Marshall DSL question - design flaw?

[Peter Clothier]

[QUOTE=AdmiralB;16620
Likewise, it doesn't seem to be present on any of the 50W models - which only populate the center two sockets.]

Erratum old chap. The schematic for the TSL60/601/602 has a 22nF across both EL34 valves!!!!!!!
Its definately a purposeful act.
(the plot thickens)

Pete.

[AdmiralB]

I meant the DSL 50. The TSL 60 is a very different amp (another thing I don't get - the TSL 100 is very very similar to the DSLs, why go so far out for the two-tube version?).

[Steve Conner]

I think feedback from plate to screen would be one way of damping out unwanted parasitics. The screen is just another grid, so there will be Miller effect that makes the cap more effective than if it were between plate and ground. I have seen schematics for UL amps where they out a series R-C snubber between the plate and screen, probably to stop oscillations that were being caused by the leakage inductances in the OT.

[R.G.]

Admiral B.
I installed a new resistor and upped the cap to 1000wvdc. End of problem.
Peter.

With no other info, I'd have guessed marginal working voltage on the cap.

As to why only one - could be that the symmetry isn't all it should be on the output transformer and damping one side works OK.

Why the screen? The screen is a second, low gain grid. At high enough frequencies, its capacitor may well let a feed from the plate kill RF response.

But I'm just speculating.

[Shea]

But why grab it inside of the screen resistor?

This is just a wild guess, and it might sound dumb to someone who knows a lot more than I do, but maybe the reason is to prevent or minimize LC resonance. If the screen side of C46 had been connected to the screen voltage supply instead of inside the screen resistor, then you'd have an AC path from the plate through C46 then through a filter cap to ground. Of course, the total capacitance from plate to ground would be very small - something less than 22p. I don't know what the resonant frequency would be for the primary inductance of the tranny and such a small capacitance, but if it's low enough that the amp would be able to sustain an oscillation at that frequency, then that's one reason to put the cap inside the screen resistor.

Shea

[AdmiralB]

LOL! I figured you might find your way to this thread!