I will be away for most of today, but I can find the old VCA stuff tonight or tomorrow if you are interested. It is a fully differential design like the Fairchild.
I'd
definitely be interested.
Last night, I found and transformed a 5670 model from a TINA to LTSpice, which only required replacing a few characters. Dropping it into my simulation in place of the 12AU7 drivers, I now
get why some of these early tube compressors used a differential design. It's not just out of convenience to accommodate a 600 Ohm mic input, using a transformer which can easily be center tapped on its secondary.
Though I'm pretty sure those transformers used contributed to the sound of the device. You read quotes about that Fairchild unit such as "Everything sounds great through it, especially vocals, piano and drums. I like it on my master buss too, even without compressing at all, just for color” and it makes you wonder.
So, putting the phase splitter as the 1st tube in a guitar amp allows subsequent circuitry to do compression. Tube compression. Then you read something supposedly attributable to Pete Townsend, referring to the Fairchild unit "They were rolled out like secret weapons to bring an electric guitar to life". I'm sure
Pete doesnt know what he's talking about, regarding electric guitar...
Manley audio says you can replace the 6386 with a pair of 6/12BA6s. The 5670 isnt like $100 to buy and I think it would be fun to try and see if it behaves like the simulation predicts. The Ampeg amp has a floating 70V winding on its OPT. In my mind, that can be full wave bridge rectified and fed to a similar attack decay circuit in pretty much that same way as in the Fairchild, which can then control the tube gain stage. Which - should - net a similar
fast attack behavior the Fairchild was most famous for - at one of the many envelope filter selections I see, just after that full wave bridge in their circuit.
Another interesting tidbit comes from a Rane service manual, for their MA 6 amp, which I have weighing down my garage floor. "Field measurements show a minimum 4 dB increase in continuous SPL, which is the equivalent SPL of a 250 watt amplifier without limiting. As many customers have proclaimed, “This is the loudest 100 watt amplifier, I’ve ever heard.” They say their "limiter" is inaudible, other than it makes the amp
sound louder.
So tubes, compression / limiting, interstage transformers; the proper mix and match is all about the sound that comes out. I - FWIW - look at these circuits and remark to myself "why would anyone build a guitar amp, any other way - it's all right there...been right under everyone's collective nose all this time!" Well,
cost. Instead, you can just follow an op-amp with a fender tone stack, a couple diodes with a switch to ground and a chip amp - done! But it's not going to sound like building the schematic below, with a few mods to accommodate a speaker and 1/4 input jack.