Plate voltage on Valvestate 8080 head

[WholeToneMusic]

Does anyone have a ballpark of what the plate and cathode voltages are on the 12ax7 of the Valvestate 8080 head?

[Jazz P Bass]

The schematic indicates 350V capacitors on the B+.

[WholeToneMusic]

Thanks J.

[J M Fahey]

Yes, that's true, but I seem to remember that the PSU voltage was around 180V, with a plate voltage around 100V.
Of course I may be mistaken.
Anyway, using a 350V capacitor there, of which Marshall must have truckloads, doesn't hurt at all.

[Jazz P Bass]

You do have a valid point about having boatloads of caps.
I was assuming a working voltage below 350V.
Sounds like it could very well be way below.
(Hey, we need a 200V cap.
Bean counter. "Use the 350's, we have boatloads")
I do not know the exact voltage.

[J M Fahey]

I remember a not too old thread about a replacement Marshall autotransformer , not sure if it was here or at SSGuitar. Too tired to search now. Help yourself.
Good luck.

[Enzo]

Unless you ar trying to build a clone, you are trying to repair one. So if the high voltage is present and not all rippled, then it is probably cirrect. If the transformer opens, you get nothing. Very rare that a transformer failure causes a wrong voltage.

That voltage could be loaded down if there is a problem. So pull the tube and measure the B+. Should now be zero current, and so zero voltage drop. If a cap is real leaky, there will be a voltage dropped across the resistor upstream of it. Got essentially zero voltas dropped across both R111, and R110? Good.

Install the tube. B+ will appear on pin 1 or 6, depending upon which one is the cathode follower side, the other side will have probably 100v lower at its plate.

A typical 12AX7 carries in the ballpark of 1ma per triode. SO that means about 100v across the plate load, or in the follower, the cathode load. And in the gain stage, that 1k cathode resistor would then drop about a volt.

If your circuit is operating remotely like that, it would probably be OK.

[WholeToneMusic]

Thanks Enzo, I am repairing one that a customer brought in saying it had low output on the boost channel, When I tested it the clean channel was weak and the boost channel was not audible, Also the reverb did not work. I tracked down an open reverb cable to the reverb problem. When testing the plate voltages they are real low. 6VDC with tube in and 8VDC with tube removed. I will try what you recommend and see whats loading it down.

[Enzo]

Disconnect the rectifier and see what the autotransformer is putting out, then move on down the line.

[WholeToneMusic]

With the rectifier disconnected I'm measuring 36VAC across R112 & R113, 32VAC across R114.

[Enzo]

R114 feeds the center of the autoformer. What voltage is between that center tap and ground. That is what the transformer wants. Now what voltage is from the top of the autoformer to ground? In other words, what AC voltage is on the pad where the anode of D3 was? We are trying to determine what voltage is getting to that D3 rectifier. Clearly if we only get a few volts of AC up there, we can;t get many DC volts either. I'd expect something in the 150-200VAC region myself, give or take some.

[WholeToneMusic]

When I had D3 lifted I tested it and it was bad so i replaced it. Here is what the test measurements are with the new diode in. The voltages from CT to ground of the auto transformer is 35VAC and the voltage from the top of the auto transformer to ground is 290VAC. Now it seems a bit on the high side. Cathode side of D3 measures 290VDC. The junction between R109 & R110 measures 196VDC. Still within spec though if we take into factor the Cap voltage ratings. The other side of R109 measures 117VDC. Thanks for the help, Iron sharpens iron.