Most Likely Source of High-Pitched Hum on JTM45 Bright Ch?

[The New Steve H]

What's the most likely explanation for a high-pitched background buzz coming from a 5F6A or JTM45? I've built both of these amps, and the bright channels tend to produce this noise.

I read up on diode noise, but I saw someone make an excellent point. The diodes (added to back up the 5AR4) may switch 120 times a second or whatever, but the actual time the switching takes is so tiny, supposedly it can't be audible.

I don't see how it could be heater hum because the pitch isn't low.

The guy I built the JTM45 for seems to think it's interference from fluorescent lights and all the crap on the stage, but I heard it in my garage. I guess the power supply could be lame.

[Structo]

Heater hum is generally 60Hz and 120Hz hum is generally from the power supply (Full Wave).

Also, check out a external source for the noise.
Ever had a cell phone near an amp? lol

Most likely it is a lead dress issue.

[Prairie Dawg]

How's the shielding?

[The New Steve H]

Shielding??? I didn't see no shielding.

[Phil_S]

Is the chassis open? The bottom plate is a shield! Or is there no bottom shield in the cab? That will let in all sorts of interference. If you have nothing else, smear some ordinary glue on the inside of the cab and put heavy duty kitchen foil over it. Works great.

[selloutrr]

You mentioned both of your builds had a similar issue. Dirty power? Try them at a friends house. Or new circuit.
If it was just one build I'd suggest swapping tubes. You may also want to check your soldering points to ground. Poor solder joints can increase the impedance creating grounding issues. What material is your board made out of? Have tested it for continuity and impedance? Did you meter your components before putting them in? It could also be a failing resistor or coupling cap. Test for leaks across the caps, and chopstick / freeze the resistors. It could also be lead dressing. You may need to shorten or move around wires. Coax may help for the input.

You can also hookp a scope and meter through the circuit to see what section the noise is developing in.

[Gaz]

Heaters can cause a bzzzzzzzz too. You should try a humdinger pronto to rule them out. all you need is two clip leads to wire a low value pot (just use the smallest you have) across the heaters with the wiper going to ground. Giver a twist and see if the buzz changes.

[The New Steve H]

I have noticed that Tube Amp Doctor 12AX7s are much less noisy than some others.

I never thought about shielding on the bottom of the box. Can't hurt.

[Structo]

You could try a piece of heavy duty foil for a quick listen test.

[marshamps]

Have you tested the Negative feedback circuit to be sure it's Negative? Remove the lead from the Speaker jack to the 27K resistor, if the noise goes away, then you have a reversal of the PI. Also, I find the lead dress on the Presence Control leads to high pitrch noise on that circuit. I will often put a 220K in series with the lead feeding the Pot. 2cents.

[The New Steve H]

That sounds like something I would do. Thanks.

By the way, the guy I made my last amp for has a Marsh. Small world.