Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on November 16, 2021, 03:24:57 pm

Title: Bias issue Fender 6G2
Post by: Diverted on November 16, 2021, 03:24:57 pm

I had a strange issue happen today and I would love to get it explained.  Friend gave me a Fender 6G2. Said it developed an electrical smell and trem hadn't worked in the two months he's owned it. He said he was told from the guy who sold it to him that a wire needed to be reconnected.
Opened it up, all wiring looked legit. Pulled the rectifier to check bias voltage. With about -30 at the bias cap (no bias pot) it was about right. Things got weird when I turned up the intensity. Bias voltage coming out of the intensity pot, at the 220K grid leak resistors junction, went as high as -6 with intensity cranked. It must have been baking those output tubes.
Started checking resistors, looking for bad solder joints and testing the coupling caps. Everything looked OK. Turned the amp on again, no change. Turned it off,  wired the tremolo out of the circuit and got normal bias voltage on the grid leaks. When I wired the tremolo back in to the circuit, the issue was magically gone. The intensity pot became an intensity pot again not a tube melting bias pot.
I'm assuming the electrical smell could have been the transformer getting very hot, or the 6V6s holding on for dear life.
I have had the amp on for a few hours now. Playing stable and fine, totally normal. Sounding great. So what could have caused the intensity pot to behave that way? I'm assuming there was a loose solder joint or something somewhere that I re-established, but I haven't been able to repeat the problem by chopsticking, etc.
I'm really curious. Thanks.

https://schematicheaven.net/fenderamps/princeton_6g2_schem.pdf

Title: Re: Bias issue Fender 6G2
Post by: sluckey on November 16, 2021, 04:21:04 pm

Replace the INT pot or at least clean it. If the wiper only makes intermittent contact or loses contact all together, then you lose bias to the output tubes.

Otherwise, the amp may be beyond repair. Send it to me and I'll bury it in my dead amp cemetery.    :icon_biggrin:

Title: Re: Bias issue Fender 6G2
Post by: Diverted on November 16, 2021, 04:30:46 pm

Thanks. I sprayed it down pretty good with Deoxit. If that doesn't work I'll send it your way for the pile.