On Tuesday, March 5, 2019 at 2:38:49 AM UTC-5, Phil Allison wrote:
Gareth Magennis wrote:




I still don't have the schematics for this amp, maybe putting the protection
diodes on the heater circuit instead of ground was a big design mistake.
I have moved them to ground.



** That the HV diodes connect to the heater line ( instead of chassis ground ) likely did not cause the failure, but it sure caused all the damage.

FYI:

A tech working for the maker / importer is NO use to you when it comes to bad design problems. I see a lot of them these days in tube amps and have to modify circuits to effect a reliable repair.

I don't give a hoot what the amp maker or even the customer thinks of this cos *** I *** am the one liable for a repeat failure both financially and in terms of reputation.



.... Phil

the standby switch opening the cathode path to ground makes me nervous.

Maybe the switch arcs when it opens.

I'd find a different way to put the amp in standby or at least put a snubber cap across the switch.

Maybe wire a switch in series with R74 and let the negative grid bias go way up to cut the tubes off.



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