Putting a Sta-Level into a guitar amp

[Ego Tripper]

So I have this tube amp from a Hammond M3. The amp is model AO-29 and boasts a nice, big power transformer, tube rectifier, and ten other tube sockets (two octal, three 9-pin miniature, & five 7-pin miniature). All of these sockets and ampacity beg to be put to use as a really sweet guitar amp. This particular socket allotment suggests to me a tweed-style amp with a Deluxe channel and a Harvard channel sporting a genuine 6AT6 thanks to the 7-pin sockets. I was looking at other things to do with the remaining 7-pin sockets when I realized a Sta-Level with a pair of 6BA6s would be a perfect use of those sockets.

Is there any issue with driving the vari-mu triodes from the cathodyne phase inverter (with coupling caps, of course)? Or do I have to buy an interstage transformer? This page seems well researched & cited when it claims that the output impedance of each half of a cathodyne inverter is equal:
http://wombatamps.blogspot.com/2011/11/on-output-impedence-of-cathodyne-phase.html

It seems like it'd be fine, but is there something I'm missing?

 

[PRR]

> it claims that the output impedance of each half of a cathodyne inverter is equal

Sigh.

Let's skip that.... why does it matter?

The problem with R-C coupling into vari-Mu grids is that feeding control voltage though the grid resistors to the coupling caps sucks-up or slows-down the gain control voltage.

If you were content with very slow Attack.... but in guitar you usually need a fairly fast attack. Fast enough to be "in the audio range".

OTOH guitar can stand a lot of bass-cut. Maybe very-small coupling caps will work.

Gross 2nd harmonic may not be offensive in modern e-guitar work. Use 6K7 single-ended as the first stage, CV RC-coupled to its grid.

Triple-grid 6K7 can also be gain-controlled at G3, which isolates signal from control.

All single-ended (and part-balanced push-pull) vari-gain schemes thump, but 2 or 3 poles of high-pass afterward may be enough.

I'm saying: this is not cut/paste (nor cut/half-paste), yet by experimentation you may find something useful.

 

[Ego Tripper]

Ah, I hadn't thought about how the coupling caps would affect the time constant. I was thinking of using 0.1 uF, a value already in use. Do you think this would be small enough?

As you point out, this application may not suffer from an excess of 2nd harmonic distortion. It might even be a good thing...

So I think I'll give the single-ended vari-mu scheme a try, but I don't think I'd use the 6K7 as I'm trying to reuse the original tube sockets, and between my power tubes and rectifier I'm out of octal sockets. Is there a factor that qualifies the 6K7 over other remote-cutoff pentodes (such as the 6BA6) or remote-cutoff triodes (such as the 6BA6 wired for triode operation)?