Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old March 5th 19, 07:38 AM posted to sci.electronics.repair
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Default Engl valve amp disaster

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


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Old March 6th 19, 10:30 PM posted to sci.electronics.repair
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Default Engl valve amp disaster

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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Old March 7th 19, 06:47 AM posted to sci.electronics.repair
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Default Engl valve amp disaster

wrote:




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.





** A switch in the supply to the output tube's screens works well - assuming beam tubes or power pentodes are in use.

Only a few mA need be switched at idle and no electros to discharge.


...... Phil






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