Phase Inverter Distortion

Phase Inverter Distortion

Postby Stevo123 » Mon Nov 15, 2010 8:28 pm

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?) :
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

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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Re: Phase Inverter Distortion

Postby tigerstrat » Mon Nov 15, 2010 9:07 pm

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?


For as long as I can remember , my Mesa Mk III's had a 12AT7 PI tube (swapped the Sovtek/Mesa out for an older Philips ECG, another one is reverb driver in my DSR), but once in awhile I notice that "MB"'s handwriting on the chassis bottom indicates 12AX7 in all five preamp sockets. For the heck of it, I should try an X as PI...

Did you try the swap? From what you say you are going for, you might want to try a lower gain tube type in V1, but you might not like it. Doesn't hurt anything to try. Fun with preamp tubes.
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Re: Phase Inverter Distortion

Postby Stevo123 » Tue Nov 16, 2010 11:36 am

Was gonna wait til the weekend to mess around with this but I got impatient and picked up an AT7 today!

I'm very surprised how big of a difference it really made. The hard-clipping my preamp would do with harder strums has a much more gradual onset. All other things equal, I can turn my mid knob up more now (lessening the attenuation I was doing earlier), getting more volume through to the power section while staying cleaner in the preamp. Seems to make the highs and lows behave in a much tighter way. That may just be because the peak signals are making it through to the power amp without being chopped up by the preamp, and the power amp is smoothing them out in a much more musical way. Glassy, clear, punchy, expressive at pretty freakin high volumes. Another cool thing is I now have a range where the speakers are moving some serious air, but it's clean enough to where I'm satisfied with the tone at that volume level. I'm very happy with the change.
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Re: Phase Inverter Distortion

Postby DeadAheadNH » Tue Nov 16, 2010 2:14 pm

I just put in used (mid seventies??) GE AT7s into my HRDeville and that cleaned it right up, that along with NOS 6L6WGB power tubes really put me in a manageable sweet spot with great power tube distortion as you wind it up! I think (IMHO) you should start with the phase inverter and then keep adding AT7s until it sounds the way you like it - I ended up doing all 3. Good luck - let us know how that works out for ya :-)
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Re: Phase Inverter Distortion

Postby jerrys.kids » Tue Nov 16, 2010 4:56 pm

hey steve, i'm running a blues jr also, but am not up to par on the tech side of things. what are your volume settings. i may try this switch also. thx. chris
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Re: Phase Inverter Distortion

Postby Stevo123 » Tue Nov 16, 2010 6:37 pm

Master volume all the way up, Volume 6-7. It's loud at that setting, enough volume to make your ears actually hurt if you're within 5-10 feet of it. That still gets into a little hard clipping with overly aggressive strums and/or big chords. Before switching tubes hard clipping started at about 4-5.
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Re: Phase Inverter Distortion

Postby tiffcheese » Mon Jan 24, 2011 9:55 am

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?) :
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

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?



Interesting.
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Re: Phase Inverter Distortion

Postby Stevo123 » Wed Jan 26, 2011 7:01 am

After some more trial and error with this I eventually decided that a lot of the breakup characteristics I didn't like were actually coming from earlier on than I thought and the power section had a bit more "oomph", richness, whatever you want to call it, with the 12AX7 in V3. The combo I settled on (for now) is a 5751 in V1 to clean it up earlier on, and the rest 12AX7s.
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