Bias drift when volume engaged.

[Ampzone]

What could be the cause of the bias dropping lower e.g. 30ma dropping to around 13ma per EL34 tube when the volume master pot 250k is fully off? If you raise the volume pot the bias draw starts building back up to 30ma.

[The Dude]

Hard to say without seeing a schematic of the circuit or even knowing what amp we're talking about.

[g1]

You might be looking at it backwards. (why is tube conducting more when volume is raised above zero)
Hum or oscillation could be reasons.

[Randall]

What is the sound of one hand clapping?

[Ampzone]

Hard to say without seeing a schematic of the circuit or even knowing what amp we're talking about.

http://i.imgur.com/6rLTy56.jpg layout that is similar.

https://drtube.com/schematics/hiwatt/hwpre1.gif

https://drtube.com/schematics/hiwatt/hwpsu1.gif

https://drtube.com/schematics/hiwatt/hwpwr100w.gif

[Chuck H]

The master in the schematic is 220k. I could assume that a 250k was used as a "nearest standard value". Or I could ask if we're all certain that the master circuit is just as shown in the schematic and the design has not been altered to a different master circuit.?.

Otherwise I agree with g1 that there could be a possibility of oscillation. Check lead dress and grounds.

[Ampzone]

The master in the schematic is 220k. I could assume that a 250k was used as a "nearest standard value". Or I could ask if we're all certain that the master circuit is just as shown in the schematic and the design has not been altered to a different master circuit.?.

Otherwise I agree with g1 that there could be a possibility of oscillation. Check lead dress and grounds.

It matches the circuit schemo, and yes nearest value. I'm leaning towards something with the grounds as well.

[The Dude]

How new are the filter caps? I'm also wondering if the amp is oscillating or motor boating. If so, it could easily be a bad filter cap or caps. Do you have a scope?

Edit: Also, what happens to your bias supply voltage when this happens. Does it stay consistent?

[Ampzone]

New filter caps. no motor boating at all, amp sounds ok. I just noticed the bias drift when setting bias. when the 250k pot was turned off it was huge drop on all the tubes. I will check the bias supply voltage now. Good idea

[g1]

So there is no sound or hum that increases when volume is turned up?

[m1989jmp]

What could be the cause of the bias dropping lower e.g. 30ma dropping to around 13ma per EL34 tube when the volume master pot 250k is fully off? If you raise the volume pot the bias draw starts building back up to 30ma.

Every bias tip I've read so far states that bias current is measured with (at least) vol down. You're measuring IDLE current that way. I suppose when you bring the volume up, something non-static happens, right?

Are you wondering why it drops/rises at all, or you're thinking it drops/rises too much?

[Ampzone]

So there is no sound or hum that increases when volume is turned up?

the negative voltage does not change when the 250k pot is adjusted. stays around -45vdc dead on. Now I just noticed that if I adjust the presence pot the same thing happens, we go from an average of 27ma down to 13ma per tube when either the 250k Vol pot or presence is adjusted or turned off. No other pot eq control affects like this. To me there should be a consistent current draw ma on each tube regardless of any pot value setting. Head scratching

[g1]

You are not answering the question about what you are hearing through a speaker.
Any hum, or frequency above or below our hearing will change the tube idle current when the master is not at zero.
You need to listen to a speaker connected, and also scope the output in case it is not an audible frequency (oscillation).
You should not expect the draw to stay the same for all settings of the pots because there are exceptions.

[The Dude]

Another question, and just to be clear: These symptoms are with no signal applied?

I have to wonder how you discovered this. I don't think I've ever fiddled with knobs while biasing an amp.

[Ampzone]

Every bias tip I've read so far states that bias current is measured with (at least) vol down. You're measuring IDLE current that way. I suppose when you bring the volume up, something non-static happens, right?

Are you wondering why it drops/rises at all, or you're thinking it drops/rises too much?

I noticed the bias kept changing, new tubes I like to check a few times. so I checked it again and when I turned the volume lower and noticed the DMmeter dropping the ma current to around 13ma per tube from around 27ma per tube when the vol is dropped fully to 0. With speaker attached there is no unusual hum, oscillation or noise save the typical hiss and noise sound floor that drops when you lower any amp volume to 0. The presence control has the same effect on the bias ma. Yes, I think a drop of almost 15ma
per tube at idle no signal /speaker attached/ with a twist of vol knob is odd

Continuing to hunt for a bad ground---