Classic Hifi Care

Everybody except me probably already knows this stuff. I never really thought about it very much. I thought biasing was something only the guitar amp guys and the AudioKarma crowd considered. I don't understand all I know yet. How they get positive cathode voltage from the chassis, which is connected to common negative. I think I understand the difference between self-bias and cathode bias. I don't understand yet how I change the voltage in a cathode bias circuit. I also think I understand that the smaller 88 series and 93 series Magnavox amps are self-biased while the bigger bi-amps are cathode biased. I haven't found where to find the specs for plate current or how to change the bias voltage. This might be a good conversation on here. Uncle Doug has several videos on this, but mostly pertaining to guitar amps. I haven't found any good videos on tube audio amp or stereo amp biasing yet. I know there are members of this forum who are more educated on this than I am. I'm not worried about making a fool of myself. I just want to learn this if I can.

I don't know about guitar amps, but some P-P stereo amps have adjustable bias so the tube pairs can be biased the same and at the correct point for optimum performance, and then adjusted as the tubes age.

Chris Campbell

Self-bias is pretty simple: resistance in the cathode return. It varies with plate current, of course, so there is a bit of distortion introduced thereby but it is always there so no tubes get whacked if it fails. Some designs use a cap in parallel to extend low frequency response via degenerative feedback.

Cathode bias relies on either a tapped or separate winding on a power transformer, rectified, then used to give the tube(s) a hard value bias negative voltage. It's advantage is it can be easily be controlled outside the tube characteristics for minimum distortion, or to bias it into a different class of operation.