KT66 power tubes in Hotrod Deville

[robrob]

You should also use the plate voltage - cathode voltage for your "plate voltage" in the dissipation calculations so that will drop your dissipation % too.

I have a tube bias calculator webpage that's pretty easy to use: https://robrobinette.com/Tube_Bias_Calculator.htm

[Micahstephens838]

You should also use the plate voltage - cathode voltage for your "plate voltage" in the dissipation calculations so that will drop your dissipation % too.

I have a tube bias calculator webpage that's pretty easy to use: https://robrobinette.com/Tube_Bias_Calculator.htm

Okay got it. How do I measure the cathode voltage? And on your calculator 80% is average, why do other people suggest keeping it under 70%?

[robrob]

The simplest way to do it is to put the multimeter red probe on the plate pin and the black probe on the cathode probe and the measured voltage is the plate-to-cathode voltage with no math involved.

You can measure the cathode voltage direct at the cathode pin to ground or from the tube side of the cathode resistor to ground.

That 80% average is for Class A (single-ended) fixed bias amps with 90% as suggested max safe.

For Class AB (push-pull) fixed bias amps 60% is average and 70% is max safe.

For all cathode bias amps I suggest 90% average and 100% max safe.

[katopan]

This is fixed bias isn't it? Then the cathode is basically at 0V (well 10's of mV through the 1 ohm resistor).

[robrob]

This is fixed bias isn't it? Then the cathode is basically at 0V (well 10's of mV through the 1 ohm resistor).

You are right, it is fixed bias so there is no difference between plate voltage and plate-to-cathode voltage.

Sorry about that OP.

With a fixed bias Class AB amp like yours most people use 70% as max safe dissipation.