Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on December 11, 2011, 11:54:58 am
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Hi,
this is my first fixed bias build since five years or so. I am nor dumb ( after all, I am wondering hey hey ) but I can't understand this one.
grid 1 : -43 V ( pot set at less negative voltage )
grid 2 : 418V
plate: 420V
cathode (pin 8) resistor to gnd : 1 OHM
max current at cathode : 15 mA ( mV on the meter )
El34, 6550, 6V6 .....same thing : can't bias hot enough.
changed OT : same thing
the amp is only wired to the PI, preamp not done yet ( is there my problem ? )
Colas
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the amp is only wired to the PI, preamp not done yet ( is there my problem ? )
Not the problem. The preamp has nothing to do with biasing the output stage.
For push/pull 6V6s -43v bias is too cold. Drop the voltage down to -30 to -35 and the cathode current should be about 20 to 25ma. (based on 420v plate voltage).
Don't know about the other tubes.
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yeah sure,too tired I guess, The schems I was following show near 500V at the plates for a bias of minus 57V. I had -57V right on the spot but I had only 420V at the plates, and I have forgotten to adjust the bias voltage accordingly. Had to lower by half the bias resistor from 47 to 22K to get the right bias. That's what happen when you twaek amps only once in a while !
regards
Colas