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.



mark

 

[John Robertson]

On 2019/03/04 2:42 p.m., Gareth Magennis wrote:

"Gareth Magennis"  wrote in message news:GAhfE.28020$LG6.15778@fx04.am4...



"Phil Allison"  wrote in message
news:340084f5-a421-404c-910f-c06582dd55d2@googlegroups.com...

Gareth Magennis wrote:



And this could have shorted the parallel 4kV diode between 3 & 2?

Or did the opposite occur?



** I only mentioned the possibility of valve arcing for completeness -  cos
it is another scenario that would cause similar damage.

Fix you Engl, come back when you have.

BTW

When the Krauts stuff something up - they do a thorough job of it.




.....   Phil



*******************************************


I have fixed the Engl.

Some of the 35v ELectro caps on the pre-amp cathodes were shorted, so
all 10
got replaced.
Kind of confirms a large global overvoltage happened I think, along with
the
burnt cathode resistor on V5.

The 6l6 heaters all survived, but I guess they are somewhat more resilient.


The Engl tech reckons V5 shorted internally.
Not quite sure how this would short the OPT diode to put HT on all the
heaters, but who knows the real sequence of events.

I don't know what the customer did, he has only said he replaced the mains
fuse with the correct one, but is otherwise keeping schtum.


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.



Cheers,


Gareth.



*************************************************


One other thing, V5 is driving the effects send and return loop, so this
is available to the outside world.



Gareth.

What I can't understand, from the schematic you linked, are the lack of
fuses between the Xformer and the bridge rectifiers. Looks to me like a
disaster waiting to happen. We add fuses in low voltage circuits like
that to protect the transformer as the primary fuse rarely blows when a
secondary bridge shorts out - at least the primary fuse doesn't
typically fail until things have gone very wrong in the Xformer...

On our old tube amps, as used in jukeboxes, the practice was to turn off
the B+ for standby, this greatly reduced heat as well as wear & tear on
components.

John :-#(#

--
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[Phil Allison]

mako...@yahoo.com 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

 

[Gareth Magennis]

"Phil Allison" wrote in message
news:c50a4e27-7cc1-41d9-8e65-5396dfd8addf@googlegroups.com...

mako...@yahoo.com 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



************************************


This seems to be a very simple and elegant solution, yet I have never seen
it in use.

Is there a reason this has not been more widely used?



Gareth.