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Mesa Studio Pre

2/20/2004 4:22 PM
Mesa Studio Pre
I was looking at a schematic for a mesa studio preamp  
and am a little stumped as far as what to change to eliminate some of the mesa buzz.  
Any suggestions?
2/22/2004 9:01 PM
Joe N.

You could start by lowering some of the values of those plate resistors.
2/23/2004 5:07 AM
I had one of those. With all the stuff going on in there (extra gain stages, reverb, effects loop and graphic EQ, etc... Alot of the grounds are on the PC board. But IIRC some are to the chassis. To top it off, I think the backs of the pots are grounded also and share the ground with some of the gain stage voltage dividers. So I think the problem is ground loops. Whew, that PC board is loaded on both sides too. It sucks a$$ to work on. Hunting down all the possible ground loops on that board is something I avoided by buying a Marshall. ;)  
Mine buzzed much too loudly on the clean channel. Not much of a distraction on the gain channel though.  
I have a schem here and I'll look it over for some other causes. Sometimes Mesa had gain stages humming into the clean channel that didn't need to be.  
I think some shielded lead on the low level signal leads and reverb circuit could help too.  
2/24/2004 6:15 AM

It's not making any electrical buzzing just you know, "Mesa Buzz". Such a versatile preamp minus the Bumblebee distortion.
2/24/2004 5:27 PM
Oh, gotcha...Fizz. That insect attracting overtone.  
Mine didn't actually do that very badly as I recall. But we may have been using different speakers too. That can actually have an effect on how much of that crappy overtone you can hear.  
I'll look over my schem again and see whats up. I think that reducing plate load resistors to 100k (there are a couple that are higher) is a good start. But that will reduce gain too :( but there are a couple of other tricks to get it back :)  
I'll post back  
2/24/2004 10:21 PM
Hi chuck BTW I posted on the repair section and nobody responded about a twinmaster I have that needs a fixing. I think the middle orange drop isn't holding a charge, I suspect this has something to do with the distortion it's making.  
Does this sound right?  
2/25/2004 12:36 AM
On the Twinmaster (BTW, what is a Twinmaster?) if you mean to say that that cap isn't blocking DC, then that might be the problem. But it's not what I'd check first. The first thing I would do is swap some preamp tubes. Then if it still does it I would start pulling them back out one at a time starting with the first one (input side). And keep going until the problem stops. Once you isolate which tube it is you can open up the chassis and use a ground probe on grids of that tube to likely isolate which triode is having a problem. At that point there are only a small handfull of parts it could be. It will ultimately be trickier than this, but this is the basic troubleshooting method.  
As for the Boogie... Try this:  
1) Add an input resistor right on the tube socket. If the tube socket is board mounted (I cant remember) then just mount it on the input jack.  
2) Change R241 to 100k  
3) Change R273 to 100k and change R231 to 1.5k  
4) Change C452 to .022uf  
5) Chance C441 to .0022uf  
6) Change C453 to .0047uf  
7) Remove C7  
Do these one at a time ( #3)is changing two resistors, do both of these at the same time) and listen in between to decide if you like it. Work carefully so that you can reinstall the stock parts if you prefere them.  
My schem is pretty smudgy looking so below is a list of what each part # should be:  
R241 = V1A plate resistor  
R273 = V3B plate resistor  
R231 = V3B cathode resistor  
C452 = V1B decoupling cap  
C441 = V3A decoupling cap  
C453 = V3B decoupling cap  
C7 = jumper cap downstream from V3B (300pf)  
This outta get rid of of SOME of the fizzies.  

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