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Hi James I put this on the site so it doesn’t get lost. https://boogieforum.com/threads/mesa-boogie-sob-mod.87143/ SOB limit control SOB presence control The 1M pot needs to be replaced with a 250K pot and wired to the secondary of the output transformer. The constant current diode...

No problem Michael, happy to assist you. Using a log pot, at “5” the wiper will be at 1K ohms. This means the bass control has the most effect. The higher the mid control the less the bass pot does. With a linear pot, at “5” the wiper is at 5K ohms. Whether you chose a log or linear pot, it...

The mid pot definitely isn’t a 250K pot, it’s a 10K pot. Suspect the 450S is the one that you are after. Have a look at the specs for both pots in the link and see what you should have. The 250K pot info from Mesa Boogie is definitely wrong. The treble and bass pots are 250K have a look at the...

A CTS pot with a solid shaft and a log taper should be fine. I prefer CTS to Alpha as the CTS look like they are made better. The Alpha pots work fine, I’m not fond of the coating on the back of them which you can’t solder onto. Regards Mark

There are a couple of circuits for the Mark 1 amp. The preamp is pretty much the same but the P. I. stage is different. One variation uses a tweed Bassman P. I. stage which has that Marshall like tone, the grid leak resistors have been reduced to 330K to reduce gain in the P. I. In later amps...

There is a reason that your SOB is bright. The cathode bypass caps on the preamp valves aren’t the same value as the Mark 1 amp. The Mark 1 amp has all 22uF@25V caps, the SOB uses quite small value cathode bypass caps on V1 (first 12AX7 near the inputs). The gain stage has a 0.47uF cathode...

There are options there. You can get a log taper (10%) 10K pot, if you are adventurous you can put a 100K log taper pot like the Santana Mark 1 model. It would work like a normal middle control from 0 - 5 and from 6 - 10 is would disengage the tone controls. If you use a foot pedal to turn the...

I have mentioned this before, but I can’t get over how different my SOB sounds to my original Mark 1 amp that I owned years ago. My Mark 1 amp was a sixty watt model with the stock Eminence speaker, and reverb. It wasn’t particularly bright sounding though it could get a lot of different...

Which pot in particular are you referring to? Treble bass or middle pot? Regards Mark

I recently saw this presence mod for the SOB amp and decided to post it up so it doesn’t get lost. I haven’t tried this mod so I can’t comment on the effectiveness of the mod. Regards Mark

So is there a link for Andy? I looked and couldn’t find one? Regards Mark

You would have the boards assembled in Asia where costs are lower. Many of the components are made in Asia so lead time should be good. The other option is to build a kit or get someone like DH Gate to build one. DH Gate Regards Mark

What circuit will you use? There are variations on the Mark 1 circuit. Regards Mark Mojotone chassis

They have reissued the Mark 1 amp at least two times. The Mark 1 reissue as well as the Santana amp. They are basically a one channel amp. Regards Mark

I don’t buy that at all. There are thousands of valves being made everyday yet these amps have discerning transformers. I have a set of Mark 2 transformers in my SOB and I don’t think they are anything special. I’m sure Heyboer could make great sounding transformers for them. Mercury Magnetics...

Well Mesa Boogie has reissued the Mark 1 amp several times and the Mark 2C+ gets a lot of attention. Mind you, unless I played one that I really liked. I would stick with my SOB.

The wire around the co-ax cables is strange. The wire goes around the input cable, the output from the first stage cable and the input to the second input. The signal on the second two cables are out of phase with the input cable. The wire from the output stage should be only seeing the preamp...

The SOB amp’s don’t have the pull brightness switch on the second gain stage or the tone stack lift on the first gain switch. I remember the first time I played one I couldn’t get over how different it was from my Mark 1 amp. It wasn’t as versatile either. They aren’t a terrible amp, but they...

The mains cable on my SOB amp went all sticky and weird. I decided to replace it with a thicker cable. It was all fine. Regards Mark

That mod is normally already wired into the amp. On my old Mark 1 amp there was a jack in the middle of the chassis, I just needed to get a cable and switch. The EQ changes significantly. I liked the gain boost but I didn’t like the change in EQ. There are other ways to get more gain from a...