Marshall JCM-800 build sounds overly distorted

[Drew9783]

Hey everyone, I recently got this vintage JCM-800 2204 chassis that someone majorly rebuilt. When I got it and plugged it in, I noticed that my guitar sounds overly distorted, with some fuzzy blocking distortion happening when I hit the strings even moderately hard. This only goes away when the master volume is set extremely low. All of the tubes are good, so I don’t know what it could be. Any advice would be appreciated!

Some gut shots

[mortatone]

Do you know if the work had been done recently or was it in good working order for a while after being built?

[mortatone]

Assuming it was recently built and never worked correctly, I'd suggest swapping the OT primary leads. It's a 50/50 shot on any fresh build getting the polarity correct.

[Drew9783]

I believe the work was done recently because the output transformer isn’t original, it’s a new Hammond replacement. I can try to swap the OT primaries!

[jgreenwd]

The build looks very similar to the layout from Ceriatone. It looks like someone just dropped in a new board and wired it up.

After checking the OT, have a look at the wire highlighted in this picture.

Preamp B+.jpg

This should be a B+ line to the plate resistors for V1. It looks like it's tapped off and running in the wrong direction.

[Drew9783]

Thanks! Output transformer primaries were indeed flipped the wrong way. The amp sounds great now, however I’ve noticed quite a bit of 60 cycle hum in the signal when turning the master volume up. I know the caps are good because they’re new JJ’s, what could be causing that?

That highlighted purple wire actually isn’t taped off, it’s running out to one of the caps!

[mortatone]

Awesome! That’s always a first check.

As for the hum, it’s chopstick time. See if moving around the input leads affects the noise. If so then put in some shielded leads and ground one end. If not then explore with the chopstick, moving wires etc.