5E3 build loud hum/buzz once warmed up

[Ashman]

Hello,

I rebuilt a 5E3 tweed deluxe that was the first amp I built a few years ago. I have built a few more complicated amps from kits and parts lists since then and have not had this problem, or actually any start up problems with those. The first time I built this amp, it worked as advertised. I have learned a bunch over the last few years of doing this hobby, but I am not a tech or EE (but I am an electrician).

Once the build was complete, I measured all AC voltages from the transformer which were good.

Installed tubes, fired it up: No odors or smoke.

Once the tubes heat up, an increasing loud HUM/buzz comes on until I turn it off for fear of blowing the speaker (though I am not sure it actually could).

Thought it could be the rectifier tube; installed new one - no change. Tried a Weber copper cap - no change.

Tried another speaker - no change

Tried replacements for all the other tubes - no change

Replaced the OT - no change

Replaced the PT - no change

I haven’t left the amp on long enough to get DC voltages, but after I turn the power off, I’ve never measured voltages on the filter caps higher than 12v.

I thought maybe filter cap(s) are bad or bad dropping resistor(s)? All my grounds have continuity. All under board wiring has continuity, correct point to correct point.

I’ll attach some photos.

Any ideas or input would be greatly appreciated.

Thanks!

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[bepone]

1. remove feedback wire from the speaker terminal

2. start amp again

3. if there is no any noise and amp works, switch off

4. change anode wires from OT to pins 3 - sockets, A1 <-> A2

5. start amp again, verify all is correct

[sluckey]

Is this jumper missing or is it underboard. It's a very important ground and if missing would explain the loud hum as soon as the rectifier warms up. If it were my amp, I'd want that ground on top of the board in plain sight.

@bepone... 5E3 has no NFB.

[bepone]

edit, this post on top is for working amp

here is major problem short circuit to the ground on Ub+ line, so new procedure:

1. switch off the amp

2. drain all the capacitors , with 10k to the ground until discharges

3. multimeter in ohms, and touch every node on + cap and to the ground, find out which cap is exploded internally , or some wrong wiring also, shorting Ub+ line to the ground

4. replace that component

5. return to the upper post

[sluckey]

edit, this post on top is for working amp

Well, it was working the first time he built it. But he recently rebuilt it. After the rebuild it no longer works, so treat it as a new build.

[Ashman]

Sluckey:

Yes the wire is under board, but thanks

[Ashman]

Bepone:

Thanks. I need to read what you have written and try to understand. Will post back when I think I get it

Everyone:

Any problem with or benefit to swapping the two anode wires from the OT?

Thanks

[sluckey]

Any problem with or benefit to swapping the two anode wires from the OT?

No problem but also no benefit. A 5E3 does not have a NFB loop.