Mesa Boogie SOB Limiter Mod?

[gregsk777]

Hi,

I have an old Mesa Boogie SOB "Son of Boogie" and it says presence on the faceplate put it is actually a limiter/attenuator. Let me just say that the limiter is terrible! I usually hate attenuators but I don't know how this useless feature ever made it onto a Boogie?

Any ideas how I could remove the limiter and wire in a standard presence knob? From what I understand of Fender amps is that a presence is usually wired to the negative feedback?

I attached a schematic I found if that helps:


Any ideas would be helpful.

Thanks,
Greg

[methodofcontrol]

i haven't compared the schematics, but this is the one i've always used. check for differences in case one is better suited to your particular amp.

The Free Information Society - Mesa Boogie Son of Boogie Electronic Circuit Schematic

i find the limiter in my SOB is very usefull. the trick is to only use it at 2 or less.
it's for taking the edge off, not for getting the amp to bedroom levels.

[J M Fahey]

Interesting.
Greg's schematic shows no negative feedback used, thus no way to wire a regular presence control, *as is*.
The Limiter just starves the PI; never tried it but it might provide farty, dull limiting, unless used sparingly as suggested.
Maybe wiring a proper PPIMV or at least a Matchless style PPIMV (single pot, plate to plate) would provide better results.
In Method's schematic, there is no limiter but a Boogie style presence control.
Is it original or a Mod?

[gregsk777]

Thanks for your responses.

The SOB is stock and hasn't been modded. From what I have read the early SOB's had a limiter installed but since they were trying to use up old faceplates it was labeled presence. There were 60/100 versions, some that actually had an actual presence and others that had updated faceplates that said limiter.

To my ears the limiter is as you describe it - pretty dull and farty. At 2 or less it sounds better then at 6 or something, but crappy nevertheless.

Installing a PPIMV sounds interesting. That would give the amp 2 volumes and two masters making it one confusing amp! Either way I'm game!

Would the PPIMV be installed between the two 330k resistors where is says "Bias" next to the power tubes on the schematic? Or am I looking completely in the wrong place?

Comparing the two schematics pin 7 shows the 12at7 phase inverter to the limiter in my schematic and pin 7 going to the presence in methodofcontrol's schematic. So could I just switch the limiter out?

Thanks again

[methodofcontrol]

my SOB has both presence AND limiter as well as 60w/100w half-power switch.
8 knobs across the front, (vol 1, vol 2, MV, T, B, M, P, Lim).
mine is from '85.
i posted about looking for a mesa mark I reverb circuit schematic a few days ago w/ no reply. i want to build the circuit into the SOB and maybe mod the rest of the amp to mark I specs while i'm in there.

[gregsk777]

Mine is an '82 version and must be one of the early bare bones versions! Actually I just saw a 60/100 version on eBay that had reverb and a limiter on the front. What was cool about Mesa back in the day is that everything was custom ordered with whatever options the buyer was willing to pay for.

Here is MK1 schematic with reverb: http://www.tubefreak.com/mk1.gif

It looks like a lot of work, but would be an interesting project.

By the way how is your presence wired if you also have a limiter?

Thanks

[methodofcontrol]

i'll try to remember to go inside the amp and take a note of that for you. still wrapping my head around parts i'll need for reverb circuit. have to double check voltages off my tranny too if i plan to use exact MK I specs. if you don't hear back from me in a week, repost a reminder. i'm pretty busy but i'm eager to help a fellow forum member.

[gregsk777]

Good luck with your build. I once had an early MK1 with dynamo tape but I stupidly sold it. The SOB to my ears sounds pretty much exactly like MK1. My MK1 also had reverb and eq. The reverb was nice, way better then a Marshall, but definitely no vintage Fender or Ampeg. Reverb would be nice but I have been getting nice tones from my Boss Fender FRV-1 so an extensive reverb mod would be way too much work for me.

I went ahead and installed a PPIMV in the SOB. It is kind of overkill but works very nice and I can get even thicker distortion sounds. I would say the biggest plus of having a second master is that it makes the amp more versatile (not that it wasn't before) and I can also overdrive the clean input 2 channel. Luckily some idiot made a hole in the back panel of the SOB before I got it so I moved the Limiter back there. I replaced the pot on the limiter and it works better then before and now I have 2 volumes, 2 masters and a limiter! Crazy, I know but sounds cool so I am going to leave it.

[methodofcontrol]

hey gregsk777,
i haven't forgot about you. i plan to open up my SOB and draw out the limiter and presence for you later today.
anyone here have reverb in their SOB? i'm looking for the tank codes for my upcoming mod. thanks!

[methodofcontrol]

opened up my SOB today...
presence and limiter are wired as expected.

presence wiper goes to a small cap to pin 7 of the PI
presence ccw goes to 8 ohm tap off OT

limiter wiper goes to ground
limiter ccw goes to weird diode(?) labeled as TCR508 to pins 3 and 8 of PI

the weird diode looks like a two legged transistor but is drawn as a diode in one schematic.
also, it's socketed in my amp maybe because they're expected to fail at some point and socketing makes it easier to replace them?

the schematic i listed as my standard reference for my SOB doesn't have the limiter drawn in, but the TCR508 is there off the PI and just goes to ground. so basically...put a 1 meg pot before ground and you have a limiter

[J M Fahey]

That TCR508 is a "constant current source", put there to guarantee that the current through it will *always* be, say, 1mA or thereabouts.
I think they were socketed to be able to factory select the best one for the particular set of tubes it had 30 years ago, remember the love of MB for fixed adjustments.
It looks like a transistor but is drawn as a diode because it is a (FET) transistor with two legs tied, don't remember now but probably source to gate.
It passes a constant current for any voltage applied from, say, 1V, up to breakdown voltage.
The limiter is a 1M pot in series which supplies *less* than the regulated current, starving the tube.
May even work ... on the scope, but musically, it's anyone's guess.

[methodofcontrol]

thanks for the insight, JM.
so...i have two questions i'll ask here and i'll repost as another topic if needed. but i figure i have some SOB enthusiast's attention with this thread, so i should milk it.

ok...so.... "S.O.B." means "Son of Original Boogie", what does "H.O.B." stand for?

also, for anyone that has reverb in their SOB... what is the tank code? (there's a stamped code on the tank that translates to input/output impedance, ground references, etc...)

[gregsk777]

Thanks for getting back to me about how the presence is wired in your SOB.

How is the conversion to a MK1 with reverb going? I think Mesa used Accutronics for their tanks. Maybe trying to get in touch with them might help or just call Mesa with some story how your reverb tank fell out of your MK1 and you didn't notice until you got home! I'm sure they would give you the code.

[edge59]

I will look at my reverb tank, its a 3 spring unit used in various Boogies, its now in my Deluxe Reverb, Wow! By the way I have a schematic of a SOB with some of the mods you are wanting to try. The presence is one of the reissue, but others can be used. If you decide to use the PPIMV, you may want to use the dual pot presence control.
Let me know what you all think.
Ed

[edge59]

I have not tried this yet, but I compared it to the Mark I schematic and added some notes, the 10pf//2.2 meg network showed a .1uf coupling cap just before it, and a 150k resistior to ground just before the next gain stage (or mixing stage if you have reverb), If you use the .047uf cap and 470k-680k reisistor to ground from the Mark I mod and its too midrangey, try
changing the .047uf to a .022uf (with 470k-680k)to keep the mids lower and the gain up. I think the .1uf and 150k to ground here should sound the same, but less mid gain, maybe the SOB cleans up better? In an old ad, Boogie said the circuit was re-designed to be guitar volume friendly. Sounds like a Blues/Rockers dream amp! Lots of possibilities here The first stage looks to be voiced to have less bass/mud gain and as you go along to through all the following gain stages, the gain gets build up gradually and the bass is added back in so its not too muddy to push the amp into a sweet overdrive. With the right mods, these amps might be sleeping giants???
Ed