Vox AC15HTVH issue...

[SonWood]

I have an AC15 Heritage Handwired head that I've owned for, oh, maybe 10 years or so. Last night it crapped out on me. Everything lights up and all, but the sound is very weak and fizzy/distorted. Just prior to this, I noticed some very faint motor boating.

I removed the chassis and fired it up again. It appears that one of the EL84s (V6) is red plating, and it's noticeably hotter to the touch than V5 after I shut things down. I swapped in another pair of power tubes and get the same result.

Where should I start looking? Bad screen resistor? I'm no tech wizard, but around 10-15 years ago I built maybe a dozen tube amps. None of them Voxes.

Here's a schematic: http://schems.com/bmampscom/vox/Vox_..._handwired.pdf

[Jazz P Bass]

A 'bad' screen resistor, if open, would prevent the tube from conducting.

Measure the grid (Pin 2) voltage levels of the EL84's.
The output stage is Cathode biased so there is not a separate bias voltage supply.
If the red plating tube has a higher voltage then, I would suspect a leaking coupling capacitor (C5 or C6) coming in from the phase inverter tube (V4).