Jcm2000 tsl602 ht issues, need direction plz

[JMMP]

Marshall jcm 2000 tsl 602, not getting plate voltage on the pre's, nor screen on the power tubes. All fuses good on board and off. Full 480v on the power tube plates. Not sure where that 480 being generated, pwr tranny secondaries not reading anything near that, doubling somewhere? What am I missing?

[Jon Snell]

Check R26.

[JMMP]

Check R26.

Yep, thanks. R26 opened up just like a fuse. No signs of damage overheating or anything. Replaced but now getting over 460 volts on the preamp plates, will check whats down line from r26. Also assuming chassis is my ground reference?

[Jon Snell]

That is about right, voltage wise.
The main cause for R26 failing is the output valves drawing too much current when the screen grid shorts out. I would check the output valves.

[JMMP]

Exactly what happened, owner replaced output tubes afterwhich amp is said to have worked for a little while".

[g1]

I don't see how you could have 460V on preamp tube plates. Better re-check that.

[Enzo]

If the preamp tube heaters are not running , then there will be no voltage drop across the plate resistors. Thus fill B+ will appear at the plate.

[g1]

But the filter cap is only 450V way back at the PI node.

[g1]

schematic attached

[Enzo]

If the amp had a full complement of conducting tubes, I'd imagine the voltage would fall into line.

[Jon Snell]

Check you have heater voltage.
Check the FIL1/2 supply from BR1.

[JMMP]

R26 replaced, tubes (pre) installed, all fell in line. Put power tubes in, biased, and all is very well. Sorry for the absence, trouble with my login.

[Jon Snell]

Pleased to be of help.