Blackface Vibro Champ AA764 6V6 issue

[cjlectronics]

Repairing this 60's Vibro Champ for a friend and it has a distorted output. I measured the resistors in the B+ circuit... they all measure good. When I turn it on and measuring pin 3 if the 6V6, I'm seeing 405Vdc and as the tube warms up, the voltage drops below 300V. At the same time I'm measuring the voltage at the cathode - pin 8 - and it is increasing way above 40Vdc and will continue to climb well above 50Vdc. That's when I turn it off. This is what I've done so far:

1. The cathode resistor measures 475 ohms. However, I swapped it with another and get the same results.
2. I've swapped out the cathode capacitor and it still does the same thing. This has been replaced before.
3. I swapped the 6V6 with another and get the same results.
4. Subbed in 20uf/500V filter caps for the first 2 caps in the B+ circuit. No change in the symptoms.
5. Pulled the Vibrato tube - 12AX7. The B+ at pin3 of the 6V6 hovers around 405Vdc when I do this.

I'm stumped. Hopefully, someone can provide a little insight on what I'm overlooking.

[sluckey]

Check the voltage on pin 5 of the 6V6.

[Stevem]

First off check this.

With the amp on and acting up what do you read for AC voltage across the speaker?

If it’s above the mv range then the amp is oscillating at a frequency above what you can hear.

If that’s not the issue then move on to these checks which are in no particular order.

If you place a short across the foot switch RCA jack does the tremolo work?

If you swap in a known good AX7 in that tremolo socket does anything change / improve?

Is that cathode bypass cap in that tremolo circuit Leaky or shorted?

With that tube out of the circuit is the amps output then normal and undistorted?

Since that 6V6 cathode bypass cap has been getting hit with way more then 25 volts it’s going to get hot and blow up in your face, so watch out!

In fact I am surprised it has not done so already!

[cjlectronics]

Thanks for the reply Stevem

- measured across the speaker and I only measure mV...

- when I short the tremolo foot switch jack No tremolo is heard. When the short is removed, I hear the tremolo when I increase the Intensity pot.

- swapped the AX7 in the tremolo socket and same result. One thing to note, when power is applied while monitoring pin 8 of the 6V6, as the tube warms up, I see the cathode voltage rise to about 10mV, then I hear a pop in the speaker, and the voltage drops close to zero. Then the cathode voltage rises from zero to runaway. I turn it off when I get past 30Vdc.

- subbed in a 22uf/50v cap in place of the tremolo AX7 cathode. Still get the same issue with a runaway 6V6 cathode voltage.

- Also I should note, this amp doesn't have the 330pf cap between pin 8 and pin 5 as seen in the schematic. Does it need it?

[sluckey]

Check the voltage on pin 5 of the 6V6.

[cjlectronics]

The voltage at pin 5 of the 6V6 is approx 2.0Vdc after warm up and as the cathode voltage increases.

[Stevem]

Well to me at this point theres 3 things that could be going on.

You have a shorted 6V6 or the 6V6 socket has arced over and has an internal short once voltages build up, because I don’t know what else would be cranking up your cathode voltages like your seeing other then the oil tag board itself leaking voltage which HAS been known to take place.

To eliminate this potential issue wire the cathode resistor and bypass cap up off of the contacts on the board.

That small cap is there to snub oscillations and since your not seeing many volts of output heading out to the speaker then your ok I would say.

I took this cap out of my 1982 VC with no consequences.

The mod I did that helped me to have no consequences in doing that was moving the coupling cap feeding pin 5 of the 6V6 off the tag board and right to pin 5 along with the 220k grid resistor that goes to ground from pin 5.

Pin 1 is not used on a 6V6 so you strap that .02uf coupling cap from pin 1 to 5.

[sluckey]

The voltage at pin 5 of the 6V6 is approx 2.0Vdc after warm up and as the cathode voltage increases.

That voltage should be zero volts. Pull the 6V6 and recheck the voltage on pin 5. If not zero, then disconnect that .02µF cap between V1 pin 6 and the 6V6 pin 5. If pin 5 now reads zero volts then replace the .02 cap.

[Stevem]

Yes!
I forgot that failed coupling cap possibly.

[cjlectronics]

I put a new .02 coupling cap between the 12AX7 pin 6 and pin 5 of the 6V6 and no difference. I replaced the 6V6 with a known good one.

A weird observation is when I apply power while monitoring pin 3 of the 6V6, it measures 500V and slowly decreases in value and the Cathode voltage on the 6V6 slowly rises. Then after about 10 seconds or so, the cathode voltage goes to 0. All voltages are steady state. Then after another 10 seconds the cathode voltage starts rising quickly and will increase past 30Vdc. I'm afraid to let it go past that. I added the 330pf cap between pins 5 and 8 of the 6V6 and that had no effect on the problem.

I would think that if I had a bad 6V6 tube socket, moving the tube in the socket would have an effect on an arcing. I've done this and there was no change in the symptoms.

[sluckey]

Time to post some hi-rez pics.

[Stevem]

In your first post you said your where seeing 405 which is in the ball park, however I can’t see how you could be now getting 500 vdc with the AC output that PT has, and especially with a 5Y3 recto !

If a socket had developed an internal carbon track from an arc over condition and is just bleeding off voltage as if the carbon track that was formed was a resistor then you would not visually see anything.