Fender Pro Reverb Bias Issue - 1970s

[misterc57]

Progress.

With V6 pulled I am at 25 mA and 425 V at the plate

V6 12AT7 tests good. Tried a different 12AT7, same previous result, high current and low plate V.

I have -56 v before the 3.3K resistor feeding the bias balance pot and -46 v at the pot.

[Helmholtz]

Result points to HF oscillation as pdf64 suspected. Did you also hear (ripple) hum?

[misterc57]

Result points to HF oscillation as pdf64 suspected. Did you also hear (ripple) hum?

I have been using a dummy load. I will connect a speaker.

I did swap three of the .1 coupling caps at V6.

Time to get the scope in play.

Attached bad drawing of my bias circuit...

[Helmholtz]

Probably no need to change the bias.

[pdf64]

Rather than the anode voltage, what does the HT voltage measure, eg at the OT CT?

[misterc57]

Rather than the anode voltage, what does the HT voltage measure, eg at the OT CT?

Same readings from OT CT and same changes when pulling V6.

However, after connecting to a speaker the issue is gone.

Weird. So then I reconnected to my dummy load, swapped V6, tried all sorts of things. The issue has not come back and I have had about 20 turn ON cycles with no issues, clean sine wave at output and no noises.

I had about 24 mA bias current at plates. Changed the 15K R to 10K at the bias pot and now have 35 mA...



Bummer that I do not know what happened to fix or cause the original bias issue. But...

As always thank you to all for the help.

MarkO