Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on December 04, 2010, 04:18:40 pm
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Hi guys,
I have a 62'ish or 64'ish blond Brownface Bassman head on the bench that I am replacing filter caps on. I also found several resistors that drifted out of spec, and 2 or 3 diodes in the rectifier circuit that were physically broken. I've replaced the filter caps, the electrolytic caps on the pre-amp tube cathodes (they looked pretty rough) and just fired it up for the first time and though "Ok, I'll make sure the power tubes are biased within reason...and...what the heck? There's no bias pot?!" So I'm under the assumption this is fixed bias, not cathode bias, yes?
A few questions:
- If fixed bias, how do I go about tweaking the bias? Or, if it sounds OK do I leave it?
- Can I use the transformer shunt method to check the bias current?
Right now the bias voltage is around -50. Oh, and here's a link to what I think is the schematic: (please notice, though, the components on the bias circuit in this amp are not the values shown in the schematic. There is only 1 cap, 20mf/50V, there is a 1K resistor where the schematic shows 10K and 27K where the schem. shows 56K...)
http://www.ampwares.com/schematics/bassman_6g6a.pdf
Thanks for the help, guys. I just don't want to risk messing up this gem ;)
Respectfully,
Mark.
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Here's your schematic... http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_6G6B.pdf
Your amp is fixed bias without an adjustment pot. I would not bother with modifying it to have a pot unless the owner wants it. That amp has run many years without mucking with bias. As long as it has about -50vdc on the grids, and the plates aren't red, I'd leave it be.
Now if I were cloning a 6G6, I'd probably add a pot just because I'm a diddler. :grin:
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Thanks, sluckey! Yep, -50 on the grids and the plates look happy so I'll leave it.
Thanks for the schematic link, too :)
Mark.