Given one 12AX7 how would you run it given the rest of the amp is SS?

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If I have to stick with 12ax7, with cold cathodes.

Frankly, with Just One Tube(tm) I'd be going for a triode/pentode tube, with the triode set up with a cold cathode and the pentode feeding into a transformer& speaker emulator to get that overdriven output stage sound.
 
So a Champ of sorts. Not bad.

The 12AU7 in P-P is something I never considered either, there are a few dual pentodes with a shared cathode. Might be interesting also.

I drew up a 12AQ5 amp with a 12AX7 with cathode follower, actually had it wired up. Just needed some input gain. I chickened out and went with two triodes in series as a BF Fender circuit.

Well we can't get any more varied than that can we? I will have to think about it some more. I have been more concerned in the last week with a 12V6 amp I want to build. I think I got it down to a sort of 5F6 Bassman - Paraphase Deluxe.
 
As cathode followers to drive the VIN- and VIN+ inputs of a bog standard LM3886 chip-amp.


But the reverb goes between the CF and the chip!


Anyway. just for kick might try this with a high voltage convertor running off a 12V laptop brick.

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I used a 12au7 on the front of a TDA7294 chip amp.

Had a few problems with oscillation and noise but fixed it with LP filter on front end.

I had reduced gain in chip amp so 12au7 could make up for gain.
Then someone told me the tda7294 has a min gain for stability hence why it was oscillating. Managed to tame tda7294 with a 1000pf across inverting and none inverting inputs.
 
Or do you have a clever scheme to drive the CF to clipping/cuttoff?
The CF is an imperfect machine, it colors the sound. If you want clean then a MOSFETis the way to go.
Valve Wizard Merlin discusses this in his article on CFs. As I mentioned previously, the CF grid must be directly coupled to the anode of the preceding voltage amplifier stage. This is a common arrangement in guitar amps and the resulting 2nd harmonics are perceived as tube warmth.
 
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If you want the tube to be more than a Marketing point (which I guess is the case with the Joyo amplifier), then it must "do something audible" to the sound.

Which means driving it hard, to clipping or at least to very high signal levels, >10 or 20V RMS.
Then it will add noticeable harmonics which will colour the sound.

A simple cathode follower, specially at the front end (where sigmal level is lowest) or output , where "everything is already cooked and flavoured" and it only has to provide about 1V RMS or less, will do very very little; detectable on a Lab but practically not audible, specially for Guitarheads who are used to lots of angry harmonics.
We are not talking a Hi Fi guy who detects small fractions of 1% in his signal chain.

Cathode followers driven hard and driving a complex load (typically the Tone Control stack) add lots of interesting distortion , unsymmetrical so lots of even harmonics, which Guitar players like, so te unwritten rule (at least most follow it) in Valvestate type amplifiers and its derivatives is to have a transistor/Op Amp front end, driving a single triode gain stage, direct coupled to a low idle current cathode follower which will starve when trying to drive the negative swing into the TC.
Best bang for the buck.

Signal output will be reasonable for a tube power stage (the design comes straight from Tweed Fenders and Marshalls) and must be attenuated to drive SS amps, any Class, but the point is to drive it hard first.
If you set signal levels so it just passes around 1V RMS or less and straight the SS amplifier, it won´t do that much either.
 
I actually have a four pole switch with no center position. Or at least I think I do, will have to find it yet. I thought about the open centre position of the other switches I have, then I quickly put that thought out of my head. I did make use of a two pole three position switch yesterday to change the voicing of the tone stack. Slope resistor 33k with two 250p caps, 56k with one 250p, then open position three resistors in series, 33k, 22k , 120k with the 250p, which gives a reasonable BF curve. Will be fun to try.
 
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