Marshall JCM 800 50W

[pnut5150]

Well all heaters are lit that’s about as far as I’ve gone. I typically do fenders, I’m just learning Marshall stuff a little

[Helmholtz]

Yes sir that is the correct transformer I have.
The old transformer somebody had hooked up was smoking when powered up, when I unhooked the 0v B+ wire it would quit smoking and the lights would come back on bright. I don’t want this Mercury to smoke lol.

Unhooking the center tap means disconnecting the power supply. The center tap is the minus pole of the supply and must be connected to ground to get B+. (In some amps the standby switch disconnects the center tap.)
But first make sure you use the correct mains fuse and that your filter caps are ok. In case of doubt use a bulb limiter.

[pnut5150]

I do have 360v AC across the two red wires with the red/black to ground. I did however unhook the two red wires while testing then

[pnut5150]

Ok we are getting somewhere now. The mains fuse they had in it was HUGE!! With yellow wire going to pin 4 on power tubes from the first 50uf 500 cap removed the amps stays on, that wire has 360v, so I’m assuming that cap is shorted out internal? No swelling or bulging

[The Dude]

1) If you suspect a cap is shorted, check it with your meter.
2) Pin 4 of power tubes is the screen grid. There shouldn't be a wire going directly there. It should be going to the screen resistors.
3) If the wire going to the screen resistors is the one you unhooked, the caps are still connected, so a shorted cap wouldn't be the problem. It would likely be tube(s), wiring, or socket(s).

[Helmholtz]

I do have 360v AC across the two red wires with the red/black to ground.

The PT should measure 720VAC between the 2 red wires and 360VAC each red wire to ground. Unloaded DC at the first filter cap should be around 500V.

[pnut5150]

1) If you suspect a cap is shorted, check it with your meter.
2) Pin 4 of power tubes is the screen grid. There shouldn't be a wire going directly there. It should be going to the screen resistors.
3) If the wire going to the screen resistors is the one you unhooked, the caps are still connected, so a shorted cap wouldn't be the problem. It would likely be tube(s), wiring, or socket(s).

Your right, I pulled wire from HT Fuse 😔

[The Dude]

Well, then there's obviously a short or heavy current draw downstream from the HT fuse. Do you have a light bulb limiter? If not, now's the time to build one before you damage something else in the amp. I'd first try removing the output tubes and see if the LBL glows bright. You could simply have a shorted tube. Is your bias voltage there? If bias is missing, the amp is going to draw a hell of a lot of current. Again, remove the tubes and see if voltages return to normal.

[pnut5150]

I don’t have a limiter, I have had the tubes out while working on it.

[Jazz P Bass]

Disconnect the OT.

[pdf64]

I don’t have a limiter...

There’s no time like now / it’s never too late, to remedy that.
My view is that for jobs like this they are as essential as a functioning multimeter.

[pnut5150]

I guess I can find schematics or diagram online to build one?

[pnut5150]

I have thought about that today, thinking it could be the culprit

[pdf64]

I guess I can find schematics or diagram online to build one?

Try this thread https://ampgarage.com/forum/viewtopic.php?f=6&t=20341

There's something on robrob's site too, his wiring diagrams are usually nice.

[pnut5150]

Figured I’d follow up on this thread, been busy hadn’t had much time. Turns out the problem was the capacitor I was concerned with. One side was shorted out. It would flash out my capacitance meter when measuring that side. So new Mercury PT, (2) F&T caps and new tubes and resistors and she’s back to life. Thanks for the help guys