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Re: What is happening???

Need to see your schematic. Nothing blew? Not even a fuse?

The shriek indicates high-frequency components accompanying some
transient event. I would suspect some kind of dielectric breakdown.

Some questions to answer:

1. Is the PIV rating of your rectifier(s) up to snuff with the
PSU voltages?
2. Are you exceeding voltage ratings of any dropping resistors?
Some resistors can only take 200V max across the terminals.
3. Do you have dielectric breakdown in your output transformer,
filter capacitors, coupling capacitors? The coupling cap to an
845 is vulnerable and dangerous point of failure.
4. If using a choke input PSU, have you equipped it with a snubbing
network to decouple the HV spikes that occur during rectifier switching?

At 750V, any fixed bias failure on an 845 might mean a dead
output transformer. You should protect it with a fuse. Rather than
put a fuse inline with the 750V (a bad idea), put it in the cathode
circuit. At 250mA fuse ought to save your output transformers.

Congrats on the 750V operating point. :) What PSU tranny are you
using?

-- Jim


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