At this point, I really don't have a lot to say about the project. It's a pretty straightforward 45 SE parafeed amplifier. The operating point for the 45, 250V Vpk, -50V Vgk and 34mA Ip, is textbook -- I did however opt for the 3kΩ output transformer rather than the 5kΩ you frequently see in 45 amps. The 56 isn't a common choice for a driver, but it does nicely here. The 6AU4GT, a TV damper diode, works well and provides a nice slow warmup for B+. If I were starting over, I'd probably change a component value here or there -- larger Rk for the 56, bigger interstage coupling capcitor -- but overall I think it's a pretty nice sounding amp.

Most importantly, it's a good base for modifications. If you look closely in the picture, you can see that I built each monoblock on three pieces of wood: one each for signal, B+, and filaments (though I'm wishing I had just split it up between signal and power supply). I have a laundry list of things I'd like to try out, and I thought by using barrier strips I could just work on the relevant sector of the amp then hook it back in easily enough. For the power supply, I'd like to try some different regulators: swap out the 0D3 dropping resistor with a 6BQ5 wired as a constant current source or alternatively with a Gary Pimm-style CCS, try one of John Broskie's Janus shunt regulators, and I picked up the SR45 kit from Bottlehead which I imagine would work well in this amp -- I should've ordered a power trannie with multiple taps, which I'll have to do in order to swing all these experiments. I'm also planning on trying a different method of generating the bias voltage, called back biasing. And, if I can come up with enough B+ I'd like to give direct coupling from the 56 to the 45 a go. Lots to try...

Schematic In all its glory...