issues with a GZ34 and open circuit anode resistors.

[bikerhifinut]

Any ideas gents?
I was running up my EL34 monoblocs prior to running a few tests before I take on the task of the off board PSU. One of them on switch on had an instant fault. The GZ34 rectifier flashed quite alarmingly, fireworks in side the bottle and then although the valve stayed lit up the amp was very obviously dead.
I was fearful that the output transformer had gone S/C to cause this but on examination and measurement all I could ascertain was that the 100R anode resistors on the GZ34 had both gone O/C but no distress marks on them at all. These are fairly cheapo white cement coffin jobs from maplin rated at 7W which should be more than ample. So is it just **** resistors or something else? All I can think is the PSU electrolytics could be to blame as they run very close to their 500V rating and have on the odd occasion gone a bit over when testing and setting the amp up (High mains volts and transformer with only 230V taps). I can find no problems elsewhere in either amplifier. So my instinct is to junk the "Ruby" USA made (I think) PSU caps from Hi Fi collective and source some better branded jobs, maybe wire em as series pairs to handle any voltage surges.
The valve is definitely sick now too so was it the valve failing that caused the resistors to go open circuit or a fault elsewhere that took the valve out too? The thing is that the flashover occured pretty much at switch on so that is a mystery too.
for the record I have gone through a few anode resistors on the rects so that may point to a fault in the PSU? All circuit voltages check out as bang on design values by the way and I am sure cathode bypass caps are good as are coupling caps.

Andy