Help identifying this flavour of hum

[TequilaKez]

Put new matched quad EL34s (ruby EL34BSTR) in a Vox AC100CPH and now I gots me some hum. It was probably there before the tube swap, one of the old EL34s was in fireworks mode so hum was the least of the worries.

Preamp is clean out of FX send and channel volumes don't effect hum. It's present with or without anything plugged in, and its present with signal direct to FX return. Master Volume effects it. So we're talking EL34 section.

Even though new, I checked each for microphonic, and swapped each indiviudallu with a known good tube. They all check out so no prob there.

It's of the mains variety, having strong 50Hz, and almost equally strong harmonics all the way up. (see attached)
Changing sockets / house / lifting ground etc does nothing. it's somewhere in the amp.

So this is the end of the line for my deductive reasoning.... so what say you EL34 lords, is there a likely culprit considering schematic and what's been ruled out so far?

Or what would be the next logical test to make? I'm thinking of investing in a scope to help investigations.... but what to scope?

http://el34world.com/charts/Schematics/files/Vox/Vox_AC100CPH.pdf

[robrob]

Your "balanced" power tube set may not be so balanced. It's possible you could reduce some hum by checking the bias of all the tubes and moving them around to get the two pairs of tubes bias balanced between push and pull.

[SILVERGUN]

I would visually inspect and test R128 and R129 (100ohm artificial center tap for power tube filaments)
 
**Of no significance--there is an error on the schematic labeling for the power tube heater connections.
They are labeled V1, V2, V3, V4 but should be labeled V101, V102, V103, V104 (see pic)

[PRR]

That strong 50Hz suggests bad main rectifier (dead diode), poor filtering (and dead diode) in the bias supply, or wall-wires passing through EL34 grid wire area.

[TequilaKez]

I would visually inspect and test R128 and R129 (100ohm artificial center tap for power tube filaments)
 
**Of no significance--there is an error on the schematic labeling for the power tube heater connections.
They are labeled V1, V2, V3, V4 but should be labeled V101, V102, V103, V104 (see pic)

Both measure 50ohms in circuit, pretty sure thats right as they're effectively parallel.

[TequilaKez]

That strong 50Hz suggests bad main rectifier (dead diode), poor filtering (and dead diode) in the bias supply, or wall-wires passing through EL34 grid wire area.

If I test all the diodes with my MM diode setting, and they read about 0.5V, does this mean they're ok?

Before one of the EL34's went into flying blue sparks mode, I remember remarking that this amp was pretty quiet, so I don't think its inherent in the design or wiring.

Is it likely that something died due to this EL34 dying in such a spectacularly bright show of colourful fireworks? If so what would it be?

I'm tempted to get a scope to inspect supply filtering. Any good infos on how to perform such an inspection?

Also I'm getting 460V at TT102 (497V on schematic), and 455V at TT101 (488V on schematic). Is that within spec?
Could there be an problem with the choke? How could I test that?

[SILVERGUN]

http://www.wincogen.com/files/downloads/winpower_manuals/winpower_diode_rectifier_testing.pdf

[TequilaKez]

Ok digging this out as I the will and time to attack this again.
Having replaced the valves and checking usual stuff again, only thing I can find out of place is the power rails on the FX Loop board. It has it's own PSU supplies 28V and 15V rails to various things including filter opamps. But voltages are out (marked on pic) so this could be my hum.

Just don't know why it's out. I replaced the rectifier and I'd previously done C201 & C222.
Couple of things I need a bit of help with:
1. What should the AC voltage be into BR200 to get +/- 37VCD. I only have 26VAC.
2. The only suitable caps I could find in stock ended up too big so I have mounted the at right angles such that C222 has rather long leads. Could the difference in lead length between 2 caps affect much?
3. I replaced R204/R235 with 1W Carbon resistors as I can't find 2R2 1W fusible resistors anywhere. Beside not fusing via lightning bolt, any issues with this?

[tubeswell]

Before one of the EL34's went into flying blue sparks mode, I remember remarking that this amp was pretty quiet, so I don't think its inherent in the design or wiring.

Is it likely that something died due to this EL34 dying in such a spectacularly bright show of colourful fireworks? If so what would it be?

Did you measure the screen grid resistor on that tube socket to see if it was open? The dying EL34 probably took it out. A blown Rg2 will stop any pentode or tetrode from conducting, and in a 4-tube PP amp, it will result in massive hum because the output stage is unbalanced.

[Leevi]

Check the negative bias voltage with oscilloscope at G1 (power tube grids) that the there is not ripple.
/Leevi