On Wednesday, February 6, 2019 at 4:09:56 PM UTC-5, Gareth Magennis wrote:
OK, I repaired an Engl Sovereign 100 recently, it's a 100 Watt combo using 4
x 6L6.

One of the JJ 6l6 had obviously overheated, and it's screen resistor was
toast.
The other 3 measured quite worn, so I replaced all 4 with personally matched
JJ Teslas I have in stock along with the screen resistor.
I also replaced some HT caps which were leaking electrolyte.

All the pre-amp valves (JJ Tesla ECC83) tested good.


The amp was fine for a few gigs, the owner was delighted, saying it sounded
great.
Then it apparently broke. Blew the mains fuse.

Customer replaced it (more than once? With the correct type? Dunno, it was
missing when the amp came back.)


Anyway I as usual first took out all the valves and tested them.
4 out of the 6 pre-amp valves had open circuit heaters.


Now, this is one of the most difficult and time consuming valve amps I have
ever had the misfortune to have dealt with.
You can't see any of the components on the main board as they are between
the PCB and the chassis.
There are 3 other PCB's hard wired to this mainboard (2 front panel
potentiometer PCBs and the rear PCB.)
No plugs and sockets, all 4 PCBs have to come out of the chassis as one,
along with all the umbillicals connecting them together, which is a total
nightmare as they only just make it.
The OPT is actually ON the main PCB, making things physically really
difficult to remove the boards.
All the power transformer windings are hard wired onto the main PCB, so you
have to desolder all these first.

One problem is I can't find the correct schematic for the board I have, and
I have a burnt resistor on a preamp cathode I can't identify.

But more to the point is the mode of failure here.

The only schematic I could find is the same one Engl sent me, and is not the
right one for this particular model, but here it is for reference.
https://music-electronics-forum.com/...ad.php?t=46947

Sorry this is so long winded, but I feel it is necessary to provide all the
pre data.


Anyway, look at the output stage.
There are 2 BY509 diodes on the Anodes of the 6L6 valves, going to the
standby switch and then to ground.
Only the amp I have here isn't quite like that.
THIS one has the 2 BY509 diodes connected to the HEATER circuit, pin 2, on
the output valves.
The diodes are connected to pins 2 and 3 of the output valves.
The standby switch connects all cathodes to ground, but is not connected to
either diode.

Now, I found one of these diodes is shorted.
Bummer.
Meaning you get HT fed to all the heaters in the amp.
Double bummer is that the heater winding has a 250 Ohm hum balance pot
across it, connected to ground, the heater winding is otherwise in the air.
So now this HT has a path through all the heaters to ground.

This balance pot is now open circuit all ways, so current must have flowed
from HT, through the heaters, burning 4 of them out, through the pot,
burning it out, which is when it stopped.

I also have the burnt resistor on a preamp valve cathode to ground, maybe
this was an internal short, putting full HT on the cathode?

There is other damage I won't get into just now, but I just would like to
understand why they put these diodes on the heater circuit, and if my
synopsis/series of events here is correct.




Nightmare.

I don't see the reason either, other than the heater circuit being close to ground (but not quite). I guess you should be grateful it didn't take out the filament winding of the power transformer.

I would consider wiring it with the diodes connected directly to the standby switch, and isolate the filament per the schematic you have. Can't help but wonder if it wasn't a factory screw up.

Another possibility is isolate the anodes to those diodes with a 125 ohm resistor to the standby switch instead of using the balance pot as the ground return.

Crazy. Good luck.