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Old 20th October 2010, 07:08 PM   #1
jonas86 is offline jonas86  Italy
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Default Need help for a bias circuit


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,

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File Type: gif bassman100.gif (117.4 KB, 102 views)
File Type: bmp bias pot replacement1.bmp (131.1 KB, 16 views)
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Old 20th October 2010, 07:56 PM   #2
DF96 is offline DF96  England
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The original circuit is strange. It adjusts the AC balance a little as well as the DC balance. I don't know how critical this is.
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Old 20th October 2010, 09:55 PM   #3
Doz is offline Doz  United Kingdom
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How many knobs on this pot ... ? 2 wipers would tend to infer 2 knobs... some Alps pots have 4 terminals on them, but only one knob. Chinese copies are available, and probably better quality than the original.
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Old 20th October 2010, 10:01 PM   #4
DF96 is offline DF96  England
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One wiper, plus a fixed centre tap?
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Old 20th October 2010, 10:39 PM   #5
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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.
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Old 21st October 2010, 02:02 AM   #6
Enzo is offline Enzo  United States
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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.
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Old 21st October 2010, 07:15 AM   #7
jonas86 is offline jonas86  Italy
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Yes the original is a pot ( so 1 knob) with 4 terminal, 1 wiper and a center tap.

Ok i will try to build a bias circuit with 2 pots with the values that Palustris has proposed.

Thank you all for yuor help!!!
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