General characteristics of cathode bias vs fixed bias?

[CitizenCain]

In a given amplifier design, what would be the general differences you could expect converting from fixed bias to cathode bias?

[Free]

In cathode-bias the signal is instantaneously amplified, because the tube does not have to "wake up” from a less than full operational state of fixed-bias. So, it makes for an amp that has a certian tonal-richness at all volumes, as well as, being very touch-responsive.
There is not as much headroom as fixed-bias because of the lower plate voltages used in cathode-bias, so it can be a virtue for players depending on amp output stage overdrive, with smoother transitioning from clean to crunch.
A fixed-bias amp will generally have more headroom and wattage with the same circuit topology, as well as a tighter bass response, all factors due to greater voltage getting to the tubes during conduction.

[CitizenCain]

Thanks, Free! Just the type of info I was looking for. Sounds like cathode bias is worth an experiment in my project amp :D That's exactly the type of behavior I'd like to get from an amp.

[Old Tele man]

...fixed-bias doesn't change with signal amplitude, it's "fixed.".

...self-bias does change with signal amplitude, especially when driven hard which causes somewhat more bias-voltage to be developed across the cathode resistor(s).

[CitizenCain]

And that changing bias on the cathode biased setup is what gives the amp that squishier, lively feel?

I gotta try this out!

[Free]

And that changing bias on the cathode biased setup is what gives the amp that squishier, lively feel?

I gotta try this out!

Essentially. In general that varying voltage (as well as lower voltages in general) is what gives "class A"-type cathode-bias amps such "lively" touch sensitivity and smooth transition into overdrive. That general compression ("squish") has to do with the lower voltages (compared to fixed-bias) going to the tube plates in cathode bias. Higher voltages (i.e. fixed-bias) give an amp more wattage, headroom and bass-response (generally), among other things.

-Mike