Marshall JCM 25/50 Low Volume & Distorted

[Aerostoon]

I have been working on a JCM 25/50 Silver Jubille and it has me a bit puzzled. The issue seems to be in the preamp. Some of my voltage reading on the preamp tubes look strange. Any thoughts?

EDIT 08/03/2017: Chart of voltage readings in this post are incorrect. Please refer to PDF file in Post #2.
The Janitor

[Aerostoon]

Here is a better more readable version.

[Enzo]

Well we have no idea which voltage readings you took and what makes them strange, so without that data how can we help?

[Aerostoon]

I attached a PDF with the readings in the 2nd post. Does it show up for you? Sorry if it doesn't.

[Enzo]

Sorry, I thought it was a more readable schematic, so I had not opened it.

What do you object to in that chart?

[dstrat]

+1 for the voltage chart , but they look pretty normal to me, what kind of tube current? How low volume are we talking here? lots of failure modes are possible.
my guess is output transformer, which is a wild guess at that.

[Aerostoon]

I guess I am more used to dealing with problems in the power tube area. I was expecting close voltages on the same pins for each preamp tube. With all of the volume and tone controls I guess that was a silly mistake.
For the transformer I did remove the power tubes and put a voltage on the primary with my variac and I measured the secondary (speaker connection) and it seemed correct. I will redo that test and post the results.

[dstrat]

you might try to put a cable in the effects loop and see if it is in the loop jacks.

[Aerostoon]

I tried a jumper cable as well as metering through the effects loop jacks. They seem very good. It was the same reading as holding my leads together.

[dstrat]

do you have another power amp you can plug the send jack into to see if the pre is sounding ok?
one thing i see is v2b pin 7 says -32.8mv is a bit odd, have you tried different tubes, i missed that last night.

[dstrat]

sorry second thought not so odd because lower plate goes grid goes up, or more negative.

[dstrat]

nope i felt like that statement was incorrect so i went to the bench and tried to duplicate those conditions,
but could not so either bad tube , resistor , solder joint on v2b.

apologies if i sent you on a goose chase.

dave

[g1]

Which are the correct voltages, the ones in the pic or the ones in the pdf? The voltages are very different from one to the other.

In any case, grid voltages can be hard to read, I would not worry about a touch negative reading there if the plate and cathode readings look ok.

[dstrat]

g1 your right much different, the pdf has pretty much what i got on the bench. i obviously failed to even look at that file.

[Aerostoon]

Thanks guys, the PDF is the correct chart. So you guys think the chart voltages are ok? I don't seem to be able to go in and delete the first chart.