Phase inverter, Mesa Boogie SOB

[Enrico]

Hello,

I recently acquired a Mesa Boogie SOB, 60W/reverb version, which is my third one (plus one that I used to have way back when and regretfully I sold). I really like these amplifiers, as you might imagine.

So, this "new" one has an issue, which is a quite obvious lack of volume compared to my other ones. It sounds good, everything works--tone stack, reverb, volume controls etc, but the volume is basically half of what it should be.

I went through the entire layout and compared it with the other two amps. The only difference I can spot is a 230pF capacitor across pins 1 and 6 on the PI tube, which has been retrofitted into the "quiet" amp. To be more precise, the amp has been modded so that the reverb circuit has its own tube, therefore instead of V3 feeding reverb and PI, there are now two dedicated tubes. I could revert everything back to original quite easily, only thing is that capacitor on the PI. Can it really account for such a drop in volume? Where else should I look? I checked all of the capacitors and they measure the same as on the good amps.

Any help is greatly appreciated!

[Enzo]

Hi Enrico, welcome to the forum. Just a suggestion. Pick one place for one thread on this amp. We don;t want to be trying to have a discussion in two places.

Having said that, in your other thread I suggested this:

Unsolder the part and lift it out of circuit and find out whether it is involved.

[Mick Bailey]

Why had the amp been modified?

It's difficult to evaluate the problem without understanding a little more detail of the modifications and whether these would have any involvement in reducing the volume. Does your amp have an FX loop? if so you could inject a signal into the return side to see if this produces full volume. Check the jack contacts for higher than usual resistance if it has a loop. These can go bad and reduce volume. A quick test is to patch a cable between send and return.

It strikes me that if everything is working ok, and it sounds good then the cascaded gain stages are getting a healthy signal and the attenuation is after the last preamp stage.

Did you get the amp like this, and do you have any contact with the previous owner to determine at what point it went bad?