Stromberg-Carlson AU-36B

This is a beast of an amp that my dad resurrected in the mid 90s. ~100W from four 6L6GCs, two 5U4G rectifiers, 6SJ7 pentode input, 6SC7 floating paraphase phase inverter, and a 6N7 driver stage for each phase after the PI.

As you can imagine from the size of the transformers, this beast is quite heavy.

Guts (so far.)

The original schematic for an AU-36 with AU-36B corrections.

The most recent revision I've typed up.

So! A whole lot of potential here, but a decent number of restrictions too, many of which self-imposed.

• I don't want to drill any holes in (the front of) the chassis. I will drill two more holes in the rear of the chassis so I can use 4/8/16 ohm cabinets. Right now it's only set up for 4 ohms, which is all I need, but eventually I think the versatility will come in handy. I could just drill one hole for an impedance selector switch, but switches are easy to bump and I don't like putting heavy currents across them if I can avoid it. 
•  This leaves me with only 4 potential knobs: bass, treble, and two gains.I tried a Baxandall stack with the two knobs on opposite ends of the chassis, but the only place in the circuit that I can put the tone stack is right after the 6SJ7 and the output impedance of that pentode is just way too high. Instead I've decided to adopt the FAC switch from Orange for a bass cut. It works really well, but the values here are still too large so I'll probably lower C4. Even though I used a shorting switch it still pops, so I'll have to put a small capacitor across it. 
• For the treble knob I was originally thinking about using a Vox style "cut" control, but I recently saw a vintage Gibson schematic of a floating paraphase inverter with a single passive treble shunt to ground on the first triode of the PI, as it feeds the second triode (and thus both phases of the output tubes.) Pretty clever! That'll also save me the trouble of putting in a series of coupling caps.
• I'm not going to bother with the master volume control - it's making way too much gain, but I think I'll try turning the 6N7 stages into anode followers instead. No idea if that'll cause problems with the global NFB loop, but there isn't a whole ton of NFB anyway so I might as well scrap it. Hey, this amp's from the 50's, it might as well sound like it! 
• The morph control is fantastic and everyone should have one. You can design the pentode to have a low screen voltage leading to an easily overdriven stage that adds crunch and a little compression, and then dial it back to a more typical triode for a more classic sound. I took that from Merlin's preamp book. 

Currently the amp's having distortion troubles - I think a resistor (or twenty) is failing intermittently and I've got to probe and scope around to find it/them. Almost all the resistors are original and apart from whichever one is failing the amp sounds like frying bacon. So yeah, vintage carbon comps are not great. The humdinger on the heater works exceptionally well though, and the overall hum and noise is manageable.

I freakin' love this amp, but I'm not going to gig with it or sell it so it's somewhat of a lower priority than the Bogen conversion which I'll mention in another post.