Bias: Cathode vs. Fixed

[Paultergeist]

Greetings, all!

I have a question about fixed/cathode bias strategies used in amplifiers. It seems that larger, higher-wattage amps tend to employ a fixed bias strategy, while most lower-wattage amps typically use cathode bias. Somewhere in the back of my mind, I seem to recall reading that fixed bias -- since it employed a negative bias voltage -- was slightly more efficient than cathode bias (which is referenced to ground potential). I could be wrong even on that point, but that is what I recall.

(Actually, wouldn't it make more sense to use fixed bias on lower-wattage amps, since one is typically trying to squeeze more out of a lower-wattage amp, while the big 100-watters are usually running on idle anyway? But I digress...)

Anyway, I have now heard a couple of times that cathode-biased amps tend to have either richer harmonics or more of the "good" harmonics -- something to do with harmonics -- or maybe harmonica players -- no, I kid. If this in fact IS true, then I wonder why those guys playing 100 watt Marshalls aren't running to their amp techs asking to have their Marshall heads modded to cathode bias for more ""mojo?" So I guess my first question is: do cathode bias amps have something going for them in the "harmonics" department, and if so, what is this benefit?

I ask this because I have very recently completed a Blackface build (AB763 circuit). I am not at all deterred by the point that the stock circuit is now performing flawlessly -- I am going to mess with this perfectly good amp, as is my habit. I am under the impression that a really cool mod is switchable bias: cathode vs. fixed. I read a post over at the Weber forum where a builder had switched his blackface over to cathode bias -- he absolutely raved about the blackface with cathode bias. I would like to explore this idea a bit further. If anyone has tried changing their bias strategy -- one way or the other -- I would be very curious to hear how it worked out.

Thanks for any replies.

Paul

[Kap'n]

Push-pull cathode biased amps tend to compress or squash. As the amp gets pushed harder, the current draw increases, increasing the current across the cathode resistor, and the resultant bias voltage.

Fixed bias gives a stout bottom end when pushed, one of the reasons for increased power in the first place.

Two different, equally valid, but opposing tonal equations.