Jcm 900 oddity

[Valvehead]

Working on a jcm 900 .. 4100 model . 5881 tubes. This has the hi / lo output switch. I've had it here before ..it eats R8 and R9 randomly. These are at the Fail Leds.
I just noticed something else...This amp has factory screen resistors at 470 ohm . They are original. The scem shows them as 2.2k . What gives ?
Should I install 2.2k ?

[The Dude]

Full schematic here:

https://schematicheaven.net/marshall..._100w_4100.pdf

R8 and R9 should have no current through them at all unless F51 & F52 fail open. So, check the fuses.
If the amp has 5881 tubes, 470 ohm screen resistors are fine. The higher value is for EL34 versions.
This is a SWAG, but my bet is that C15 (the bias V coupling cap has gone bad/intermittent causing lack of bias voltage and too much tube current. Then, blowing the cathode fuses and burning the resistors. I wouldn't call it "common", but I've seen that cap fail often. Check bias voltage to see if it's where it should be.

Edit: That cap for the bias AC supply is C12 in the 5881 version- not C15. It's on page 3 of the schematic I linked. Also, that page 3 schematic probably better matches the amp you are working on.

[Valvehead]

I'll check it. Bias is fine measuring at Ot. Yes it did have a blown cathode fuse and r8 was burnt . Amp is running fine . Maybe I'll swap out c15 while it's apart

[The Dude]

Just a heads up that I edited my post above. Not sure if you noticed. Maybe that will help

[g1]

What is the wattage of those R8,R9 resistors? I don't recall ever seeing one burnt. It is designed so if one tube goes bad, the pair is removed from the circuit and you go on playing, with the LED on.

[Valvehead]

Not sure , but ive replaced them before. I used 1w