Need help modifying Musicman 75

[PepsiFX357]

Hey everybody,

I'm working on modifying a MusicMan 75 amp. This amp is in poor condition so now, it's my new project amp. Right now, it works fine, but there is a capacitor that came off of the tube socket. I believe it's the C40 470pF 3KV cap. Not sure how this effects the output. It's been like that since I've had it. I've noticed that the second speaker output on the amp doesn't work. Maybe this is why. So I guess is a MusicMan 37.5 instead of a 75. lol

I'm learning more and more about building circuits but I'm still a little new to it. So far my experience has been repairing simple problems like broken power connectors and building 2 SCA N72's, 2 SCA B16s, and the DI box from SCA. I'm also in the works, trying to build the Dr. Boogey pedal from scratch.

Here's what I want to do. I was wanting to add an FX loop, remove the existing pre-amp section and replace it with something like the Dr. Boogey, if I end up liking the sound. I've looked at different FX loop circuits and I understand that it's a custom thing for each amp and I also understand that the Boogey is a pedal and probably doesn't have the correct output voltage to tie into the power amp section. I've included the schematics of my amp on this post for convenience.

My amp's schematics are located on page 3 of the pdf. I've concluded that the input of the power amp is located at Bi-Polar C13 right before the Master Volume pot. I'm not sure if the capacitor should be included in the new circuit or removed, and tie the new pre amp directly to the master volume pot.

I have many questions....but that's my plan.

Can anyone help me out with this?


Thanks

[drtechno]

the 470 pf cap is to shunt the noise from the diode of the cathode.

the cap in the master control is like that on purpose to isolate DC so the input does not upset the biasing characteristics of the driver stage especially when the other input cap is taken out of the circuit due to the deep switch engaged. this cap should be of good quality like a solen or an orange drop.

[PepsiFX357]

Originally Posted by drtechno

the 470 pf cap is to shunt the noise from the diode of the cathode.

the cap in the master control is like that on purpose to isolate DC so the input does not upset the biasing characteristics of the driver stage especially when the other input cap is taken out of the circuit due to the deep switch engaged. this cap should be of good quality like a solen or an orange drop.

Thanks, I still haven't gotten around to replacing the cap. I plan on doing that sometime this week.