Marshall DSL Plate Voltage Chart

[AmpFix]

Does anyone have a DSL plate voltage chart for all the tubes in this amp? I know that the voltages can vary from one amp to another but I'm wondering what they are spec'd at for all the tubes.
I'd like to have something to compare to.

[Enzo]

The main B+ will be whatever it is. You can calculate it from the transformer voltage, which is easily measured.

The screens will be a little lower in voltage, not a lot.

The preamp stages will probably run on 300v or so, I forget, and with B+ in that range, I generally expect preamp plates to run in the 150-200v range or thereabouts. Not rocket science. 350 B+? OK, move those up to 175-250v.

Cathode follower stages of course will have the full B+ on their plates. The cathodes will have high voltage on those.

WHen I check out an amp, I go down the row of preamp tubes and check all the plates and cathodes. If the plates are even roughly in that range, I assume they are OK unless I have reason to think otherwise. If the plates are at the same voltage as the B+, then I know the tube is not conducting. Likewise the cathodes. ON regular gain stages, the cathode will sit at a volt or 2 most times. And in fact, if you see for example 1.5v on a cathode with a 1.5k cathode resistor, then Ohm's Law tells us there is 1ma of current flowing. SInce current is the same through a series circuit, I can then look at the plate load resistor, and if for example it is 100k, then I take that 1ma of current, and Ohm's LAw tells me that there will be 100v dropped across 100k at 1ma. SO if the B+ was 300v, 100v dropped fropm that leave 200v on that plate.

That is how I know what to look for. The cathode current tells me what to expect on the plate.

Cathode followers are special, the cathode will sit at the 150-200v or whatever. The grid will be about a volt lower if you measure between cathode and grid directly.

And the phase inverter also will be special, the cathode will sit a lot higher than a gain stage.

[AmpFix]

The main concern I have is a little over 400vdc on the cathode follower plate. The cathode is at approx 240vdc so my assumption is that the anode to cathode to voltage is actually OK and the 12AX7 is operating within spec.

In terms of the preamp, yes, they tend to hover at the 300vdc mark.

[pdf64]

That's outside of the 12AX7 spec, which from memory is 200V limit cathode to heater. The 200V would include any Vac signal peaks superimposed on the Vdc.
240V may cause problems with some tube brands, which has been noted in the past (EH?).