Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeAmpDegen on December 16, 2025, 11:19:42 am
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So I have an early revision Crate BV-60 that's got an non-adj fixed bias. It seemingly uses this weird transistor circuit to monitor the bias. The amp is biased very cool and overall the system is trash imo. I'm thinking of clipping the transistor circuit and installing a trim pot bias.
Rough idea is to cut out the following. R94, r96, d9, d10. This would isolate the entire q16 circuit. The only thing left in the negative voltage line would be r88/r89 > r1/r3 to pin 5 of power tubes and then 25a fuses with r90/R91 to ground coming out of pin 1/8 of power tubes.
Change r90/r91 (12ohm resistors) into 1ohm 1% for bias reading.
Lastly running resistor + trim pot parallel to c58 for a shunt bleed setup.
Now I'm not very experienced in extensive bias mods like this so I'm not positive on the viability of this and I'm hoping I could get some advice. If I'm going about this the wrong way please don't pull any punches.