Hot looking anode/plate resistors.

[bassetrox]

Hi,

I've got a Marshall DSL 100 with 4 cooked looking resistors... R48, R2, R3, R1. What could cause this?

R2 and R3 are in the anodes of the PI valve (V4)

R1 is the load resistor for V3b, a gain stage. R48 is on the cathode of V3a, a cathode follower.

I don't know what wattage they are. Assuming 1/2W it would take just over 3mA to take it outside spec.

jcm2-60-02.pdf

Can you help?

[bassetrox]

Any help with the cause of this?

The amp in question has been blowing fuses, and whilst I suspect the power tubes, I'd like to get to the bottom of this too.

Thanks,

[pdf64]

Might the VB+ have risen, thereby causing excessive dissipation?
eg amp operated without a full compliment of (functional) power tubes, intentionally or otherwise?
Pete

[bassetrox]

Thanks Pete,

do you think that one dodgy tube would really make that much difference? I don't know how much the rails jump by, but say 20V or so would only cause an increase in current of .2mA

:s

[pdf64]

Agree one tube wouldn't impact, but some guys run with 2 of the 4 removed; then if another blew its screen grid resistor there would only one tube functioning. Or if there's a pre-amp output, some remove the output tubes for quiet recording etc.
And if the node voltage rises then the whole LTP will draw more current, so it's a cumulative effect.
I concede that it's a bit fanciful.
Maybe someone tried an ECC82 in there.
Replace the resistors and hopefully all should be ok.
Pete

[bassetrox]

Yeah I see where you're going with it. Makes sense. Hmm. I guess like you say I'll just replace with higher W resistors and go.

[Jazz P Bass]

Faulty power tubes can take out the PI resistors, ala Fender HR amps.
Personally, I would ditch the PI tube & change out the power tubes, after replacing the resistors.
You wouldn't want to go too high on the wattage rating, as the resistors act as fuses in a power tube melt down.
So it's either the resistors that open or the copper trace that melts.

[pdf64]

'Faulty power tubes can take out the PI resistors, ala Fender HR amps'
Yikes, not aware of that - any idea what the mechanism is to causes it?
Pete

[Jazz P Bass]

The mechanism?
That I do not know for certain.
I see it a lot on Fender HR amps.
Somehow the faulty power tube pukes back into the PI.
Or makes the PI tube overconduct.
Again, I am only guessing.

[Enzo]

I don't know how the power tubes would have anything to do with it, the PI plate resistors are isolated from the power tubes by capacitors. And I don't often see those caps bad. Just my opinion, but I write it off to under spec'd plate resistors. I could be wrong.