Volume Control problems

[EETStudent]

Im not exactly what you mean by leakage path, nor crosstalk. could you explain?

[Enzo]

OK, the signal comes in the front and steps stage by stage throught the amp. Now lets say somewhere in the middle a coupling cap fails open, so the signal path from the plate of one stage is broken on the way to the next grid. The signal in the earlier stages - those before the break - are enough to radiate some signal around inside the chassis. The rest of the circuit after the break has enough sensitivity and gain to pick up some of this scattered signal. SO you hear a much weaker signal coming out the speaker. This is what he called leakage, I call it crosstalk.

Crosstalk is where two channels interact. Imagine on a stage where you sing into a mic. When you are not singing, your vocal mic still picks up some of the drums from behind you. That would be a form of crosstalk, though not electronic. or now and then your local TV news show has the weather guy start his forecast, but they don't turn his mic on. But you can still hear him in the background coming through someone else's mic. That is also acoustic crosstalk.

In poorly designed audio mixers or multitrack tape decks, sometimes a hot signal on one channel will bleed into the channel on either side a little. That is electronic crosstalk in the board.

In the guitar amp this is happening all the time, but the signal itselg is WAAAY stronger than any crosstalk, so you never hear it. it might have some small contribution to the sound of an amp, but that is another story for another time. But when the main signal cannot make it through the amp, all that is left is this crosstalk.

Now you have an example where it does matter some. Your volume control all the way down has cut off the main signal, but there is still audible crosstalk coming through. it isn't coming through the volume control, it is just leaking around things. That is what you hear when the control is down. As it turns out in your amp, the main signal and the crosstalk are out of phase at that point.

So as you turn the volume up from zero, the main signal starts to increase. But it is adding itself to the crosstalk already there. Since they are out of phase, they start to cancel. Full cancellation happens when the main signal is up at the same level as the crosstalk. That is the nothing that comes out at 2 or whatever. Then as you continue up with the control the main signal gets louder and louder, and crosstalk is swamped and forgotten.

[pdf64]

Try a different 12ax7. I've got a NOS Mazda ECC83 from Watford Valves (with the cryo treatment, though I don't think that's the cause) which does the exact same thing. Somehow the input from the jack is getting onto the grids of both sections, though I can't see how. As the amplified signal from the first section is inverted compared to the unamplified signal that's getting on to the grid of the second section, the 2 signals cancel out at about 2 or 3 on the volume control which is between them. If the scope is triggered from the sig gen, the signal on the second plate can be seen to fall to zero, then invert, and then get full amplification, as the volume is turned from min to max.
Other identical Mazda ECC83s don't do this, the other cryo 12ax7 (EH) I've got doesn't do it.
So try a different tube and see if the problem goes away.
Which is always the golden rule for troubleshooting a tube amp anyway.
Hope that helps - Peter

[EETStudent]

yeh man you guys are incredible. So basically ive got a leakage somewhere. thats another step in the right direction. lol Enzo thanks for all your little analogies. Ill have to let you guys know when I fine tune this amp and get it cooperating. I cant even begin to explain to anybody the amount of knowledge Ive gained over the past month or so. Schematics and concepts used to terrify me, but now i got more confidence and understanding than i thought i would ever have. thanks for everyones help