• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12b4 hybrid

I came upon this over on Tubecad. My question is will the 12b4 actualy
work ok at those low voltages and would anyone care to quess what the resistor values would be ?


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Joined 2003
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Thank you.

What sets the DC current in the MOSFET? Apparently the DCR of the choke lifting the tube cathode. If we assume 28V peak swing we want 3.5 Amps. He has a caveat about choke DCR higher than a third of an ohm, which would be 1V (already high). So we need a tube sensitive to much less than 1V change.

The 12B4 at Mu=6.5, working at 24V, needs 4V of bias. A part-Volt change makes very little difference.

I think this was an end-of-blog throw-away circuit, not fully thought-out. I can "make it work" with a higher-Mu tube, but it feels much too fussy about all device parameters; also I am seeing slewing at 1kHz. (A newer MOSFET would be better, but not a miracle.)

The low voltage is not a killer problem. Tube current falls faster than supply voltage but the MOSFET gate is nominally "zero current". I can get it to stabilize with much less than a mA in the tube. I also have to run it that cool to reduce DC errors in the NFB resistors. However a MOSFET's gate capacitance is large and a cool tube won't drive it fast.

EDIT this works in sim but I do not trust it. Also with the large antique MOSFET the slew is asymmetric and only 0.6V/uS on the downswing.


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