overview of the plan

So here's what I'm planning to do with the VTX, referring to the schematics I linked before.

Power amp:

I will keep the existing 4x6L6 tubes, their power supply, and the output transformer.

Peavey has the 6L6 tubes connected to a very peculiar circuit, which I haven't seen anywhere else.  The grids, where audio signal usually comes in, are simply hooked to a DC bias voltage, no signal.  The signal (two mirrored phases) comes in to a totem-pole of power transistors, which modulate the current through the cathodes of the tubes, two pairs in push-pull.  It looks to me like they started with a purely solid state power amp, and then tacked on the tubes to increase the voltage swing and thus output power.  Clever, innovative, but no thank you!

Instead, I will hope and assume that there's enough leeway in the rather beefy power transformer to handle two additional 12AX7 tubes (the issue is the heater currents).  One of these will be used as a conventional phase inverter, as found in the Fender AB763 circuit or the Marshall 2203.  The input to the power amp will go through a new master volume pot.

I'll operate the 6L6s in fixed-bias, with the cathodes grounded (or maybe through low-value power resistors like 1 Ohm, to facilitate setting the bias).  Peavey has the screens fed from what I'd call "ultra-linear" taps on the output transformer, and I guess I'll keep that initially, unless I encounter any reason to change it to fixed voltage drive, like the AB763.

Pre amp:

One of the sticking points for me in the past, as I tried to figure out what to do with this VTX amp, was the Peavey solid state preamp.  On one hand, I didn't want to expend energy "correcting" the power amp circuit, only to then keep it married to the funky Peavey preamp.  Particularly given that I'd be adding 12AX7 tubes, how could I resist providing a real tube preamp?  Yet, the Peavey preamp has a number of cute features, which I didn't necessarily want to lose: spring reverb, a phase shifter, and a variable-frequency midrange EQ.  These features could be especially useful for the many non-guitar applications I forsee for this amp, such as bass and electric cello.  The phase shifter is particularly unique, in that it has a pull-switch to disable the VCO, allowing you to dial a fixed phase shift setting, which can be a way to create specific, unusual voicings.

So, best of both worlds!  I will keep the Peavey preamp, but underneath it on the front of the amp will be another panel, same length and height, which will provide an AB763 style preamp (vol, bass, mid, treb), and a 3-way switch to select which preamp (VTX, AB763, or both).  This panel will also house the master volume pot, which will be functional for both preamps.

In addition to the four knobs, the tube preamp will have two switches: a Fender-style "bright" switch, and a "voice" switch.  This latter selects the cathode circuits for the two 12AX7 triode sections: either Fender-style, 25 uF and 1.5 kOhm, or Marshall-style, 0.68 uF and 2.7 kOhm.  Or, a third choice, 2.7 kOhm and no cap.

One remaining issue with the Peavey preamp.  I seem to recall that it must have the footswitch connected, otherwise some of the functions are not available.  I forget if this was reverb, or phaser, or what.  But in any case, this is awkward and I will fix it: either I'll change the behaviour so the features are on-by-default, or I'll put toggle switches on the new front panel, so that the amp can be operated with no footswitch.  (C'mon, Peavey!)  (Later: actually, all the functions are available without connecting the footswitch, I was wrong about that.  With no footswitch, both channels are active, and one simply turns one of the two "post" volumes down to select the channel.  Similar for the reverb and phaser, just turn down a knob to "disable" the function.  Simple, perhaps a bit cheesy, but intuitive.  My apologies, Peavey!)

Speaker baffle:

As I mentioned before, I will convert from 2x12 to 1x12, by cutting a new plywood baffle.  I'll cover it with the same Peavey black grill cloth.  There's enough room in the case, that I can mount the new, centered 12" speaker a little lower in the baffle, thus leaving space for the new preamp control panel above.

But what about the big, bold "Peavey" logo?  This will no longer be a real Peavey, to its credit.  But neither will it be fully my design and construction, so I don't feel I can put the "McGrath Audio" logo on it.  So what will I do?  Let's just say, I have a plan for that.  Stay tuned...