.  As a precaution, you can put a 10ohm resistor between the power supply 0V and the the preamp 0V.   The resistor doesn't do much in normal operation.)

I have tried two more things:

1. connect my preamp to ground at input jack only (grounding scheme exactly like the old SS preamp)
2. connect above + ground from triple power connector on my preamp's PCB, to star point on amp's PS, thus definitely creating a ground loop

Both behave completely the same. :(

I will try to change volume pot right now, as Merlin suggested.

Quote from: Rob Strand on November 17, 2017, 04:26:54 PM
Can you run all wires for the heaters back to the +rail and the ground point at the caps? 

It was connected that way for the first time I connected preamp, but the hum was the same as with later connection schemes. If I understood you well - ground for preamp connected directly to star point between filter caps on amp PS, and power wires for preamp connected directly to +/- rails at filter caps? But I later decided to separate heater supply, because it dropped rail voltage too much. However, that change didn't affect hum at all.

There's a schematic in PDF: http://www.cad.rs/d/ECH83v1.pdf
and PNG:
(http://i68.tinypic.com/2qa68tj.jpg)

I have changed output volume pot to A50k (didn't have smaller A type at the moment) and capacitor before it to 10uF and it helped to fix one thing - hum is not maximizing when volume pot is at 0, but gradually grows as I turn it to maximum. Maybe I could leave C18 to 100n, cause it forms HP filter with knee at 31Hz with 50k pot, but never mind, I can restore it later, just to get rid of this hum.

I have also added one 470 ohm resistor before filter cap C8, but it didn't seem to alter the hum a lot, then I removed ground path from star point to power connector (red gnd) on preamp and it decreased hum a lot, but not completely.

I will now try to add another 470 ohm resistor between caps C8 and C9, thus forming another ripple filter, and I will also try to completely isolate input jack and connect ground to star, to see what's happening in that case.

Lots of things to do :)

I have a feeling about this one.
Can I suggest a simple test. 

You have two wires, or a coax between the input of the power amp and the output of the preamp - yes?
Without having any preamp present, but with those two wires left in circuit,  short those two wires together.
Do you hear a hum?

If yes that means the power-amp input has the hum and you need to find a new ground point which has the least hum.

Look at this circuit,
http://sound.whsites.net/p3a-f1.gif

Often the best ground point is to connect near the ground side of C3.
[Edit:  Just to be clear, some circuits will have C3 and R4 reversed. In that case you want R4's connection to ground.]

Many power amplifiers will keep the wires for C3, C2, R2 on their own PCB track and the output from the preamp (ie. the power amp input) connects to there.   Some power amplifiers will even put a 10 ohm resistor from that point to the 0V in order to force good grounding and avoid ground loops.

Quote from: Rob Strand on November 18, 2017, 03:50:04 PM
I have a feeling about this one.
Can I suggest a simple test. 

You have two wires, or a coax between the input of the power amp and the output of the preamp - yes?
Without having any preamp present, but with those two wires left in circuit,  short those two wires together.
Do you hear a hum?

No. I've removed new preamp, and when I did what you suggested (short amp input and gnd), amp was absolutely silent. There was some hum when those wires (ground and amp input) were NOT shorted. I will remove old preamp completely and see what happens then.

I have also noticed that chassis is connected to circuit ground by input jacks, and there is no chassis-to-earth direct connection, see image below. So I will first add earth-chassis connection and replace all the jacks with plastic ones, as described in Merlin's script.

(http://i65.tinypic.com/2i0vfo9.jpg)

Original preamp is powered form + rail only, and it is connected to the rail at the very beginning of it. I will try to fix all the grounding, but if I don't succeed with eliminating hum with preamp, I will build separate PS for pre only, with additional transformer.

And one more question: if I add hum-loop block network, as proposed in Merlin's script, may I use 18 ohm 5W resistor instead of 10 ohm, because I have some of those in stock?

QuoteAnd one more question: if I add hum-loop block network, as proposed in Merlin's script, may I use 18 ohm 5W resistor instead of 10 ohm, because I have some of those in stock?

The resistor value isn't that important.

However,  that circuit doesn't apply directly to you case.  It is used when all the *sockets are isolated* from ground.  Then you bridge *circuit ground* and mains ground using that  network.  It prevents ground loops when you connect devices to the amplifier.   If you have non-isolated sockets that connect the ground to the chassis then that circuit won't work.

Look at figure 4 on this page, the circuit doesn't connect to the chassis and connector (in fact all connector) is isolated from the chassis:
http://sound.whsites.net/earthing.htm

You could change all your sockets to isolated ones, remove all circuit ground connections from the chassis, then add one of those networks Merlin proposed between the circuit ground and the mains ground.   I would say this configurations is *the* most successful configuration in removing all types of grounding issues.   In fact when I was young I experienced many issues like yours on systems with grounded sockets.   I changed to using the isolated sockets and those ground networks and never had any problems (that was for guitar and HIFI systems.)


Edit: I just realized you are in the process of changing to isolated sockets.  However, this is still a bit weird to me:

The thing I'm puzzled about is, you don't get any hum with the "shorted wire" experiment but you do get hum when you connect the power amp input to the preamp PCB and turn the output volume knob to 0.   What other connections are there to the PCB when you connect the preamp?  because one of those connections is the cause!

Something else I'm not 100% clear about:  For the input socket are you using separate input sockets hanging off your preamp  PCB?  Are they mounted to the chassis?  Or, are you wiring the preamp input to the existing sockets mounted to the chassis.

I haven't read through the thread, so forgive me if this is totally irrelavent, but did the solid state amp work well (hum free) before adding the tube preamp?  Those filter caps look original and decades old.

OK, I have finished isolating jacks, and connected ground to chassis at the same spot where it is connected to earth (still no hum breaker circuit, I'm waiting for a PCB). All the connections from jacks to preamp and from preamp to amp are done with shielded cables. I also have external preamp input, so my preamp can be overridden.

Ground for both preamp or preamp override jack is connected at the same spot - as shown on schematic with red letters (not drawn by me, I just marked it). I didn't try to connect it between PSU filter caps.

(http://i65.tinypic.com/23r4e2d.jpg)

If I connect my preamp to power rails from main PS filter capacitors, and I play through it, I still have strong hum. Remarkably less than before isolating/shielding etc, but still too much to enjoy playing. Heater is powered from another DC source and heater ground is not connected to amp ground, so heater use completely separate circuit.

If I connect external tube preamp based on 6J6 tube (built using a little bit modified schematic from here (http://www.diystompboxes.com/smfforum/index.php?topic=111622.0)), hum is very weak and almost unnoticeable. External preamp is powered from external power supply. But this preamp is somehow darker than this one based on ECH83, so I am really eager to make it work well. :(

If I turn volume pot on any of the preamps to the minimum, there is no noise or whatever in my amp, it's totally quiet. At least one achievement! :)

So I have only one idea left - to provide completely separate PS for preamp and see what happens then.

Regarding capacitors - they are old indeed, but I have measured them and their leakage is very low, so I didn't bother to replace them. And they don't make much hum when using external preamp or internal SS preamp so I am almost sure they are not causing hum on ECH83 preamp.

Thanks to explaining all the details it helps a lot.

QuoteIf I turn volume pot on any of the preamps to the minimum, there is no noise or whatever in my amp, it's totally quiet. At least one achievement!

Ok great.   That means one of the causes is gone.

QuoteIf I connect external tube preamp based on 6J6 tube (built using a little bit modified schematic from here), hum is very weak and almost unnoticeable. External preamp is powered from external power supply. But this preamp is somehow darker than this one based on ECH83, so I am really eager to make it work well.

That's a good test.  The small amount of hum might be from the transformer.  You could see if it reduces when you move the preamp (and wires) away from the transformer.   However that problem isn't the main issue right now.

Quotef I connect my preamp to power rails from main PS filter capacitors, and I play through it, I still have strong hum. Remarkably less than before isolating/shielding etc, but still too much to enjoy playing. Heater is powered from another DC source and heater ground is not connected to amp ground, so heater use completely separate circuit.

This is the problem to solve.

Maybe you have two ground paths?   What I'm not 100% clear about is:
1) On the amplifier, does the red ground point on the schematic connect back to the power supply 0V; by either a wire or a PCB track.
2) For the signal do you connect the ground of the preamp output to the power amp (red ground point on the schematic) , and also connect the preamp 0V (the red mark on you layout pic) to the power supply 0V (the point between the caps)? 

If you have (1) and both items in (2) there will be are two ground paths, in this case you can lift one.   You might need to experiment to find which is best.    You don't want to lift the ground if there is no 0V connection in (1)  that will cause trouble.

The other issue is the power rail might have too much ripple and the power filter to the tubes isn't enough.  Maybe try replacing the 470R with 4.7k and see if it makes *any difference*.

QuoteSo I have only one idea left - to provide completely separate PS for preamp and see what happens then.

This usually fixes all problems except hum from the transformer's magnetic field.
You shouldn't need to go this far.  If you look at the original amplifier they got it to work with one power supply and grounded sockets.  (An unrecognized achievement in many amplifiers like this!)

The best way to check caps is to measure ESR.

The preamp 50V should be decoupled from the raw 50V, say with 5K and 100uFd.

Quote from: Rob Strand on November 22, 2017, 05:00:38 PM
Thanks to explaining all the details it helps a lot.

Thanks to all of you for your time and knowledge! :)

Quote from: Rob Strand on November 22, 2017, 05:00:38 PM
Maybe you have two ground paths?   What I'm not 100% clear about is:
1) On the amplifier, does the red ground point on the schematic connect back to the power supply 0V; by either a wire or a PCB track.
2) For the signal do you connect the ground of the preamp output to the power amp (red ground point on the schematic), and also connect the preamp 0V (the red mark on you layout pic) to the power supply 0V (the point between the caps)?

If you have (1) and both items in (2) there will be are two ground paths, in this case you can lift one.  You might need to experiment to find which is best.  You don't want to lift the ground if there is no 0V connection in (1)  that will cause trouble.

1) There is only one path on amp PCB from the red point, where my preamp connects, to the amp ground (to the star point on PS between main filter caps). So yes, they are connected, but no loop there.
2) I connect preamp at single point only, so no, I don't connect it directly to PS star point from my 0V point on preamp PCB. I did it before, but not now. There is only one ground connection for signal and supply between my preamp and amp, the one marked red on amp schematic.

Quote from: Rob Strand on November 22, 2017, 05:00:38 PM
The other issue is the power rail might have too much ripple and the power filter to the tubes isn't enough.  Maybe try replacing the 470R with 4.7k and see if it makes *any difference*.
The best way to check caps is to measure ESR.

OK, I'll try better filtering, as proposed by you and PRR. I will also check power rails with oscilloscope. I have measured ESR of those main capacitors, but it was few months ago and I am not sure about values. I will do it again.

I actually have rail filtering done like this:

(http://i63.tinypic.com/2aj5efs.jpg)

I thought it shall be enough. But I will try to replace 470 with 4k7 resistors.

This is oscillograph I get at preamp output, no input signal at all:
(http://i66.tinypic.com/13z7qiv.jpg)
Almost perfect sine wave at 50Hz.

This is how positive rail looks like (this is with x10 probe):
(http://i63.tinypic.com/2qa3i0x.jpg)

Negative rail (the same probe) looks the same as positive (I've changed voltage display size):
(http://i68.tinypic.com/2z539lh.jpg)

So it seems that PS is the main trouble here. I will now disconnect my preamp and see what happens with rails without it.

Update: when I disconnect my preamp, this is what I have on plus rail of amp PS (x10 probe).
(http://i64.tinypic.com/2q3wz2u.jpg)

Negative rail is almost flat (can't measure smaller signal with my oscilloscope):
(http://i65.tinypic.com/2luqlqv.jpg)

:icon_question:

QuoteThanks to all of you for your time

No problem at all.

Quote1) There is only one path on amp PCB from the red point, where my preamp connects,

OK, great.  Should be OK here.

QuoteOK, I'll try better filtering, as proposed by you and PRR.

The more I think about it, it probably is a good idea to increase the resistors.  When the amp is running on full the ripple can be quite high.

Quotehis is how positive rail looks like (this is with x10 probe):

The ripple looks pretty normal.   (For some reason the waveform shape looks flipped upsidedown.)

It is normal for the ripple to increase as the load increases.  That's why it drops when you disconnect the preamp.

QuoteThis is oscillograph I get at preamp output, no input signal at all:
Almost perfect sine wave at 50Hz.

Yes the shape is suspicious.   It doesn't look like ripple *but* it could definitely be filtered ripple.  It is possible 50Hz getting into the circuit.  Sometimes the tone controls can affect the shape.

Does your input jack short the input to ground when you pull out the input jack?

At this point I'd be trying to narrow down which stage of the preamp has the 50Hz.
If you can change the hum level with the tone controls that is good indication that at least some of the hum is getting into the first tube stage.

My guess is the hum is probably getting in around the tone controls. (if it isn't caused by ripple)

I'd perhaps try to isolate which stage the hum is getting like this:
1) Disconnect the tube side of C2 and connect C2 to ground.
    Restore the C2 connection then,
2) Disconnect the wiper from treble pot and connect it to ground (with very short wire that doesn't form a large area loop.)

Use the oscilloscope and also listen.

Have you tried moving the preamp board away from the transformer?

I have tried with bigger filter resistors (4k7) and hum is the same :(

Quote from: Rob Strand on November 23, 2017, 05:04:11 PM
Yes the shape is suspicious.   It doesn't look like ripple *but* it could definitely be filtered ripple.  It is possible 50Hz getting into the circuit.  Sometimes the tone controls can affect the shape.
Does your input jack short the input to ground when you pull out the input jack?

No. And the sound of hum is the same if the guitar is plugged in or there's nothing plugged in.

Quote from: Rob Strand on November 23, 2017, 05:04:11 PM
At this point I'd be trying to narrow down which stage of the preamp has the 50Hz.
If you can change the hum level with the tone controls that is good indication that at least some of the hum is getting into the first tube stage.

Yes, they do affect hum. So you are right, first stage is suspicious.

Quote from: Rob Strand on November 23, 2017, 05:04:11 PM
My guess is the hum is probably getting in around the tone controls. (if it isn't caused by ripple)

I'd perhaps try to isolate which stage the hum is getting like this:
1) Disconnect the tube side of C2 and connect C2 to ground.
    Restore the C2 connection then,
2) Disconnect the wiper from treble pot and connect it to ground (with very short wire that doesn't form a large area loop.)

Use the oscilloscope and also listen.

Have you tried moving the preamp board away from the transformer?

No, I didn't try that, cause wires are not long enough :( But I may try to add some kind of shielding, another metal box connected to ground. I'll report what happens in all those new cases :)

QuoteI have tried with bigger filter resistors (4k7) and hum is the same

OK.  It was worth trying.  Probably a good idea to leave in the 4k7.

QuoteNo. And the sound of hum is the same if the guitar is plugged in or there's nothing plugged in.

Maybe at least see what happens with the guitar volume set to zero, and/or shorting input.

QuoteYes, they do affect hum. So you are right, first stage is suspicious.

OK, that's something to look into.

QuoteNo, I didn't try that, cause wires are not long enough :( But I may try to add some kind of shielding, another metal box connected to ground. I'll report what happens in all those new cases :)

I'm running out of options.  You might need to play around to see if you can provoke a change that will help narrow it down.   With such a large hum signal there must be an identifiable cause.    Supply ripple and grounding have been ruled out so it's likely to be a single cause now.  You could perhaps wire the first stage to the power amp, ideally you want a buffer in between as the gain will be reduced due to loading from the power amplifier.

If the problem is magnetic you will struggle to shield it.   Moving the preamp away and changing the PCB orientation are the best ways to identify magnetic fields as the cause.

[Edit: One thing worth trying.  Turn all your tone controls down to zero.  With that tone control it will short the input to the second tube stage.  It should have no hum.]

I have tried to move and rotate preamp towards transformer and away from it (as far as current wiring allows) and it didn't affect hum in any way. So it's not magnetic, I believe.

Turning all tone pots down did cut hum completely, but there is also almost no sound at output. I will try to wire only particular stages to amp to see what's going on. In a day or two my PCBs for hum breaker and separate PS for preamp will arrive, so I hope it will fix this problem if anything else doesn't.

QuoteI have tried to move and rotate preamp towards transformer and away from it (as far as current wiring allows) and it didn't affect hum in any way. So it's not magnetic, I believe.

That's actually good news.

QuoteTurning all tone pots down did cut hum completely

That's good too.

The problem is around the first stage and tone control for sure.
Like I mentioned before see if you can provoke a change.

Perhaps *totally unrelated* but worth checking.
I just realized your CRO plots show the ripple is 50Hz (20mS).
It's a full-wave rectifier so the ripple should be 100Hz (10mS between peaks).
Perhaps test your rectifier and also the soldering on the rectifier.

Something weird going on here.

One more idea - I don't have screen bypass capacitor here, could that be the problem? I didn't put it because I didn't want too much gain, to avoid distortion.

QuoteI don't have screen bypass capacitor here, could that be the problem?

It did cross my mind but there's plenty of designs out there without them.

Maybe just try it.

You could be coupling AC from the screen and the control grid which it is connected to.  But maybe the decoupling cap should go to the positive rail instead of ground.  As the B+ goes up during the ripple, coupling the B+ to the grids would tend to pull the plate voltage down.  You may find a coupling cap to ground and one to the B+ would allow you to neutralize hum just by varying the ratio of the two.

Quote from: amptramp on November 24, 2017, 07:32:44 PM
You could be coupling AC from the screen and the control grid which it is connected to. But maybe the decoupling cap should go to the positive rail instead of ground.  As the B+ goes up during the ripple, coupling the B+ to the grids would tend to pull the plate voltage down.  You may find a coupling cap to ground and one to the B+ would allow you to neutralize hum just by varying the ratio of the two.

I just have tried to use 1u decoupling capacitor between B+ and grid/screen and nothing changed :(

I have also tried to use a little TLC555 boost circuit which converts 9V to 37V, but it was also supplied from B+ rail and the result was still the same. I have used this boost converter while breadboarding this and it worked fine.

So I am really out of ideas what to try next, until my hum breaker and separate preamp PS PCBs arrive, so I will try with them.

I will also check ripple frequencies (my o-scope is not the best one) and if they are not as expected, I will try to replace bridge.

I have also tried to short op amp section and send triode output directly to amp input, and I believe it has less hum, but it may be subjective due to less amplification. I must test this further.

50Hz hum?  Do you have any other tubes to try?  Maybe even from a different manufacturer?  I learned the hard way recently that some preamp tubes have excess leakage from heater to grid, putting large amounts of heater hum into the signal.

Nevermind, you are using DC on the heaters I think.

QuoteSo I am really out of ideas what to try next, until my hum breaker and separate preamp PS PCBs arrive, so I will try with them.

On your layout I noticed the input is a long way from the tube.  Also, the input ground point is not near the ground side of R8 and C13.  I would solder a short wire from pin 2 of the tube (the input) to the ground point near C13 and R8.  Even if that doesn't solve the problem I'd leave it there until the problem is solved.

When you separated the heaters is there any connection between the heater ground and the preamp ground on the preamp pcb?  (I'm fairly sure there isn't. I'm just asking.)

QuoteI just have tried to use 1u decoupling capacitor between B+ and grid/screen and nothing changed

You should try to ground also (ground near the ground side of R8 and C13)

How confident are you that the tube second stage has no hum and the first stage has the hum? If you short the input of the second stage and there is no hum but when you route the first stage tube to directly to the power amp hum is present, then to me the first stage is clearly the problem.   Knowing the first stage is the cause removes a lot of circuitry from the equation.

If you look at only the first stage there's not circuit much there.  If you include layout then that's another variable.  With problems like this you need to change things to provoke a change.  It is better if you have a theory why the change might do something.    Another approach is to look at what is different between the other preamp that works and the one you have with the issue.

Do you think that this layout would be better regarding tube connections? It also contains integrated boost supply for 50V and ±15V supply for op amps, so I can try different power and grounding options.

(http://i64.tinypic.com/2cgmqv4.jpg)

Quote
How confident are you that the tube second stage has no hum and the first stage has the hum? If you short the input of the second stage and there is no hum but when you route the first stage tube to directly to the power amp hum is present, then to me the first stage is clearly the problem.   Knowing the first stage is the cause removes a lot of circuitry from the equation.

I will try first with all those decoupling capacitors and ground tube input, as you proposed, and I will try to isolate the problem.

I have also replaced PS filter caps with completely new 10.000u/80V. But I believe that old caps were fine, since their ESR was even better than that of the new ones. Never mind.

QuoteDo you think that this layout would be better regarding tube connections? It also contains integrated boost supply for 50V and ±15V supply for op amps, so I can try different power and grounding options.

It looks tighter but to be honest right now I can't look at it with the detailed eye that it needs.   We are getting work done at our house and I have to get some stuff done for tomorrow morning.  I've just stopped for 5mins to had a cup of tea!.   I won't have a computer tomorrow.

QuoteI will try first with all those decoupling capacitors and ground tube input, as you proposed, and I will try to isolate the problem.

I definitely recommend finding the issue before you do another PCB.    Even if you cut tracks and use air-mounted components close to the tube to prove the point.   If you can find and solve the problem with the PCB you have then you *know* what needs to be changed on the next version.

You were right about rectification problem - I didn't pay attention to ripple frequency. Now I have checked both rails simultaneously and they are at 50Hz with half cycle phase shift, which most probably means that a pair of diodes in bridge is dead. Positive rail has much bigger ripple than negative rail, approx 10 times bigger. It could be connected to the fact that amp's VU meter is powered from positive rail only.

I have also noticed that input jacks have some kind of 50Hz hum when nothing is plugged in. I have no idea why. This is the case even when I remove the tube (and even then I have the hum at output, but a little lower). Maybe opamp section produces this hum? Or it's power supply with LM7x15 pair? How to check it?

So I desoldered the bridge and measured it - and it is fine - all four diodes are measurable with multimeter and measure V_f of about .59V. Both transformer secondaries show the same voltage (62V AC) when bridge is out. I have checked all connections from transformer to bridge and from bridge to filter caps and all appear to be fine.

Then I returned the bridge and removed preamp: so without preamp, the rail ripple remains the same (50Hz instead of 100) and bigger on positive rail than on negative, just as it is when preamp is in.

I am completely puzzled with this...

Quote from: Rob Strand on November 23, 2017, 05:04:11 PM
Does your input jack short the input to ground when you pull out the input jack?

Actually they do; jacks are isolated and connected as usual Fender way (with 68k and 1M resistors) for high and low input. So when plugs are not plugged, inputs are grounded.

And I have tried those four combinations:
1. Removed tube, there is some very weak 50hz hum at output even when input jack is not connected to op amp, but only audible at maximum; no hum at input jack.
2. Removed op amps, tube in place: a lot more hum then above; hum at input jack.
3. Removed both tube and op amps, still hum but very, very weak. No hum at input jack.
4. With tube and op amps all in place, strongest hum; hum at input jack too.

So it is quite sure that the tube is producing hum. Investigating further!

QuotePositive rail has much bigger ripple than negative rail, approx 10 times bigger. It could be connected to the fact that amp's VU meter is powered from positive rail only.

Different loads on different rails are a common cause for imbalanced ripple.  I actually thought it was the tube heaters.

QuoteI am completely puzzled with this...

It's a little puzzling to me too.  If it's not the rectifier,  it could be *one* of the windings not connecting.  It could be the side of the winding that goes to the rectifier or where the two windings join.   Sometimes the enamel coating on the wire hasn't been removed correctly leading to a bad joint.

If you get ambiguous results measuring power supplies it a good idea to take the all measurements with a load.   Maybe 1k or 100 ohms or both; 100 ohms will get hot at 50V.   The multimeter's high impedance sometimes leads to false measurements.

If the voltage on the transformer windings aren't exactly the same you might see this behaviour.  Normally the windings are done in a way that guarantees the voltage to be the same.

QuoteActually they do; jacks are isolated and connected as usual Fender way (with 68k and 1M resistors) for high and low input. So when plugs are not plugged, inputs are grounded.

OK. (Good)

Quote1. Removed tube, there is some very weak 50hz hum at output even when input jack is not connected to op amp, but only audible at maximum; no hum at input jack.
2. Removed op amps, tube in place: a lot more hum then above; hum at input jack.
3. Removed both tube and op amps, still hum but very, very weak. No hum at input jack.
4. With tube and op amps all in place, strongest hum; hum at input jack too.

I'm not sure when you removed stuff if you just pulled that item out, or if you bypassed it with wires. (Sorry)

Edit:  forgot a couple of things:

Quote4. With tube and op amps all in place, strongest hum; hum at input jack too.

Did you try shorting the tube input to ground, near the ground side of R8 and C13

QuoteIhave also noticed that input jacks have some kind of 50Hz hum when nothing is plugged in. I have no idea why. This is the case even when I remove the tube (and even then I have the hum at output, but a little lower). Maybe opamp section produces this hum? Or it's power supply with LM7x15 pair? How to check it?

If you don't have the "ground network" in place and there are *no* ground connections at all then sometimes you get hum with things connected to the ckt.

Power transformers are not necessarily wound with balanced resistance.  If you have one side of the push-pull secondary winding wound around the one for the other half of the push-pull output, you can get a difference in resistance.  I have a 15-tube Philco 37-116 sitting behind me right now that takes 165 watts of input power and produces 15 watts of audio output from a pair of 6B4G triodes.  One side of the power transformer plate winding is 45 ohms, the other side is 47 ohms.  A more typical 5-tube radio with a power transformer has 120 ohms on one side and 130 ohms on the other.  Check the power supply for resistive balance.

Quote from: Rob Strand on November 27, 2017, 03:42:57 PM
I'm not sure when you removed stuff if you just pulled that item out, or if you bypassed it with wires. (Sorry)

I pulled them out of the sockets. :)

QuoteDid you try shorting the tube input to ground, near the ground side of R8 and C13

I just did and hum is the same. :( Does it mean that first tube stage is OK? I will try to short treble pot lug with ground to see what happens then.

And I have checked once again input hum - there's no input hum, I made a bad ground connection with probe clippers, so it actually floated when I tried to measure input hum.

I have also measured tube's second stage output, when first stage input is shorted to C13 ground, and this is what I get:
(http://i64.tinypic.com/avfvbk.jpg)

Actually, when I look at all those graphs, it seems that there is a lots of different noise frequencies, but what bothers me the most is the base frequency of 50Hz (frequency reading on graphs are not OK).

Now I have shorted input of tube's first stage with output of the second stage and there's almost no hum at all (barely heard when volume at max).

However, I have noticed few things in that scenario:
1. If I don't connect tube heater to it's supply, then I get minimal hum, as described above.
2. If I connect tube heater to it's supply, but with supply off, hum is maximized. It seems that the cable is working as antenna.
3. If I connect tube heater to it's supply and turn it on, there is some very weak hum, a little bit stronger than in case 1.

My heater's ground is not connected to audio or amp ground, it is completely separated and DC powered. Is it possible that all the hum is created by the heater? How to solve it?

Quote from: Rob Strand on November 25, 2017, 04:10:41 PM
On your layout I noticed the input is a long way from the tube.  Also, the input ground point is not near the ground side of R8 and C13.

I have tried to connect input ground to C13 ground directly and it didn't change the hum :(

Quote from: rankot on November 28, 2017, 04:53:33 AM
My heater's ground is not connected to audio or amp ground, it is completely separated and DC powered. Is it possible that all the hum is created by the heater? How to solve it?

Heater must be referenced to ground.

It can be referenced directly to ground (one side of AC heaters).
It can be center tapped AC heaters.
It can be artificially center tapped (two 100 ohm resistors).
It can be referenced to a DC voltage (helps supress heater to cathode leakage).
It can be referenced through a humdinger pot (to adjust reference balance for least hum).

No matter what, you should not leave the heater voltage floating with reference to the rest of the circuit.

Quote from: Phoenix on November 28, 2017, 07:44:48 AM
Heater must be referenced to ground.

It can be referenced directly to ground (one side of AC heaters).
It can be center tapped AC heaters.
It can be artificially center tapped (two 100 ohm resistors).
It can be referenced to a DC voltage (helps supress heater to cathode leakage).
It can be referenced through a humdinger pot (to adjust reference balance for least hum).

No matter what, you should not leave the heater voltage floating with reference to the rest of the circuit.

Pal, you nailed it! I have connected my heater's ground to audio ground using 18 ohm 5W resistor (the one I had at hand) AND NO HUM AT ALL! Thing fixed!

Thanks again to everyone for help and support!

:icon_mrgreen:

One more thing to solve, and it is rectification problem.

QuotePal, you nailed it! I have connected my heater's ground to audio ground using 18 ohm 5W resistor (the one I had at hand) AND NO HUM AT ALL! Thing fixed!

Thanks again to everyone for help and support!

Very cool!

FYI:

QuoteI just did and hum is the same. :( Does it mean that first tube stage is OK? I will try to short treble pot lug with ground to see what happens then.

What that mean is the hum doesn't come from the long tracks at the input of the first tube.  It eliminates all that part of the circuit from the equation.

QuoteI pulled them out of the sockets.

Nothing wrong with that.  In general if you leave inputs floating they are susceptible to picking up hum and buzz.  They can also make the circuit more susceptible to producing hum & buzz when you connect something to the circuit.  It's best to ground floating inputs, you don't want to add to the problem!

Quote from: rankot on November 28, 2017, 11:52:44 AM
One more thing to solve, and it is rectification problem.

I read above about your rectification issues, and you're seeing 50Hz ripple despite the amp using a bridge rectifier and that rectifier testing fine out of circuit right? Could you sketch us out the power supply, the bridge rectifier could be just used for half-wave rectification for bipolar rails if the high voltage winding on the power transformer is not center tapped, in which case everything would be working as expected, and there's nothing wrong. If you were getting too much ripple though, you'd have to increase the bulk reservoir filter capacitors (if the transformer has enough current to spare).

I have a Stromberg-Carlson 207 public address amplifier that has no connection to the tube heaters other than a 0.1 µF capacitor to one side of the line.  I haven't used it yet, but I might try setting the heaters to some voltage.  This beast has 6L6's in the output, so if you get hum, it can get LOUD.

QuoteI might try setting the heaters to some voltage.

See the list that Pheonix posted above.
Watch out though, the methods only work if the heater transformer windings are floating.

Some of the Fender amps actually had a hum balance pot.  This works well when adjusted correctly, it can also work against you if someone puts it off the null point.

Quote from: amptramp on November 29, 2017, 07:11:11 PM
I have a Stromberg-Carlson 207 public address amplifier that has no connection to the tube heaters other than a 0.1 µF capacitor to one side of the line.  I haven't used it yet, but I might try setting the heaters to some voltage.  This beast has 6L6's in the output, so if you get hum, it can get LOUD.

There's probably enough leakage current in that cap (even when it was new) that it provides a weak reference, and that's all the heaters really need, they don't need to have a low impedance reference. The reference just minimises random heater-to-cathode leakage.
Different if you want balancing of course, in which case you want the lowest impedance you can afford (like a center tap or pair of 100R resistors).

Elevation of heaters (other than sometimes being necessary to avoid heater to cathode breakdown) can saturate the heater to cathode leakage, which makes it more steady and less random, reducing noise, so might be worth a try in your 207.

Quote from: Rob Strand on November 29, 2017, 07:33:13 PM
Some of the Fender amps actually had a hum balance pot.  This works well when adjusted correctly, it can also work against you if someone puts it off the null point.

I'm not a fan of the pot.  I mentioned it the other day in this thread or the other one, but those pots don't hold up IME.  Fender used a 1/2W carbon pot, Ampeg used a 5W (I think) wirewound, almost no difference.  Seems the problems tend to occur at the wiper, or where it contacts the resistive track.  The two 1/2W resistors almost never fail, unless they break physically in the Fenders because they are mounted to the lamp holder which works loose.

QuoteSeems the problems tend to occur at the wiper, or where it contacts the resistive track.  The two 1/2W resistors almost never fail, unless they break physically in the Fenders because they are mounted to the lamp holder which works loose

IMHO, it would be better to have the 2x 1/2W resistors then a higher value pot in parallel.  That way it's basically good even if the pot wasn't there.  You just fine tweak it to get a good null.

This is schematic of amp's power supply, as asked by Phoenix:

(http://i68.tinypic.com/i29iz8.jpg)

Ok, should definitely not have 50Hz ripple on that if all diodes in the bridge are functional. Have you double checked it's wired exactly as drawn there?

I did, but I will check once again in the morning. :)

(https://s33.postimg.org/426vtjgln/48_VBplus.gif) (https://postimg.org/image/426vtjgln/)

I actually have those filter resistors and capacitors for preamp tube stage, but I didn't show them here, because I have a problem with rectification :)

QuoteI have a problem with rectification

Try deliberately disconnecting one side of the transformer from the rectifier then the other.  Measure the DC voltage accurately in each case.

This will show any imbalance in the winding + rectifier voltages.   If you have a bad connection, one of these cases might not even work.

When I disconnect one side of transformer, I get +53.9V and -54.8V.

When I connect that side back and disconnect another one, I get +54.1V and - 55.1V.

Also, both coils have almost the same resistance (measured when disconnected) of abou 1.6-1.7 ohm.

I will try to replace bridge with a new one.


I have another problem that I didn't previously notice too much because of hum - there is some HF noise getting into the preamp through the guitar cable. When I touch the cable (even outside isolation) it gets stronger, and when I touch strings, it is even stronger. Turning treble pot makes it stronger or weaker. I tried to parallel 1M input pulldown resistor with a small capacitor (100p) but nothing changed.

QuoteWhen I touch the cable (even outside isolation) it gets stronger, and when I touch strings, it is even stronger. Turning treble pot makes it stronger or weaker. I tried to parallel 1M input pulldown resistor with a small capacitor (100p) but nothing changed.

Try a 10nF to 100nF connected from the input socket ground to the chassis at the input socket.

Do you have your ground network connected up yet? (the one with the cap + 10ohm + diodes)

Quote from: Phoenix on December 01, 2017, 10:12:20 AM
Ok, should definitely not have 50Hz ripple on that if all diodes in the bridge are functional. Have you double checked it's wired exactly as drawn there?

Is it possible that something else is making those ripples? Dead output transistor, or something like that?

QuoteIs it possible that something else is making those ripples? Dead output transistor, or something like that?

It's unlikely for an amplifier circuit to do that.

You could disconnect the amplifier and preamplifer from the power supply, place a dummy load on the power rails then measure the ripple.  Ideally you would put loads equal the amplifier's current draw on each rail but any reasonable load should be OK.   Drawing more current might help also, because ...

From your measurements the rails differ by 0.2V to 0.3V, so it is possible if the load doesn't draw enough current the slight imbalance in the voltages will cause the ripple to be 50Hz.  When the load is increased to the point where the ripple exceeds say 0.6V you should see the other winding suppyling power and the ripple might return to 100Hz.    The rectifier voltage drops might adding the problem but the measurements you gave a while ago looked OK - when you have weird problems it doesn't hurt to experiment.

Quote from: Rob Strand on December 02, 2017, 07:16:47 AM

QuoteWhen I touch the cable (even outside isolation) it gets stronger, and when I touch strings, it is even stronger. Turning treble pot makes it stronger or weaker. I tried to parallel 1M input pulldown resistor with a small capacitor (100p) but nothing changed.

Try a 10nF to 100nF connected from the input socket ground to the chassis at the input socket.

Do you have your ground network connected up yet? (the one with the cap + 10ohm + diodes)

Yes, I have built and installed ground network (just like in Merlin's script), but it seems to do nothing. Noise's the same regardless I connect chassis through it or directly :(

And to clarify this with cable, I didn't wrote well - noise is bigger when I touch jack (or guitar strings), it doesn't change when I touch cable isolation.

QuoteAnd to clarify this with cable, I didn't wrote well - noise is bigger when I touch jack (or guitar strings), it doesn't change when I touch cable isolation.

If it was only when touching the strings I'd suspect the grounding of the bridge in the guitar.  However, since it also occurs when touching the jack it's not that at all.  In fact it's quite a tricky problem.

At this point I'd probably take a few steps back in order to work out where the problem lies. See what happens with these, do them one at a time maybe not together.   Don't worry about hum, focus on the buzz.
- Replacing the ground network with a short.
- Replace the input jack with a non-isolating type.

Another thing to try is to solder a ground wire to each pot then try connect that wire to the chassis, then try connecting it to the circuit's 0V.    Ungrounded  pot casings can causes weird noise issues.

One thing I"m not 100% clear on:  What other jacks are there?   Are they all isolated types?   (don't forget to include speaker jacks)

Quite possible the amp's input jack is wired incorrectly.

Quote from: Rob Strand on December 02, 2017, 08:13:22 PM
- Replacing the ground network with a short.
- Replace the input jack with a non-isolating type.

Another thing to try is to solder a ground wire to each pot then try connect that wire to the chassis, then try connecting it to the circuit's 0V.    Ungrounded  pot casings can causes weird noise issues.

One thing I"m not 100% clear on:  What other jacks are there?   Are they all isolated types?   (don't forget to include speaker jacks)

I have already tried to short ground network and it didn't help.

Since all jacks are isolated (there's no speaker jack, it's cable is soldered to PCB), I will try to restore non-isolated one for input and see what happens. I will also try with grounding pot casings.

OK, here is what I've done:
- added 56n cap between jack ground and amp PCB ground (can't solder it to front panel, it's made of aluminium and solder doesn't stick to it);
- added 100p cap between input and ground, just before first stage's grid;
- tried with and without hum breaker.

After that:
- Hum breaker works OK, I have double checked and it definitely kills most of the hum.
- There is still hum that I produce when I touch strings or jack, but this hum is very low when hum breaker is connected; what is important with this hum is a fact that when I touch stings for the first time, there's a loud pop, and when I play it doesn't pop again, but if I leave a guitar for a few seconds and touch strings again, it pops again. The trouble is that I want to rock, not to pop :)
- No more radio.
- I have killed too much of treble with that 100p cap, I will try without it or with smaller value.

I have also noticed that amplification is not as strong as before and the sound is distorted. Probably that 100p cap too.

This is schematic of input section as it is right now:
(http://i67.tinypic.com/286v9ly.jpg)

Quoteadded 56n cap between jack ground and amp PCB ground (can't solder it to front panel, it's made of aluminium and solder doesn't stick to it);

Normally you would use a large ground lug.  The input socket thread passes through the hole.  Not these exact ones, but ones that look like this,
http://www.guitarpartsresource.com/images/large/EP-4968-000.jpg

Quote- No more radio.
- I have killed too much of treble with that 100p cap, I will try without it or with smaller value.
I have also noticed that amplification is not as strong as before and the sound is distorted. Probably that 100p cap too.

Agreed.  Maybe 47pF.   Try to work out the largest value before you start losing treble.  Hopefully that's big enough to remove the radio.
The tube has some input capacitance (not sure what it is for that tube) but it is probably around 100pF.
With 2x68k's on the input that already has some HF roll-off due to the tube capacitance.

Quotewhat is important with this hum is a fact that when I touch stings for the first time, there's a loud pop, and when I play it doesn't pop again, but if I leave a guitar for a few seconds and touch strings again, it pops again.

Yes, that is probably the most important thing to fix.   It is not normal and it points to a problem.
Sounds like a capacitance is charging up.  Exactly what capacitance and how, I'm not sure at the moment.

A broken ground in your guitar cable or somewhere in the amp might be the cause.   With the amplifier turned off, use your multimeter and check continuity from the socket on the guitar to various ground points in the amplifier.

How confident are you the ground on your mains socket is OK?

QuoteI have also noticed that amplification is not as strong as before and the sound is distorted. Probably that 100p cap too.

That's a bit strange too.
If you have strong RF or something is oscillating you might see this type of thing.  In fact oscillation could cause the weird pop behaviour as well.  Can you monitor the output of your power amp and various points in the preamp with an oscilloscope?  Use a x10 probe.   Unfortunately when you connect the CRO ground, or the CRO probe, it changes the circuit sometimes even stopping the oscillation!

The ECH83 characteristics are here:

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/ECH83.pdf

The input capacitance of the hexode section from Grid 3 to everything else is 6.0 pF and from Grid 1 to everything else is 4.8 pF.  This is not going to have much effect on the high frequency rolloff.

With any guitar/amp that I'm aware of, there should be less hum when you touch the strings or other grounded metal part.  Getting more hum suggests somewhere the "hot" and "shield" are reversed.  The pop IMHO is from static on your body.  It is winter after all.  Something like that will be exaggerated with the "hot ground" issue.

QuoteThe input capacitance of the hexode section from Grid 3 to everything else is 6.0 pF and from Grid 1 to everything else is 4.8 pF.  This is not going to have much effect on the high frequency rolloff.

Thanks.  The Miller effect is very low then, so we only have to deal with the 4.8pF.
That's means adding 100pF is about right.  Right in the sense of emulating the input capacitance of a 12AX7.
The HF roll-off should be noticeable but not extreme.

I was too busy with regular job those days, now I build separate PS for this preamp. I decided to debug it outside amp first, and when it's completely fine on its own, to try to put it inside.

I'm back on this baby! I've built a separate power supply, and it is working almost fine. Almost means that there is hum sometimes, sometimes not! PS is supplying 9V DC to my preamp, and that is later used for heater supply (regulated with 7806) and for ±15V and 50V generated with LTC1054 based boost DC/DC converter. The most interesting thing is the fact that if I plug my preamp together with amp, and at the same time, hum is annoying. If I unplug preamp, then plug in again, hum gets reduced. HOW TO HELL IS THIS POSSIBLE?

When I put my oscilloscope to 9V DC, it shows that there is a small (±20mV) sine wave still present after regulator, but I presume it is acceptable value.

Besides that, I have tried to add bypass capacitor parallel to screen resistor and it just produced distortion, so I removed it. I have also put two 100p bypass capacitors to ground behind pentode and triode and that completely killed all the radio noise I had.

QuoteThe most interesting thing is the fact that if I plug my preamp together with amp, and at the same time, hum is annoying. If I unplug preamp, then plug in again, hum gets reduced. HOW TO HELL IS THIS POSSIBLE?

I'm not sure if you are plugging in the signal lines or the power.

There's something weird going on for sure.  That and the weird click problem you were seeing before points to something is wrong.   Generally when you see this type of thing something *is* wrong.   It's always hard to find because it is often caused by something you are assuming is correct is not correct.  Something as simple as two wires connecting, like a ground.  At this point I can only suggest checking connections throughout the system.  It is a slow process proving what you think is true is actually true.

Quote from: Rob Strand on December 27, 2017, 05:56:44 PM

QuoteThe most interesting thing is the fact that if I plug my preamp together with amp, and at the same time, hum is annoying. If I unplug preamp, then plug in again, hum gets reduced. HOW TO HELL IS THIS POSSIBLE?

I'm not sure if you are plugging in the signal lines or the power.

There's something weird going on for sure.  That and the weird click problem you were seeing before points to something is wrong.   Generally when you see this type of thing something *is* wrong.   It's always hard to find because it is often caused by something you are assuming is correct is not correct.  Something as simple as two wires connecting, like a ground.  At this point I can only suggest checking connections throughout the system.  It is a slow process proving what you think is true is actually true.

I am plugging in the power. :)

QuoteI am plugging in the power.

You could  measure the current draw and see if there is a consistent difference in the humming and non-humming states.  (Sometimes oscillation can appears as a hum.)

OK, I will.

Well, I thought that it could be the problem with underpowering the preamp, so I changed some things (put bigger capacitors on voltage doubler, replaced LM317T with LT1083, but no progress yet. I have also tried to add few 100n capacitors between DC power and ground at some places to kill some oscillations, but it didn't help either.

Here is the clip (https://clyp.it/rcguc4b1)

So the first 4 seconds are just the preamp's own noise, after that few seconds when I touch the stings and the noise gets stronger (seems I behave like an antenna, so you can call me Radio Ranko), and after that few tones. I like the sound of it very much, but this noise is freaking me out completely!  :icon_twisted:

Thanks for the clip.    Yes, it's annoying at higher volumes.

I had a look at the spectrum of the noise/hum.    There are harmonics of 50Hz across the entire spectrum (all the way up to the 8kHz limit of the mp3).  It is present in the good case and the bad case.  When you touch the strings it all rises up by about 8dB.  However at 100Hz and 200Hz (but not at 50Hz) it rises up quite a bit more say 20dB;  I can't explain that at this point.

From what I can see the problem is related to grounding.

Have you tried a different guitar, different cable, or shorter cable?

Does the noise/hum decrease if you touch the mains ground (say the amp chassis) with the palm of your hand while touching the strings?

Does the hum change when you set the guitar's volume to zero.  And with the guitar's volume set to zero, does touching the strings have any effect?

[Edit:  One thing you can try is increasing the caps from the circuit's 0V to the chassis.   Normally you wouldn't have to do this.]

Rob, preamp is still in modular/experimental phase:

(http://i63.tinypic.com/23ur3q0.jpg)

So no chassis yet. It seems I will have to make one to continue with debugging. :) I tried to shorten DC cable from PS to preamp, but it didn't change a thing.

But I will certainly try all chassis unrelated suggestions before that!

QuoteSo no chassis yet.

Ah! The lack of chassis may actually be the cause of the hum!
It is quite common to get the problem you are seeing with unshielded circuits.

Try putting a square of aluminium foil under the boards.
Make it extend past the edges of the board.
Then connect the aluminium foil to either 0V or mains ground.

If you want to be safe put a piece of paper between the circuit and the foil.

I'll put a cardboard, just to make better isolation, but I have also tried what you suggested and here are the results (have in mind that when I mention pots here, I relate to pots on guitar/bass, not pots on my preamp):

- I work as an antenna regardless volume control on guitar/bass. Actually more on bass (seems it somehow knows that's MINE instrument) :) So when volume is at zero, I touching strings on both instruments increase hum.
- If I unplug guitar and left cable plugged into preamp, hum is HUGE and it seems to be clear 50Hz; touching cable tip with finger reduce that huge hum to previous level (as with bass/guitar plugged in).
- Volume control of my bass doesn't affect hum a lot, but if I plug my guitar, hum seems to be strongest with volume at middle, reducing or increasing volume decrease hum level.
- Tone control on bass/guitar affect hum level a lot, even with volume at zero - turning tone pot to high adds high noise to hum, turning to low it does reduce higher harmonics of hum.

Is your bass active?

The guitar in the middle being worst is "normal" when crap is getting into the signal lines.

I just noticed on your picture that the input wires are not shielded - is that right?
You should use coax.  For the interim make them short as possible also twist them tightly together.

[Edit:

Quote- If I unplug guitar and left cable plugged into preamp, hum is HUGE and it seems to be clear 50Hz;

I forgot to mention that is normal.

Normally what I do to get a baseline for hum/noise is short the input with a *very* short piece of wire.  The hum/noise usually doesn't get any better than that.    Next is to put in a lead and then short the instrument end of the lead.   In this configuration you should be able to touch the ground without the hum/noise increasing; the more junk you hear the poorer the grounding.
]

Have a look at the pic in post #5 by Stallik,

http://www.diystompboxes.com/smfforum/index.php?topic=116423.0

That type of set-up avoids a lot of problems with hum and noise. 
You can try to approximate the same idea with foil and cardboard.

I have replaced simple input wires with shielded ones and hum is still the same.

This is how I built PS, I had only 6V transformer at hand, so I decided to build a voltage doubler based PS. Could it be the cause of 100 and 200 Hz hum?

(http://i64.tinypic.com/35ckb47.jpg)

QuoteThis is how I built PS, I had only 6V transformer at hand, so I decided to build a voltage doubler based PS. Could it be the cause of 100 and 200 Hz hum?

It may be contributing but at this point it is hard to know.

One way it can contribute is the transformer itself is too close to the circuit, or the guitar.   If you can move the guitar and the preamp circuit about 1m away from the transformer it would be a good test.  Really you need to move at least upto C1 away as well because you don't want the current pulses from the transformer on the long wires.  Best would be to move the whole PSU circuit upto the 9VDC part away.

Another way it can cause trouble is by putting current pulses through the ground.  However, from the previous discussions the way you connect the power looks OK and should not cause such issues.

Yet another way is junk is coupling through the transformer.   Normally this isn't a problem.  That would be reduced by connecting the transformer chassis to mains ground (and possibly adding small caps from the transformer outputs to mains ground.)
Is your transformer chassis connected to mains ground?

The reason why I think it is grounding/shielding related is the fact the problem gets worse when you touch ground.  This is a common characteristic of a grounding or shielding problem.  From your sound sample there is hum present before you touch it.  That can be PSU or grounding.

Fixing these types of problems takes a lot of patience and experimentation.  The aim is to narrow down the problem to a smaller zone.  You need remove all possible causes of noise, like shielding the wires, placing it over a ground plane, moving the PSU away, grounding the pot cases.   While that might fix the issue, there's chance it might not.   After that, through a sequence of experiments,  you start narrowing down where in the circuit the noise is coming from.   At any time you might discover the problem is one silly thing.    Sometimes you just have an idea try it and it fixes it but that relies on luck.  It's much like how we narrowed down the hum problem before to one tube, and then the grounding of heaters.   (There could still be a problem with the grounding of the heaters!)