Sticking a BF Princeton and a 5E3 in a Champ Chassis. Sort of.

[printer2]

I know everyone has been clamoring about it, how does it sound? Not as quite as scooped as Fender but I knew that from the start. More like it gives bass and treble controls without taking too much middle away. Come to think of it I think I simulated closer to the Marshall curve. Works fine with the 8" and small cab, mind you I might like the more scooped Fender sound with it also? I'll have to put one of my other amps into the speaker. In to the Tweed position. Cranking it does give that classic sound, no PI funnies that I can tell though. I can tell the 10W line transformer is just hanging in there. I matched the tubes to one mA at idle. I think it is the limiting factor in the amp. The amp does have enough gain to get crunchy with the tone stack also, most likely as the mids are not are not taken out that much. Tried the amp with a 12" Fender something or other Eminence (I am told) and it does get loud cranked. Cleans, well it is under 10W. Anyway that is about it. I'm out of here.

[Gnobuddy]

it seems it (ed: cathodyne PI) has a rather big effect on the tone and distortion of the amp. I assume the PI stage affects the tone quite a lot. I assume that the PI stage has lower headroom than the preceeding gain stage, causing it to clip when pushed.

Until you overdrive it, I think the cathodyne PI, because of the 100% negative feedback, has inaudibly low distortion compared to the other stages in the amp. And when you do overdrive it, it tends to make various nasty-sounding types of distortion (Merlin Blencowe discusses this in detail in his preamp book). The big challenge with the cathodyne PI seems to be, to prevent it from making musically undesirable sounds when it's overdriven. Merlin says that using a huge (up to meg) grid stopper is the key to making a cathodyne behave.

I used an LND 150 MOSFET "source-o-dyne" in my last build. It is very much like Printer2's version, with the source biased to about one-quarter of the B+ voltage for maximum clean signal swing. The only real difference is that I have series back-to-back 12V zener diodes between gate and source for overvoltage protection, and my component values are a bit different.

I also made sure that the source-o-dyne cannot be overdriven - the previous (valve) stage can be fully overdriven without ever clipping the "source-o-dyne". This was done by putting a bit of fixed attenuation in between those two stages to cut down the signal level just enough. Since the MOSFET stage never clips, it's impossible to generate any of that harsh clipping that semiconductors are notorious for.

With that precaution in place, the source-o-dyne is completely transparent to my ears. Does its job quietly, and stays out of the way, just like a properly designed cathodyne with a 1Meg grid stopper. But the little TO92 MOSFET takes up much less room, uses no heater power, costs much less, et cetera.

IMO a cathodyne stage is the perfect place to swap in a MOSFET in an otherwise all-valve amp.

-Gnobuddy