Mods for Mesa Boogie S.O.B.

[Mark]

I have been thinking what to do with my Mesa Boogie S.O.B.

The stock S.O.B. was pretty ordinary, The amp has had several incarnations, it has been a Mark 1 for a while but I found that rather sterile and not exciting at all.

I did turn the amp into a Dumble Bluesmaster for a while but the S.O.B. wasn't the right platform for the Bluesmaster. The O.D. tones were fine, but the clean tones weren't good enough.

Any thoughts on mods for this amp? I should point out that I intend to use a regular longtail pair and not use the weird Mesa Boogie S.O.B.

[thetragichero]

got a stock schematic so we've got an idea of what you're working with?
what kinda sounds would you like it to be?

[Mark]

Thanks for your reply. Attached it a hand written circuit diagram.

IMG_1327.GIF

[Mr. Lime]

I didn't like the boost stage in front of the clean channel either.

How about relocating the stage behind the tone stack and make the last two gain stage's cathodes cold clippers?
So in clean you would basically have something Trainwreck kind of and in drive mode it's similar to a Lee Jackson GP-1000.

At least that's what I am working on..

[Mark]

Thanks for your reply Mr Lime, I had entertained the idea of the cold clipper instead of the reverb return stage where I have no reverb.

What I'm wondering, is what is a good sounding P.I. stage?

The 5F6a P.I. is good, but it has quite a bit of gain and it has "that" sound about it.

[Mr. Lime]

Thanks for your reply Mr Lime, I had entertained the idea of the cold clipper instead of the reverb return stage where I have no reverb.

What I'm wondering, is what is a good sounding P.I. stage?

The 5F6a P.I. is good, but it has quite a bit of gain and it has "that" sound about it.

Depends on what sound you are after..
Would you like maximum output swing or PI distortion?
Also the negative feedback around the PA is a variable feedback control instead of presence. You might want to change that as well.
More feedback tightens the bass frequencies and a presence control reduces the feedback on high frequencies. Try higher values like 680n as presence cap for more midrange if you want to change that.

IMO the reverb network between V2a and V2b without a reverb is nice for clean sounds but probably not helpful for overdriving a cold clipper.
What value do you have at the grid leak after 2M2//10p?
Looks like 80k, SOB should have 150k but I got my MkI clean preamp built into a Mesa DC-5 with 220k grid leak and I love it.
80k would attentuate way too much I guess.

[Mr. Lime]

Here's what I'm going to try soon:

SE KT88 Amp.PNG

[Mark]

That looks good, why single ended though?

I was thinking of losing the reverb recovery stage as it serves no real purpose, thus the cold clipper stage instead.

The caps in the power supply are bubbling so I shall replace them. I will reduce the main filter cap from two 220uF caps to two 100uF caps. The 30uF caps will be 22uF caps. The resistor values may remain stock per the reissue circuit diagram.

https://schematicheaven.net/boogieamps/ ... eissue.pdf

[Mr. Lime]

I've got an old AX84 SEL amp which I converted to a Splawn circuit but it has oscillation problems due to the strange construction of the preowner.

I have to build it new from scratch and got very good sounds in the past from a single KT88. I think a preamp like this can benefit from the fast attack of a single ended PA and 20W is just enough to get a usable clean sound.