Stevo123 wrote:swapping the 12AX7 for a 12AT7 (in PI)... Maybe someone else has been down this particular rabbit hole before and can share what they've found?
Stevo123 wrote:I'll preface this by saying I am starting to understand the inner workings of tube amps a little bit, although I may be wrong about certain things and if I say any here I would absolutely love to be corrected!
So I've been doing a lot of trial and error with different variables on my blues jr. setup (tricked out like crazy - billm style), trying to optimize the combination of preamp vs. power amp distortion. My goal is to avoid pushing my preamp into "hard-clipping" while driving my power section as much as possible. I know the amp is not really designed to do this, but I've been having some success. So far I've found two interesting principles:
1. I've discovered, for one, that using my available "boosts", the clean boost module I had installed from billm, as well as the "fat" button that comes on the amp, as long as bass and mids are dialed back properly (I use an EQ pedal before the amp, and the tone stack to do this), will keep the preamp relatively clean, but give much more juice to the power amp tubes.
2. I've also found that dialing back my tone stack as much as possible (bass at 0, mids at 3-5, treble at 6-7, which still really gives PLENTY of bass and midrange due to the way the modded amp is voiced) keeps the preamp tubes cleaner as well.
Using these principles I'm starting to get some very sweet, gradual power tube breakup that manages to escape the ugly hard-clipping of the preamp tubes, up to a certain point. However, I'm wondering if I can go even "furthur" with this. On my amp, I understand V1 is the initial driver, I think V2 drives tone stack and reverb, V3 is the phase inverter (all 12ax7s, stock). Based on the latter principle that dialing back the tone stack (attenuating lots of signal between V2 and V3 from what I understand), it's keeping V3 cleaner. It seems to me that V3 gets hit the hardests out of any of the preamp tubes, because it's dealing with a signal that has already been significantly boosted. So I'm pretty sure this tube is my limiting factor in keeping my preamp cleaner at a given volume level, as it probably is in most amps.
I read an interesting article about the PI tube here which is where I'm gathering some details of my current working knowledge (perhaps to my detriment?) :
Apparently older Fenders (including Twins, I believe) use 12AT7s for the PI. They don't provide as much gain, and they are apparently able to handle current much better. I'm curious (and I plan to experiment with this in the near future) to find out if swapping the 12AX7 for a 12AT7 or similar in V3 (which I believe to be my current "choke point") will offer increased headroom in the preamp, therefore allowing an even greater ratio of power amp distortion to preamp distortion. Maybe someone else has been down this particular rabbit hole before and can share what they've found?
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