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Gareth Magennis[_3_] February 6th 19 09:09 PM

Engl valve amp disaster
 
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.








N_Cook February 6th 19 09:18 PM

Engl valve amp disaster
 
On 06/02/2019 21:09, 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.








Can you find a flea-bay "spares or repair" one with a different fault to
compare with?


John-Del[_2_] February 6th 19 09:34 PM

Engl valve amp disaster
 
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.

John-Del[_2_] February 6th 19 09:38 PM

Engl valve amp disaster
 
On Wednesday, February 6, 2019 at 4:09:56 PM UTC-5, Gareth Magennis wrote:


Now, I found one of these diodes is shorted.
Bummer.
Meaning you get HT fed to all the heaters in the amp.



I wonder if the new output arced causing the diode to short.

Phil Allison[_3_] February 6th 19 10:04 PM

Engl valve amp disaster
 
Gareth Magennis wrote:



These are 4kV diodes,



** WRONG !!!!

Their rating is 11kV at 4mA and should never have ben used in the amp.


..... Phil



hard to find.
I had to order them from Europe, Farnell, RS etc don't have stock.




Phil Allison[_3_] February 6th 19 10:07 PM

Engl valve amp disaster
 
Gareth Magennis wrote:





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


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.


Sjouke Burry[_2_] February 6th 19 10:35 PM

Engl valve amp disaster
 
On 6-2-2019 22:09, 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.







Are you sure the right tubes are there?
The wiring suggests the tube pins are not matching the surrounding
wiring and components.

Phil Allison[_3_] February 6th 19 10:44 PM

Engl valve amp disaster
 
Gareth Magennis wrote:




This makes more sense now.



** NB: Have a good look ( and a sniff ) of the output valves - one of them may have internal arcing damage inside the plastic base between pins 3 and 2.




..... Phil




Phil Allison[_3_] February 6th 19 10:49 PM

Engl valve amp disaster
 
Gareth Magennis wrote:




OK, so I am still of the opinion that nothing could have caused that diode
to fail other than the diode itself (?)




** There are a couple of scenarios* that allow the 4kV rating to be exceeded & if the spiking is at a high audio frequency, the diodes may dissipate enough heat during switch off to cause failure.

Fact is, such diode DO sometimes fail and ought not cause collateral damage.


" Like plugging a jack lead from speaker output back to an input causing massive NF oscillation with no speaker load.



..... Phil



Phil Allison[_3_] February 6th 19 11:00 PM

Engl valve amp disaster
 
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



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