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Hi Kevin
So I made a mistake in the simulation and had the screens tied to Va (in addition to the OT). Once I changed that the current in the off tubes looked like it was following the plate signal up and down about the idle current, albeit with some really nasty looking waveforms. So we're good there.
But right, that technique is shown multiple times in the TUT series, I know this is something you've done, and we see this in a few commercial designs. And it's funny, I had read the TUT series so much prior to actually hearing/playing through EL34s, so I know how you feel about them. But when i first played with them, their tone was exactly as I imagined them sounding reading your books prior...so great job describing them! And ya, I hear that little bit of distortion too.
When I observed the build after posting, I did see some voltage on the grids and thus cathodes, and I do have a tiny bit of feedthrough, but I do see voltage on the grids and the cathode that looks like the ones in the sims when I open the wiper to gird connection. Interestingly, changing Fixed/Cathode bias and Triode/Pentode on the "off" tubes definitely affects the "on" tubes with the wiper lifted too. Triode mode seems to cut the output power of the on tubes as if I had set their screens to triode mode.
And because I can't observe this distortion on the scope with a function generator, I tried scoping while playing. At first I didn't see anything, but I turned down the power scaling control slightly and it looked like crossover distortion appeared. I can tell that's it by the way it decays; it sort of oscillates like you're turning the rate down on a trem/vibrato and when it ends it sort of collapses like a gated dead battery. That's exactly how the signal behaved on the scope.
This is as far as I've gotten but it's curious because I'm biased to ~60% fixed, and I dropped the cathode resistor to 330R (~75%) just to experiment; I wouldn't think the tracking regulator would shift into cold bias that quickly (using the 'Standard' circuit). But we're on to something at least!
Re: Coil Snubbing. Ooooohhhhh, ya, I put it across the coil, not that way! That makes sense. I remember wondering while I was adding the zener where that 'extra' 12V came from in your first suggestion, lol.
Thank you so much Kevin!
So I made a mistake in the simulation and had the screens tied to Va (in addition to the OT). Once I changed that the current in the off tubes looked like it was following the plate signal up and down about the idle current, albeit with some really nasty looking waveforms. So we're good there.
But right, that technique is shown multiple times in the TUT series, I know this is something you've done, and we see this in a few commercial designs. And it's funny, I had read the TUT series so much prior to actually hearing/playing through EL34s, so I know how you feel about them. But when i first played with them, their tone was exactly as I imagined them sounding reading your books prior...so great job describing them! And ya, I hear that little bit of distortion too.
When I observed the build after posting, I did see some voltage on the grids and thus cathodes, and I do have a tiny bit of feedthrough, but I do see voltage on the grids and the cathode that looks like the ones in the sims when I open the wiper to gird connection. Interestingly, changing Fixed/Cathode bias and Triode/Pentode on the "off" tubes definitely affects the "on" tubes with the wiper lifted too. Triode mode seems to cut the output power of the on tubes as if I had set their screens to triode mode.
And because I can't observe this distortion on the scope with a function generator, I tried scoping while playing. At first I didn't see anything, but I turned down the power scaling control slightly and it looked like crossover distortion appeared. I can tell that's it by the way it decays; it sort of oscillates like you're turning the rate down on a trem/vibrato and when it ends it sort of collapses like a gated dead battery. That's exactly how the signal behaved on the scope.
This is as far as I've gotten but it's curious because I'm biased to ~60% fixed, and I dropped the cathode resistor to 330R (~75%) just to experiment; I wouldn't think the tracking regulator would shift into cold bias that quickly (using the 'Standard' circuit). But we're on to something at least!
Re: Coil Snubbing. Ooooohhhhh, ya, I put it across the coil, not that way! That makes sense. I remember wondering while I was adding the zener where that 'extra' 12V came from in your first suggestion, lol.
Thank you so much Kevin!
