Tube supply question

[Rob Strand]

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.

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.

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 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.

[rankot]

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...

[rankot]

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!

[Rob Strand]

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.

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

I 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.

[Rob Strand]

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.

OK. (Good)

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.

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:

4. 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

Ihave 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.

[amptramp]

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.

[rankot]

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.

Did 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:


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).

[rankot]

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?

[rankot]

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

[Phoenix]

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.

[rankot]

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!



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

[Rob Strand]

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!

Very cool!

[Rob Strand]

FYI:

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.

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.

I 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!

[Phoenix]

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).

[amptramp]

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.

[Rob Strand]

I 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.

[Phoenix]

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.

[thermionix]

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.

[Rob Strand]

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

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.

[rankot]

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