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Need help for a bias circuit

jonas86

Member
2010-10-10 5:04 pm
Hello!

I'm trying to build a bassman100 amp but i can't find the 10k pot with 4 terminals to set the bias.
I was wondering if i can do it in an other way with 2 normal 10k pots (Attched file),
could somebody tell me if it is wrong and maybe how it could be done?

I will appreciate any help,

Jonas
 

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That is not the way I would have done it, but that's not the first time I've seen that bias setup. Its real advantage is that you have the current, in this case about 3.5mA, only going through a small portion of the pot, therefor it drops little current and can be low wattage and cheap. The double wiper pot really has only one function: to set the current through each tube to the correct value.

You could do a slight re-engineer to provide the same functions using two separate pots. I would try two 20k pots in parallel; one end to B- (omit the 3.3k resistor but not the cap) the other to the 15k resistor. The wiper goes to the 68k resistor. The pot will have to dissipate 2.7mA * 35V = .1W, so any well constructed 1W or 5W, 10k linear pot should do. I can't guarantee that the voltages will work out exactly as I described, but by juggling the 15k resistor, maybe 7.5k on either side of the pot, you should be able to get the pot in the center of its range when the amp is biased properly.
 
Leo Fender never paid extra for matched power tubes when he built amps. Random tubes were installed. The balance style bias control was there for hum null. It let you balance the amp with the random tubes, at least improve the balance. Set the control for least hum.

Many many guys have rewired the thing to a straight bias level control for all the tubes, and they just install matched sets.

You can find that odd pot and duplicate this circuit, or build something else for bias. You can make something that adjust each tube separately, or something that has an adjustment for each side, or just one overall control. Only thing you MUST do is wind up with proper bias voltage on pin 5 of each power tube.