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previous: Josh in a cathode biased amp, why is the... -- 8/29/2000 10:36 PM	View Thread

Re: cathode bypass cap on power tubes. why?

8/30/2000 1:23 AM
dpcoyle	Re: cathode bypass cap on power tubes. why? It only has to be there if you prefer its sound. Self biased amps are self regulating, meaning that as the plate current increases, the cathode resistor's voltage goes up and shuts the amp off some. This is distortion or compression depending on how you feel. A bypass cap in common with a single resistor for both tubes has a time constant and the larger it is the more the amp compresses. If you have a big power supply cap it exacerbates/enhances this effect. Hifi designs avoid caps here. You can put a switch on it, as I believe Fender did somewhere. Dan

8/30/2000 1:23 AM

dpcoyle

Re: cathode bypass cap on power tubes. why?

It only has to be there if you prefer its sound.

Self biased amps are self regulating, meaning that as the plate current increases, the cathode resistor's voltage goes up and shuts the amp off some. This is distortion or compression depending on how you feel.

A bypass cap in common with a single resistor for both tubes has a time constant and the larger it is the more the amp compresses.

If you have a big power supply cap it exacerbates/enhances this effect.

Hifi designs avoid caps here.

You can put a switch on it, as I believe Fender did somewhere.

Dan

Replies:

Graywater And,The flow of cur... -- 8/30/2000 1:36 AM