bf vibrochamp circuit with low low output

[mac dillard]

Check the .02 coupling cap that goes to Pin 5 of the 6V6. Also the 330 pf cap between Pin 5 and Pin 8 if you have it (I don't see it in the pic). Should not have DC on pin 5..

[mort]

Subbed in a .02 (or 2 parallel .01's rather) and still neg DC on pin 5 with volume increase. I noticed that the 6v6 cathode voltage starts dropping as the volume gets turned up. Starts out at 21vdc with the volume down and then hits 18vdc with a quarter turn. I don't know WHAT the heck is going on here... weirdest thing I've seen yet...

And I've tried subbing in a different 6v6. No change.

[mac dillard]

I believe that I would disconnect the NFB and the vibrato circuit and see if the base amp works.

[mort]

I've already done that a couple of times and does not change the issue.

Here's what I'm thinking now... All the cathode caps read within spec when lifted. But the 6v6 cathode cap reads a little high at 26.2uF. I can't look at it right now but if that cap were performing unpredictably under load, could I see goofed out voltages across the v1b and 6v6 cathodes, grids and plates, possibly effected by the amount of signal let in by the volume and intensity pots?

[eschertron]

I've already done that a couple of times and does not change the issue.

Here's what I'm thinking now... All the cathode caps read within spec when lifted. But the 6v6 cathode cap reads a little high at 26.2uF. I can't look at it right now but if that cap were performing unpredictably under load, could I see goofed out voltages across the v1b and 6v6 cathodes, grids and plates, possibly effected by the amount of signal let in by the volume and intensity pots?

If you suspect the 6V6's cathode bypass cap, then remove it. If you suspect the vibrato circuit is affecting the voltages on V1b (and maybe that's normal, I wasn't paying attention before) then disconnect it. The fewer unknowns you have the easier it will be to find the true issue.

If there is 3vac on the grid of the 6V6 with no signal applied (and volume down) then there may be parasitic oscillations. Does your DVM have a frequency meter function? Also, as Mac pointed out, some of these amps have a small value cap from cathode to grid on the 6V6 to address that very issue. Add that or a grid stopper and see if the amp behaves differenty.

[mort]

The 3vac & neg voltage on grid pin 5 begin to show up as the volume is increased, with or without input signal. It isn't present with volume down and no signal.

And just to clarify, exactly what issue does the grid to cathode pF cap address?

[eschertron]

The 3vac & neg voltage on grid pin 5 begin to show up as the volume is increased, with or without input signal. It isn't present with volume down and no signal.

And just to clarify, exactly what issue does the grid to cathode pF cap address?

If the amplifier is self-oscillating under certain conditions, and that oscillation is a HF (generally above the audio range), then a cap - Fender used 330pF - between the grid and ground (grid and cathode is close enough for rock n roll) will dampen those oscillations. There are several Champ designs that include that cap. My SF champ from the CBS era is one that does.

The point I'm exploring is what is that 3vac (what freq, what waveform) and where does it come from? Did you say that it was there even with the vibrato circuit disconnected? What lead did you lift to disconnect the vibrato?

The neg volts is even more troubling. With appropriate AC coupling, there should be no DC appearing on the grid. Taking a step back, I have found that the high impedance at a grid pin often prevents me from taking any accurate measurements on that pin with the cheap meter that I have. Yours may be better than mine, but it might be giving you ghost readings.

What if you pull the 6V6 tube and measure the voltage/signal on the grid pin. Does it look OK? Does it sound OK?