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FDP Forum / Amp Mods, Repairs, and Projects / bias voltage on output tubes drifted WAY apart


New York

Jan 20th, 2017 12:58 PM   Edit   Profile  

I inherited a Fender Concert 6G12-A. It didn't make any sound and I ended up taking out the circuit board, storing it safely away, and building a new one. It worked and everything looked good. About a month or two later I happened to notice one of the output tubes (winged C 6L6GC) was really glowing hot red. I switched the tubes and the glowing tube switched sides also. Looking at the layout, I have -55v coming off the bias pot I installed, and the two 220K resistors check out ok, but if I get one tube to 35mv the other registers 23mv; previously they were 35mv and 34mv. Is this something that just happens or was it my new circuit board?



Jan 20th, 2017 05:04 PM   Edit   Profile  

Redplating can damage a tube.
Metal structures built to extremely fine tolerances may become mis-shapen by the heat, detrimentally affecting the tube's performance / characteristics.
I know it's very disappointing when that happens but you may need another good matched pair.

Contributing Member

The spotlight

looks like a prison break
Jan 20th, 2017 07:07 PM   Edit   Profile  

Since the problem is location (vice tube) -dependent, that indicates a possible problem with the socket.

Check your solder connections on the tube socket. Try re-flowing all of them and see what that does.

Contributing Member

So. Cal. USA

Jan 23rd, 2017 11:31 AM   Edit   Profile  

" I switched the tubes and the glowing tube switched sides also."

I doesn't sound like your new board. If the glowing red plating tube moved when you swapped sides the issue is with the tube it would seem. Too bad, I like the way winged C tubes sound.

I'd check for equal bias voltage at pin 5 of each power tube socket then replace the power tubes and rebias.


New York

Jan 25th, 2017 08:32 AM   Edit   Profile  

Thanks for the help, everyone. I'm going to touch up the tube sockets and replace the winged C's. I also loved their sound, but now unfortunately, they are out of my budget.

FDP Forum / Amp Mods, Repairs, and Projects / bias voltage on output tubes drifted WAY apart

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