Marshall DSL100 conductive pcb

[Gus]

Someone I know has a DSL100 that burns up output tubes. I did a search and found.


https://drtube.com/en/modifications/jcm2000-stable-bias-mod

http://www.ampwares.com/schematics/marshall/jcm2000_100w_dsl100.pdf

http://www.amparchives.com/folder/1741/

https://groupdiy.com/index.php?topic=67684.msg860152#msg860152

I was thinking the repair might be
remove the output tube sockets
drill out pins 5 PCB locations for good clearance around the pin 5 leads on the socket
install the sockets on the PCB
Remove the 220k grid 1 resistors from the PCB
Install new grid resistors at pin 5 at the  sockets
run wires to the - bias voltage node

What I am not sure of is there more issues on PCB besides G1 pin 5 locations?

 

[evil grill]

Thanks! Great info! If I only had been aware of this 15 years ago.

 

[CJ]

had one of those in the shop,  drilled holes in the pc board in between screen and grid pins,

that was not enough , we had to isolate the bias circuit also, pretty weird,

a little clear coat was involved but did not help,

it is a real slow creep, like a lion in the brush, plate current inches upward,

Marshall said it was their fault and would fix it for 250 bucks for a new board out of your pocket, then you install,  wth?   

 

[Gus]

CJ

How much of the Bias circuit did you need to place on another board?

You drilled between the pins with the sockets installed?

I wonder what is happening in the PCB with heat. Glass transitional temp (Tg) related?

I was thinking removing the sockets drilling out pin 5s install the sockets(maybe pealing back the traces that go to pin 5)
install 5.6K grid resistors at the sockets.

Also read on the web there is a C46 cap that should get changed.

A little off topic
Heat
conduction, convection, radiation
Heat conducts via the tube leads to the socket.
Some radiates from the plate thur the vacuum to the glass.
I should look for an old book that probably has information about how much heat by pin conduction and plate radiation in tubes

Heat, solder, vibration and tube mass.

 

[Whoops]

Hi Gus,
why don't you just buy the pcb from Dr.Tube?

it seems to be really well made an thought of,
or you could also do the same approach a do a pcb yourself

There just seems to be 12 components of the Dr.Tube pcb

 

[Gus]

Whoops

Not my amp

The owner is ls looking for a budget way to repair it. The person will build the bias circuit on perf board if needed.

Gus