How does the Mesa Boogie SOB P.I. stage work?

[teemuk]

Are any of schematics circulating in the Internet* "factory"? I doubt they are all entirely accurate.

*There are two, one, "open loop" circuit with limiter control to "current starve" the phase inverter stage and reduce its headroom, and another, sans limiter but with a presence control, which evidently controls amplitude of a a hi-pass filtered signal employed as negative feedback. (AFAIK, that should suffice as a tone control).

This is one of those rare amps where, by using the limiter, you actually DO get that praised "phase inverter clipping" instead of usual power tube overdrive. I have found that people rarely praise the tone of these circuits when they actually hear what they sound like, or even more, experience how they fail to react to picking dynamics.

[Reeltarded]

The absolute first version of it is insanely good. It was what everyone I knew wanted. smooth Marshall with just a bit more gain.

MkIIB from same period is the anti-SOB. Terriblist amp they ever made until they made some more.

[Mark]

Best Boogie ever made for ROCK. Un-Series

Mk0

Seriously great amp if serviced to original specs.

Which amp are you referring to?
I’m not aware of it?

Is there a circuit for this amp?

[Reeltarded]

Best Boogie ever made for ROCK. Un-Series

Mk0

Seriously great amp if serviced to original specs.

Which amp are you referring to?
I’m not aware of it?

Is there a circuit for this amp?

The very first version of SOB, Mark. The controls do great things, the channels don't walk on each other. From the perspective of 1983 and playing early Marshalls, SOB was the answer. They did legato right out of the box. Seriously good.

Unfortunately for me, I played the first one at Metro Music in Atlanta for 6 hours and ordered a pair of MkIIB stacks based on being told they were even better. Most expensive mistake in my amp history. The MkIIB preamp was hateful. I ended up driving those amps with Marshalls on dead loads I was terrified of. An old aluminum suitcase with three i/o sets that was scary hot..

I have no idea what the circuit is. I have never seen drawings that look like the sound I heard. There HAD to be some tiny snubbers. It did not lack treble but it was smoother than a whipped banana. I wish I bought that amp. No kidding here at all. I wish I had that amp. It was a combo.. I am not a combo guy. With a PAF 335 and a Mockingbird the thing was a Dumble crusher. Muted notes punched like a champion and free notes sang into harmonic insanity. The clean was adorable.

They made another amp that was similar much later, at least one channel. I think MkIV drive channel almost does it.. not quite.

Damnit.

[Mark]

That sounds like an amazing amp. It might have been the schematic listed below. That appears to be the earliest schematic.

My S.O.B. sounds reasonable but not anything amazing. The diode in the P.I. was stuffed when I got it. I suspect my one is the later circuit with pot in series with constant current diode.

http://guiguijones.free.fr/Electronique ... ie/sob.gif

[Reeltarded]

Hard to read all the values there but this is the one of two I have seen that look like a maybe.

All the attenuation and huge grid leaks. Weird animal.