Morgan wrote:
Search long tailed phase inverter if you want a detailed explanation. That’s the classic big-boy amp phase inverter used by just about all manufacturers after 1960. Your Marshall 18 watt probably uses one too.
I don’t really understand how some guys get away with using 80-100 watt amps. I’m always being told to turn down and I play 20 watt amps. So, that being said, I’d go low powered twin. But maybe that’s just me.
The hpt is basically a tweed bassman with 2x12s and 2 extra output tubes. You could build it as a 2x12 bassman with a 45 watt output.
The lpt is a different circuit, akin to the tweed super, pro, and bandmaster except with the ridiculous dual rectifiers. They didn’t have solid state rectifiers back then, that’s why they used 2.
I like the lpt better, but I’m a weirdo. A hpt will give you a very classic sound with great cleans and great breakup.
Okay, did a bit more reading. I'm working through Valve Wizard's book on guitar pre-amps (excellent btw).
Let me make sure I have this right...the cathodes of the long tailed phase inverter are connected at the top of the 470ohm resistor. I'm not exactly sure what the resistor network is for aside from biasing the phase inverter, but I can worry about that later. The ".1-2.00" cap from the lower triode in the long tail grounds ac signal on the lower triode grid, letting the push-pull nature of the long tail do it's magic.
As for the 80W HPT, I agree that I would pretty much never use that much power unless I take it home to the barn and play for the cows. Would it be too sinful (or difficult) to do a half power switch? I've seen a lot of modern amps that do this. This is a big assumption, but based on the hpt schematic, I suppose I could more or less wire a switch such that the heater voltage would be removed from two of the four power tubes when enabled. With a master volume, that would give quite a bit of control.
Thanks,
Grant