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I'm considering implementing a multi-stage 12A?7 based preamp in my next guitar amplifier. I'm not sure how to set it up, I have a number of thoughts on this. I also want a "Blues Mode" switch to bypass all stages beyond the first and do single-tube preamp.
My first idea is reverse-order overdrive. Basically, I figure I'll set up voltage dividers between each tube stage such that each tube gets more signal (i.e. less goes to ground). The overall amplification of the first preamp (after attenuating its output), for example, should be 1. The second should be, say, 10 (compared to the input signal, not the output of tube 1). The third, 20. Etc. The last tube, of course, overdrives first because it gets the biggest signal in.
The second configuration I have in mind is forward-order overdrive. Same deal, except let the first tube start overdriving while the signal that reaches the second (after being attenuated) still gets amplified clean (of course, it's clean amplification of harmonic distortion). Again, as the volume goes up, more preamp overdrive kicks in through more stages; though this way we're coloring harmonics through the still-clean tubes. This may sound more pleasant?
In either case, the volume control will consist of a pot wired as a voltage divider between stages 1 and 2 of the first preamp (12A?7 dual triode). Because the rest of the preamps use fixed, non-adjustable components, it's safe and sane for me to flip a switch that bypasses the other tubes (DPDT, link the output of tube 1 to the output of tube N and open the output of tube N before this junction). In the reverse-order overdrive model, the first tube would have a large amount of clean headroom (biasing etc); in this one-preamp mode, we now have a blues amp (rely more on power tube overdrive).
I'm also thinking of doing this as a push-pull class AB1 amplifier, with each power output consisting of one octal and one 9-pin power amp socket. Basically, this would be a push-pull of a pair of 6V6 or EL84, or one of each, or a pair of each. Each power tube gets its own bias pot, and the user can pull the tubes he doesn't want to use (i.e. load 6V6 or EL84 depending on desired tone).
Just some thoughts, I'm planning this out right now. I can't do this just yet, I don't even know how to build a proper push-pull.
My first idea is reverse-order overdrive. Basically, I figure I'll set up voltage dividers between each tube stage such that each tube gets more signal (i.e. less goes to ground). The overall amplification of the first preamp (after attenuating its output), for example, should be 1. The second should be, say, 10 (compared to the input signal, not the output of tube 1). The third, 20. Etc. The last tube, of course, overdrives first because it gets the biggest signal in.
The second configuration I have in mind is forward-order overdrive. Same deal, except let the first tube start overdriving while the signal that reaches the second (after being attenuated) still gets amplified clean (of course, it's clean amplification of harmonic distortion). Again, as the volume goes up, more preamp overdrive kicks in through more stages; though this way we're coloring harmonics through the still-clean tubes. This may sound more pleasant?
In either case, the volume control will consist of a pot wired as a voltage divider between stages 1 and 2 of the first preamp (12A?7 dual triode). Because the rest of the preamps use fixed, non-adjustable components, it's safe and sane for me to flip a switch that bypasses the other tubes (DPDT, link the output of tube 1 to the output of tube N and open the output of tube N before this junction). In the reverse-order overdrive model, the first tube would have a large amount of clean headroom (biasing etc); in this one-preamp mode, we now have a blues amp (rely more on power tube overdrive).
I'm also thinking of doing this as a push-pull class AB1 amplifier, with each power output consisting of one octal and one 9-pin power amp socket. Basically, this would be a push-pull of a pair of 6V6 or EL84, or one of each, or a pair of each. Each power tube gets its own bias pot, and the user can pull the tubes he doesn't want to use (i.e. load 6V6 or EL84 depending on desired tone).
Just some thoughts, I'm planning this out right now. I can't do this just yet, I don't even know how to build a proper push-pull.