[GrimJosef]

Thanks for the replies everyone.

Yes the EZ80 socket and, indeed, the EZ80 are both still in circuit. But under no-fault circumstances the BY127, with only a volt or so dropped in the forward direction, bypasses it. It doesn't protect the EZ80 from reverse breakdown of course but there seems to have been no gross failure of the EZ80 (shorts in either direction, bits of metal or mica rattling around inside the envelope, loss of vacuum etc).

I did worry about reverse breakdown of the BY127 but the same device has been in place for at least 6 years and, most of the time at least, still works fine. Then again if it could survive a few hundred milliamps of reverse current that would be sufficient to destroy the resistors.

Assuming the huge Al-clad 50k resistor between the two 47uF capacitors is OK then it's hard to see how anything downstream of that could be causing the 830 ohms (1k2 in parallel with 2k7) to burn out. The same average current would have to be flowing through them both and the 50k would limit it to ~10mA.

Breakdown of the EZ80's heater winding insulation inside the mains transformer would explain the symptoms. My limited experience of transformer insulation breakdown is that once it fails it tends to stay failed - the carbonised paper and plastic don't recover their insulating properties. But I suppose it could be arc breakdown across an air gap.

Since the BY127 is doing all the work I'm minded to replace it with something with a higher reverse breakdown voltage - I think I've got some GP10Y-E3/54s around somewhere, or even a couple of UF4007s in series - and then disconnect the high voltage from the EZ80 altogether. It'll still be left glowing orange above deck but it won't be threatening the bias voltage's integrity.

Again my experience of breakdown in electrolytics is that it tends not to be reversible. But simply as a matter of good practice I'm inclined to replace the first 47uF with a couple of 100uF 350V units in series, balanced with a pair of resistors.

As with all infrequent but persistent faults it can take literally years to discover whether or not you've fixed them. But that doesn't mean you shouldn't try.

Thanks again.

Cheers,

GJ