Tube supply question

[rankot]

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.



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), 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.

[Rob Strand]

Thanks to explaining all the details it helps a lot.

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!

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

If 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.

f 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*.

So 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.

[PRR]

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

[rankot]

Thanks to explaining all the details it helps a lot.

Thanks to all of you for your time and knowledge!

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.

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.

[rankot]

I actually have rail filtering done like this:



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

[rankot]

This is oscillograph I get at preamp output, no input signal at all:

Almost perfect sine wave at 50Hz.

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


Negative rail (the same probe) looks the same as positive (I've changed voltage display size):


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).


Negative rail is almost flat (can't measure smaller signal with my oscilloscope):

[Rob Strand]

Thanks to all of you for your time

No problem at all.

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

OK, great.  Should be OK here.

OK, 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.

his 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.

This 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?

[rankot]

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

[rankot]

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.

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.

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

[Rob Strand]

I 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.

No. 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.

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

OK, that's something to look into.

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

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.]

[rankot]

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.

[Rob Strand]

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.

That's actually good news.

Turning 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.

[Rob Strand]

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.

[rankot]

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.

[Rob Strand]

I 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.

[amptramp]

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.

[rankot]

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.

[thermionix]

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.

[Rob Strand]

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.

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.)

I 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.

[rankot]

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.

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.