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Post subject: Fender Twin Biasing question
Posted: Tue Mar 08, 2011 5:47 pm
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Joined: Tue Mar 08, 2011 5:43 pm
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I need help on finding the "recommended" bias / balance numbers for my '94 Fender Twin amp. As the manual indicates, I switched to the LOWER POWER OPTION by taking the INSIDE 6L6 power tubes out and setting the IMPEDANCE switch to 8 ohms.

However, in the manual in setting the output tube bias adjustments, it indicates the recommended setting for the BIAS at .08VDC and the BALANCE at 0.0 VDC. These settings are when you have all FOUR 6L6 power tubes installed. With the lower power option, you take the inner tubes out but it does not state what the NEW BIAS / BALANCE numbers should be. Does anyone know?


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Post subject: Re: Fender Twin Biasing question
Posted: Tue Mar 08, 2011 7:23 pm
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Hi flemingbt,

You really shouldn't need to readjust the bias when only using 2 output tubes. If you notice on the schematic the bias reading at the TP is across a 1 ohm resistor on the Cathode of each set of output tubes (2 test points, one for each side/pair). So with only two output tubes installed the bias reading will be approx half of what it is with all four tubes installed. If you adjusted the bias with all 4 output tubes installed to 80 mV at the TP, then with only 2 output tubes installed you would want to see approx 40 mV at the TP. If you adjusted the bias and balance pots with all 4 tubes installed, then pull 2 tubes, the bias should still be OK and no adjustment necessary. Do not try to adjust the bias to the same reading that you had with all four tubes installed, it will be too much for only 2 output tubes. If you want to check/tweak the adjustment when using only 2 output tubes, you will look for approx half of the voltage reading at the TP that you had with 4 tubes installed.

The balance test point reading can be adjusted in the same way with 2 or 4 output tubes (0 volts at TP). If you notice more hum when using only 2 output tubes, that is an indication that the balance pot needs adjustment. If no extra hum, then the balance is probably OK also. :idea:

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