Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chuggy on March 02, 2011, 09:25:59 am
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I plan to change the bias balance to adjust on my SF 100 watt Twin Reverb. The schem is page 2 of the twin_reverb_sf_100_schem that is readily available. My questions-
1. Is the AB763 bias circuit a good one to use?
2. The bias pot of my amp has a fourth tap that recieves the tremolo signal and this is different from the AB763 schem. Why is the forth tap there? What should be done with the trem connection with the AB763 bias circuit?
3. Would replacing the 50 mfd cap of the AB763 bias circuit with a 100 mfd 100 v cap interfere with the operation of the trem, since the trem works by swinging the bias voltage?
4. Would replacement of the 3.3K resistor with a 10K pot in series with 470 or 1K resistor be a simple and effective alternative that would accomplish both bias balance and adjust?
5. I was surprised to find the 6L6 plate V to be lowish at about 425V (but at highish idle current), then noted that later schems show the plates at 405V. Is this typical?
Thanks-
Jim R
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1. Is the AB763 bias circuit a good one to use?
yes
2. The bias pot of my amp has a fourth tap that recieves the tremolo signal and this is different from the AB763 schem. Why is the forth tap there? What should be done with the trem connection with the AB763 bias circuit?
Look closely at the schematic. The fourth tap you're talking about is the wiper of the pot. It goes thru a 15K resistor to ground. The tremolo does not go to the bias pot.
3. Would replacing the 50 mfd cap of the AB763 bias circuit with a 100 mfd 100 v cap interfere with the operation of the trem, since the trem works by swinging the bias voltage?
No. The trem does not swing the bias voltage. See #2 above.
4. Would replacement of the 3.3K resistor with a 10K pot in series with 470 or 1K resistor be a simple and effective alternative that would accomplish both bias balance and adjust?
Sounds good to me. Try it and see if it gives you the voltage range you want. If it can swing the grid voltage between -40 and -60 it should be fine.
5. I was surprised to find the 6L6 plate V to be lowish at about 425V (but at highish idle current), then noted that later schems show the plates at 405V. Is this typical?
As you back off the idle current, the plate voltage will rise.
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Replacing the 50 mfd cap with a 100 mfd cap in the AB763 bias circuit DID inactivate the tremolo. The trem worked fine with the original circuit and with a 50 mfd cap in the new AB763 bias circuit.
Jim R
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Which circuit are you talking about? There is no 50µF cap in the bias circuit of this amp...
http://www.el34world.com/charts/Schematics/files/fender/twin_reverb_sf_100_schem.pdf
But anyhow, changing the bias cap even in that amp or the AB763 TR will not kill the trem. If you look at the circuit you will see that the bias voltage going to the trem circuit thru that 2.2m resistor is shorted to ground by the footswitch whenever the trem is turned on.