Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yosser on February 02, 2026, 06:45:41 am
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I'm replacing the bias circuit on a Carlsboro 60par amplifier with a better marshall bias circuit and will be using the Duncan lord valve website one ohm cathode resistor procedure to set the output valve bias of the amplifier.
The first stage of the method states this: "Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of any power tube socket. You should see a negative voltage in the -35 to -50 volt range if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s, 6L6s, or KT66s. KT88s, 6550s, KT90s, and KT100s can have bias ranges that go as high as -100 volts. Amps which use 6V6s will usually have bias supplies which produce voltages that are similar to EL34 amps...but not always. Note that you should *not* have any power tubes installed in your amp yet."
The Carlsboro 60par amplifier does not have a standby switch, if the amplifier does not have any tubes installed at all, would that be the same state as being in standby?
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Just remove all power tubes. The idea is to not damage the power tubes if the bias voltage on pin 5 is not correct. Also, while you're monitoring the bias voltage on pin 5 adjust the bias pot for maximum negative voltage.
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Thanks!