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In Reply to: Biasing Fisher amp - can anyone shed light on this? posted by PeterI on March 11, 2007 at 21:41:04:
pete
esentially what the AC balance pot does is match both sides of dual triode driver to produce equal AC signal voltage going to the grid of each output tube.
What a lot of people do is get tubes that test equal on both sides and set the pot right smack in the middle.
Here is what I would do since you dont have a scope or a signal generator.Get a cd with 1khz test tone on it and get yourself some 8 ohm or 4 ohm resistors 40 watts or more and hook them across the speaker outputs.Turn the volume up about half way with the tone running thru it and put your DVM on AC volts, and put your meter pos on pin 6 of the 7591 and ground and set the AC voltage so its equal on both 7591 grids
or as close as you can get it.For instance if you have 18.2vac signal going to the grid one output,you want 18.2vac going to the other output.I usually use a scope and look at the sine wave and I do it at low power and mid power and full power..
Now I gave you simple way to set those pots without hopefully not confusing you..Im sure there will be some that snark at this method but since you dont have a lot of the equipment like a sig gen and scope,this will get you in the ball park.
Follow Ups:
NT
Have you ever heard of the hummmmm method? Scott has a proceedure for feeding the speaker output into the other channel (don't remember exactly how it goes) then adjusting for minimum hum. I tried it with my 340B but never had success so I just set the pots back to the original position, also does the balance have to be reset when changing output tubes? It certainly doesn't seem as easy as adjusting bias without the proper equipment.
The AC balancing system in Fisher pieces is different from the AC adjust on Scott pieces. On the Scott, you are adjusting the center tap of the 6.3VAC heater voltage to produce the lowest hum through the speakers. On the Fisher pieces, the 12AX7's split load phase inverter, rather than using two equal resistors, one from the cathode to ground and the other from the plate to the V+, Fisher used an oversized resistor on one leg which was paralleled with an adjustment pot. Although the stock adjustment was to produce the least IM distortion, Mikey's method is plenty good enough.The upshot of all of this is that if you sit with your ear next to the speaker and adjust the pot on your Fisher waiting for hum to show up, you'll wait for a long time and run the risk of burning up the 12AX7 section.
In either the Scott or the Fisher, the important adjustment is the bias adjustment. It's very important to get that right. These AC adjustments are just there to confuse women, children and dogs. Cats won't care. They've been into solid state for years.
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