what is this SOB??

https://reverb.com/item/26155200

i bought this on reverb recently. i love the 100 watt SOBs... but this one is different. says HOB on the side like I'd expect... but has an extra preamp tube in it. it's not a reverb model (no RCA jacks). the parts they used all look vintage-mesa correct except for that extra preamp socket is riveted instead of screwed. what the heck is going on here? mesa mod? the work of a great tech?

i haven't even plugged it in yet... just opened it up to make sure it wasn't simul-class as some of them get mislabeled as 100 watters...

I've got 14 days to decide if i want to keep it or not. if it's some obscure mesa mod I'd be interested, but if somebody tried to make it a SLO or something I'll send it back

The link doesn't work for me.

okay i fixed the link, first time posting here.

I've already pretty much decided this isn't the work of Mesa and asked for a refund on it. But any info you guys have as to what is going on and what they're trying to accomplish would be appreciated. The soldering and craftsmanship is really impressive... and looks so much like Mesa's work from that time period. But it's a big step backwards in tone, in my opinion...

The limit has the effect of another master volume in this setup. I'd assume the added preamp tube is there for more gain but its actually got less gain than a stock SOB. It's still sonically in the realm of that blackface/mark territory... but just... kinda duller and squishier?

I don't think this is MESA's work but should be fairly easy to have it returned to stock?
As for the Limit control.This actually starves the Phase Inverter allowing you to get more power tube overdrive at quiter levels.
Not really a master volume in the true sense but does act similarly in that you can turn the master up and keep the amp quiter by turning the limit up.

Strange that the amp has less gain as that whole extra preamp tube should add quite a bit more gain I would've thought?

MK IIB said:

I don't think this is MESA's work but should be fairly easy to have it returned to stock?
As for the Limit control.This actually starves the Phase Inverter allowing you to get more power tube overdrive at quiter levels.
Not really a master volume in the true sense but does act similarly in that you can turn the master up and keep the amp quiter by turning the limit up.

Strange that the amp has less gain as that whole extra preamp tube should add quite a bit more gain I would've thought?

Click to expand...

i worded that poorly. it's not a limit on this one... it's like a redundant master volume. I've got two SOB 100 watt heads (this would've been the third). i know how the limit control is supposed to work.

i actually think the limit control is awesome between 1-3 on the dial because it kinda behaves like a cold clipper. i can get some really tight/chunky distortion from an SOB that i think even the IIC+ struggles with (i had them both for a while). that, and the Princeton-style phase inverter has more sparkle/spank on the cleans than the beefy/smooth thing most Marks have. The SOB is hands down my favorite Mesa... just trying to figure out what the heck this thing was haha

Ah ok!
I have a 60 watter SOB and love it too.
I would like to have the limit converted to a mark I/II style presence control to "warm" it up a bit as I believe without the presence control it's like having presesence on 10 which is a bit brighter than I like on the overdrive channel/input 1.I do love this amp though.
Did you try it out to see how it sounded/behaved?

MK IIB said:

Ah ok!
I have a 60 watter SOB and love it too.
I would like to have the limit converted to a mark I/II style presence control to "warm" it up a bit as I believe without the presence control it's like having presesence on 10 which is a bit brighter than I like on the overdrive channel/input 1.I do love this amp though.
Did you try it out to see how it sounded/behaved?

Click to expand...

oh yeah... the presence control is suuuuper important for the SOB. the distortion is real ratty and harsh without it. i went through a couple 60 watters thinking they sounded bad before i realized the presence control was the secret sauce for high gain tones. it's like $2 in parts and is less complicated than a tele wiring harness... do it!

https://music-electronics-forum.com...ie-sob-mod-how-to-add-a-real-presence-control

the limit control is way more useful with the presence control too. it still gets too squished and compressed past 3 on the dial... but i it adds a great harmonic content on the dirty tones...

Can't really help the OP about the extra tube, all I can see is that the amp has been worked on with a couple of changed caps, and oddly some (but not all!) replaced electrolytics.
If you have other 100w SOB's as models it should be easy to revert that head to stock, those are fairly rare.

Regarding the lack of presence on the short chassis, I wouldn't replace the limiter with a presence pot, but that's certainly possible and easy to do, I would keep the limiter as it is pretty useful if you don't have an attenuator.
There are holes punched in the back of the chassis, you just need to drill the back plate to install a knob there, as Mesa would have done it.

I have a '85 SOB Simul-Class and that's really a great no-frills amp.
I have a question: I'd like to use an A/B box to go from input 1 to input 2, but the inputs have interrupting jacks (if one is plugged in, it disconnects the other).

Would it be possible to replace those jacks with normal grounding jacks, or would that be an issue, maybe extra noise or something?