Two-Channel Matchless, Spitfire + Lightning?

[jhaas]

Looking at the Spitfire and Lightning schematics, I see two simple preamps, similar (if not identical) poweramps with no NFB.

It seems to be quite feasible to build an amp with a Spitfire channel and a Lightning channel, each with it's own input, joined at the PI (one channel into each leg).

An A/B switch could be used to switch between the two inputs.

Anyone try this, or see any potential problems?

[Gee]

Should be fine. I have been toying with the idea of converting my Lightning clone into a similar design with a single triode for the Spitfire side and a single triode for the Lightning V1 as I want to keep the tube count the same.

[jhaas]

V1 is identical between the two models so you could split the signal after V1 thereby keeping the same tube count and retaining paralleled V1 for both channels.

In my case, starting from scratch, I'm fine with the extra tube for the sake of simplicity.

[Philoffline]

I was thinking of doing the very same thing on my project amp, which right now is pretty much a Spitfire with an empty socket and some space in the middle. It would need another pot to keep both the tone knob and T/B stack intact, though.

FWIW Trinity makes a variation on the theme that's AC-15 on one channel, Lightning-ish on the other. I've coveted that kit for a while now.

[KT66]

Since the preamp from those 2 amps are out of phase with each other and you plan on using both sides of the LTP PI, this would make them in phase and you could wire it so that both channels can be on at the same time instead of using a switch to toggle between them. There are a couple of ways to do that : you could jumper the grid resistors together and feed the signal from the lone input jack there, or you could use a 3 input jack setup with 1 jack for each channel and 1 that links them together like in the HiWatt linked-input schem below.

[Philoffline]

That's a very cool idea, thanks. I always forget about phase differences.

I don't think it'll work in a Matchless clone, though, because both amps use two triodes in parallel as the first stage. (As I understand, this is to emulate the high gain of the old EF86.)

[jhaas]

That's true regarding the V1 parallel triodes, but the Lightning has another dual triode between the first and the PI which does invert the phase (only once, the second triode in that envelope is a cathode follower which does not invert the phase).