Need help troubleshooting...

[C-Bone]

I recently acquired a 57’ tweed Pro amp that has quickly become a “desert island” amp for me. After playing thru the amp for several days with no problems, I noticed a strange hum-like sound at the start of a session. The amp still has the ground switch / death cap circuit, so I flipped the ground switch, the noise didn’t change much, so I flipped it back. That’s when I noticed that when I touched the chassis bolt-head on the top of the amp, it popped a little and the hum went away. This is the chassis-to-cabinet bolt that is attached with an L-bracket to the PT. If I touched the chassis / control panel anywhere else, the hum remained. It’s only when I touch that one bolt that the hum goes away. I thought that was odd, so I got my meter out, set it for AC and measured 142 volts from the bolt-head to the ground on the input jacks. (or just anywhere on the control panel) I get 142 volts with the standby switch off as well as on, and zero volts with the meter set to DC. With meter set for continuity, I get nothing from bolt-head to chassis, but I do get continuity from the bolt-head to the PT laminations. Basically, I have 142 volts from the PT case to ground... I've tried removing tubes and moving PT wires to make sure no wires were making contact with PT case.

Thoughts?… Is my PT dying…. could I have a partial short in one winding side of the PT?

Don't the internal windings come into contact with the laminations / PT case?

Any help on this would be much appreciated.

BTW, I have since removed the death cap and wired up the 3 prong AC cable correctly.

[Structo]

You need to rewire that amp to a three prong cord and get rid of the death cap ASAP.

Be sure to ground the green wire to it's own screw and nut through the chassis.

[selloutrr]

you can bypass the ground switch with the new 3 prong wiring it's no longer functional. leave in the switch for looks or use the hole for a mod.

[C-Bone]

You need to rewire that amp to a three prong cord and get rid of the death cap ASAP.

Be sure to ground the green wire to it's own screw and nut through the chassis.

Thanks, I appreciate it....

In my orig. post I mentioned that I've already removed the death cap and wired the AC cable exactly like your diagram.

Any thoughts on the PT issue?

[Gibsonman63]

Is you PT somehow isolated from the chassis? It should be bolted directly to the chassis making it electrically the same potential as your (grounded) chassis. For that matter, is the chassis truly grounded? I would check it against the ground prong of the power cord with a meter. Also don't rule out funky wiring at the electrical recepticle.

[C-Bone]

Is you PT somehow isolated from the chassis? It should be bolted directly to the chassis making it electrically the same potential as your (grounded) chassis. For that matter, is the chassis truly grounded? I would check it against the ground prong of the power cord with a meter. Also don't rule out funky wiring at the electrical recepticle.

Yes, the PT is isolated from the chassis by the fiber washers between the chassis and PT bolts. (all stock, factory stuff)

No continuity between chassis and PT case.

Chassis is truly grounded, recepticle wiring is good.

[ToneMerc]

Is you PT somehow isolated from the chassis? It should be bolted directly to the chassis making it electrically the same potential as your (grounded) chassis. For that matter, is the chassis truly grounded? I would check it against the ground prong of the power cord with a meter. Also don't rule out funky wiring at the electrical recepticle.

Yes, the PT is isolated from the chassis by the fiber washers between the chassis and PT bolts. (all stock)

That still doesn't make it right, at one time they also installed the fuse on the neutral side as well. The PT casing should be connected to the now grounded chassis and should not be floating, especially since you have rewired it correctly(and sans the death cap) with the fuse on the hot side.

That PT has 50+ year old paper as a winding insulator. If it was mine, I would yank it and check it for shorts. If I was to guess, I would assume the primary is shorted to PT casing, however since it's floating and not connected to ground, the fuse isn't blowing.

TM