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famousmockingbird 7th March 2011 10:33 PM

Marshall JCM 800 50W PI circuit

Skip to page 11 on the link.

My friend got the amp for free and here is why, even with the preamp and master volume on 11 the amp sounds like it is on .5. He bought new tubes which I checked for him on my hickok 539b and they are all good.

After some DMM probing this is what I found, V1A plate is at 53.8v when it should be 185v, grid is 50v when it should be 23v, cathode is 49v and should be 39v.
V2B plate 229v should be 175v, grid 38v should be 24v, cathode 49v and should be 39v.

Could a bad .022 coupling cap before the grid cause symptoms like this?
Any advice would help a lot:cool:

famousmockingbird 7th March 2011 10:47 PM

Here is the exact schematic for the amp, the PI circuit looks the same but the first link has voltages for a reference.

V4 pin 6 one of the plates has 15v , that doesn't seem right either.

porkchop61 7th March 2011 11:15 PM

I think you need to follow your B+ line to see where the voltage drops. Is it correct right out of the diodes? This is basic troubleshooting. Could be a coupling cap.


EDIT: EL34's do not have a connection to pin 6, so it must be used as a tie/solder point for something else in the circuit. You're going to have to follow the schematic.

gingertube 8th March 2011 01:32 AM

A bad (leaky) 0.022uF coupling cap from V3B could certainly account for what you see.
The input side of the PI (V5a on schematic, V1a in your description) is conducting way too much current which shows up as the low anode voltage. This would also tend to turn off the other side of the diffamp and you see that as its anode voltage being way too high.

So either the input side grid is too positive (or more correctly not negative enough) with respect to its cathode (from a leaky 0.022uF coupling cap)
The other side grid is too negative with respect to its cathode (a shorted 0.1uF across the 10K in the diffamp tail [cathodes] may do that).

Replace the coupling cap first - if that doesn't fix it then change the 0.1uF


famousmockingbird 8th March 2011 01:57 AM

Thanks gingertube, and yes I meant V5 ( I wrote V1 for some reason? )

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