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I have a Fisher tube amp, number 680-A from a console. I am starting to restore it; it has 4 - 7591 power tubes. I have not touched the bias adjust or the 2 "AC balance" screws on top of the chassis, but after replacing a few components, I will want to check that the bias is correct. I will use the same 7591 tubes.
I did a search on "Fisher biasing" but didn't really find an answer - (but it was an instructional read.)Can anyone tell me how to adjust the AC balance and bias screw - or should I just leave 'em alone. I have meters, but no o-scope.
Thanks,
Pete
Follow Ups:
If you're using the amp alone, you need to add a 240 Ohm 10W (I'd use 25W) resistor to sub for the preamp tube heaters that were powered from the 5751 cathodes. Bias is adjusted for 33V at the 7591 cathodes. I'd leave the AC balance alone (actually I wouldn't, but I have a basement full of test equipment).
There is a yellow wire that supplies the heater current to the 7591 output tubes. Then it continues to the preamp. I'd put a 240 ohm 10W (or more) resistor in that wire to the chassis? Does this correct the load?
Yes, I meant 7591s... there are three preamp tubes that get heater current from the 7591 cathode current. There's a 600 Ohm resistor to ground from there, according to schematic - the 240 Ohm goes in parallel with that, cathode to ground. 244 in parallel with 600 is 171.5 Ohms, 33V across that is 192 Ohms, or 48 mA per tube. I'd also add a 10 Ohm resistor in series with EACH cathode to give a point to check current per tube. Would be nice if the original 7591s are still matched, but I'd check.
Thanks.
I will put in the 240 ohm resistor.
My Triplett tube tester won't test these 4 nice Sylania 7591s - they're too 'modern'. I wish my Eico 667 tester wasn't acting up but I can't trust the readings. That's a good tester.Pete
nt
Thanks, Tom and others, for info.
It isn't obvious to me, looking underneath the actual amp, where that resisitor goes. One of the wires to the preamp obviously - I will study the schematic and see what leads supply the preamp tube heaters.
This amp doesn't have 5751's. 12ax7 is called for in those locations. I have 2 spare 5751's if they are better.
Pete
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.
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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