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Changing the PA Cathode caps is like changing the preamp cathode caps. You're changing the low end cutoff and resonance point of the amp.

But the preamp triodes are operating in class A so the average current (and bias point) doesn't change much with signal level. The class AB output stage is very different in that respect, so the bypass cap has another role to play.

So, can they be switched out or not?

I don't see why not. You don't get the tone-shaping really (I think that was the point Peter was making?), but you can definitely change the feel by switching power amp cathode caps.

Can you define "feel"? Is that the tightening of the low end that was talked about? I have a friend that plays a lot of metal, and his belief is that the best amp has zero bass... And I must say that his playing is very, very good. I cannot play worth a pot, but I have several friends that do, and I must say that amp building is utterly intriguing to me.

Please, I appreciate the education in this aspect.

There's a general understanding that smaller caps = less bass. Also there's three areas to mod - preamp cathode bypass, coupling caps, and power cathode bypass. Preamp cathode and coupling caps tend to have a noticeable effect on bass response, but their effect is different. Coupling caps roll off bass and the slope continues down to zero response at DC. Preamp cathode caps roll off bass but level out again at some frequency below roll off such that there is a lower gain all the way down to DC. This can depending on the values have a different sound compared to coupling cap changes. As for power cathode bypass, you generally have to go really small before you start effecting the low end of guitar frequencies. So the size of this cap has more to do with power amp bias shift and recovery with signal. This has its own sound with changes in value, and is the 'feel' that has been mentioned.

All three can be switched, and often you need a high value resistor like PeterS mentioned.

Cknopp wrote:Can you define "feel"? Is that the tightening of the low end that was talked about?

Fixed bias amps tend to have firmer (less boomy) bass and faster response compared to cathode-biased amps. You can get some of that feel from a cathode-biased amp with a very large cathode cap. HTH

PeterS wrote:

Changing the PA Cathode caps is like changing the preamp cathode caps. You're changing the low end cutoff and resonance point of the amp.

But the preamp triodes are operating in class A so the average current (and bias point) doesn't change much with signal level. The class AB output stage is very different in that respect, so the bypass cap has another role to play.

I'm aware of the effect the PA cathode cap has on the bias current, but that's not the only effect.

If different values aren't changing the low frequency cut off point, why do we hear it?