I used to have a 52 Gibson GA 40 (regretfully no longer) and to me that had a very nice jazz/early blues tonality, you could overdrive it by plugging into the mic input, but plug an archtop through the instrument channel, and you pretty much had that warm clean-ish jazz sound with some nice overtones.
For those of us who are into period-appropriate audio circuits for particular electric guitar styles, yes, that's an amp that might have been used by legends like Barney Kessel, Tal Farlow, Jim Hall, Jimmy Raney. Wouldn't it be great to capture something of that sound in a preamp to send to whatever power amp/speaker cab combo was appropriate for the situation?
I've come to believe that the tone control, phase splitter, OPT, and speaker/cab have more to do with the 'clean tone' of this or that guitar amp than the preamp or output tube types chosen. But I could be wrong. I often am.
I think I'd like to try a 12AX7 or 12AT7 style hi-mu triode as the input stage, into a pentode stage set to generate a good amount of THD and dynamic compression of the audio signal, with a tone stack or a simple one-knob tone control, and some kind of output buffer or OPT for impedance matching to power amps. I think getting that 2nd stage pentode to 'enrich' the tone might be worthwhile for getting an unusual sounding clean tone.
[Edit to add:]
A low-mu triode driving an OPT with a step-down ratio of 4:1 or so might be just what's needed here --- A cheap, limited bandwidth Edcor or similar SE OPT might be perfect.
GA-40 schematic is attached. It uses a 5879 pentode input stage into a 12AX7 paraphase phase splitter (with the tone control in the phase splitter section) driving push-pull 6V6 pentode output stage. That might be a good place to start...
Does anyone see details of chosen operating points, etc. that might have a large influence on THD, etc?
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I've come to believe that the tone control, phase splitter, OPT, and speaker/cab have more to do with the 'clean tone' of this or that guitar amp than the preamp or output tube types chosen. But I could be wrong. I often am.
I think I'd like to try a 12AX7 or 12AT7 style hi-mu triode as the input stage, into a pentode stage set to generate a good amount of THD and dynamic compression of the audio signal, with a tone stack or a simple one-knob tone control, and some kind of output buffer or OPT for impedance matching to power amps. I think getting that 2nd stage pentode to 'enrich' the tone might be worthwhile for getting an unusual sounding clean tone.
[Edit to add:]
A low-mu triode driving an OPT with a step-down ratio of 4:1 or so might be just what's needed here --- A cheap, limited bandwidth Edcor or similar SE OPT might be perfect.
GA-40 schematic is attached. It uses a 5879 pentode input stage into a 12AX7 paraphase phase splitter (with the tone control in the phase splitter section) driving push-pull 6V6 pentode output stage. That might be a good place to start...
Does anyone see details of chosen operating points, etc. that might have a large influence on THD, etc?
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Pre clipping I'd say you're right. But I'd also add where the pre-amp valves are biased is also very important as it sets how the 2nd and 3rd kicks in, particularly on the first part of the note, which is probably over loading something somewhere in the amp
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