[AMPS] Power Supply Diodes

[Steve Harrison]

At 05:10 AM 2000-02-03 -0700, measures wrote:
>>One more time, the failure DID NOT occur on start-up. It DID NOT occur 
>>during a big-bang or other disaster. The whole thing was loafing along at 
>>about 1/3 design limits when the damn diodes just rolled over and died. 
>
>I have seen very brief hv arcs to gnd that somehow did not result in a 
>big bang.  
>
>>Never before have I experienced diode failure at anything but turn-on or 
>>big-bang. I have almost convinced myself that it was just a bad batch of 
>>cheap diodes, but yet at the level it was running at, anything should have 
>>survived.
>
>If you had a 50% or better piv safety factor, this failure is not easy to 
>explain.  
>>
>>So the question remains, if 3 amp diodes blew with only 700 ma. load, what 
>>does it take to keep it running at 2 amps?
>>
>First off, I would use a high-pot tester to make sure the diodes used are 
>confirmed good and roughly matched in actual piv.  .  

I gather that Larry has apparently not replaced the rectifiers as yet and
operated the unit again. So perhaps something DID blow, taking out the
diodes, which he will discover when he tries to fire it up again. I would
say it would be worthwhile installing new diodes and at least getting the
tube back into idling current again just to see whether something else
actually has gone away. He could do this with smaller fuses than required
for full-power operation to limit the decibels and shrapnel of a possible
immediate Big Bang. Even a hi-pot tester would give some indication of a
problem, particularly with the tube filament off. Perhaps the filter
capacitor broke down; the peak voltage with capacitor input filter is about
1.2 times the average DC output voltage; so a filter cap operated near its
max average DC voltage rating might have been overstressed by a
relatively-small and unnoticed AC line transient once too many times.

73, Steve K0XP

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