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FDP Forum / Amp Mods, Repairs, and Projects / 91 Bassman 70 no negative voltage

mjguit

USA

Oct 26th, 2018 11:00 AM   Edit   Profile  

V6 power tube is red plating. Tried different tubes still the same. Plate voltage is 520vdc. show no negative voltage to pin 5. Pin 5 on V5 gets -30vdc on pin 5 and does not red plate. what do i check next

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Oct 29th, 2018 04:36 PM   Edit   Profile  

Do you know where the bias cap is in this amp? In older Fender amps, a bad bias cap can cause your power tubes to red plate. That's all I got.

pdf64

UK

Oct 30th, 2018 11:31 AM   Edit   Profile  

What you do is remove the power tubes until you get the issue sorted.
-30V seems likely to be insufficient to achieve a suitable idle plate current even on the non red plating tube, it should be twice that. So yes, try replacing the bias supply ecaps.
The V6 68k grid leak and 1k5 grid stopper resistors may be open circuit.
Or maybe the 0.1uF coupling cap from V4 is leaking dc.

schematic

mjguit

USA

Nov 7th, 2018 11:17 AM   Edit   Profile  

I have it fixed. Was a output balance pot. Cleaned it and it works great. Thanks for the help


pdf64

UK

Nov 13th, 2018 06:41 AM   Edit   Profile  

Good work finding that; pots are a horrible weak point, consider adding back up resistors in case the pot fails again, eg R3 of linked page.

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