Hum appeared in valve amp (C-J MV-55)

[6SN7WGTB]

My MV-55 has developed a hum equal in both channels. On this amp the output tube bias is set with a pot by each tube with an indicator LED. They're all trimming mid-range which is correct.

If the bias is wound down the hum disappears, suggestign that it's coming in from the earlier stages of the amp (12AX7 and 6SN7).

Tried rolling all tubes to rule out any obvious issues.

HT all looks fine, with minimal ripple.

So, to the heater supply: this consists of one winding which provides AC to the four parallel heaters on the output tubes, and then is bridge rectified (discrete 1N500x type)  and smoothed (4700µ) which then feeds the small signal tubes with DC.

The DC heater supply is referenced to the HT ground (via a 220k resistor and parallel cap), but I do not believe this is relevant (the line disappearing off in the schematic below

Measurements show, however, that on the supposed DC supply I've got about 4V DC plus 1.2V AC superimposed. This rail when 'scoped shows the 1.2V RMS AC at 50Hz (UK) not 100Hz.

This suggests to me that I have a duff rectifier diode, presumably open circuit? Am I on the right path?

It's so simple I cannot see what else could be wrong...

All guidance appreciated. Thanks.

[6SN7WGTB]

Answering my own post, I find that my MV-55 has a fuse in the heater winding circuit so by removing the EL34s I can test the rectifiers avoiding a complete engine out.

Result - one is o/c.