Marshall JCM800 50MV w/ reverb repair

[Opry Audio]

Hello! I’m having quite the time figuring out what’s wrong with this JCM800. It’s lacking high end and the breakup sounds “flubby”. I went ahead and took all critical voltages which I will list below. The only thing that’s been done to it is new JJ 50/50 can caps. I guess the originals were no good. I’m not very familiar with Marshall’s and what normal operating voltages should be although I know it can be figured out from the schem. I appreciate your help! Maybe the cathode voltages seem a bit low. Could it be sending some of the preamps into cutoff?

Here’s a link to a schematic: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM800-Splitch-Rev-50W-4210-Schematic.pdf

Voltages: (all heaters read 3.3vAC)

V1:
1- 139v
2-0
3-.925
6-86.1
7-0
8-.5

V2:
1-122
2-0
3-.75
6-233
7-122
8-123

V3:
1-128
2-0
3-2v
6-148
7-0
8-1.2

V4:
1-283
2-0
3-1.9
6-160
7-1.7
8-.7

V5: phase splitter
1-190
2-0
3-32
6-185
7-21
8-32

V6, V7 (el34.. voltages match)

3-390
4-383
5- -29
6- 390

[Opry Audio]

Note: made an edit since I left out the phase splitter by mistake

[Pietro]

the stated voltages seem plausible..
But how on earth are you getting 3.3volt on the heaters? Thats about half of what the HV should be.

please re-measure and report back.

[Opry Audio]

the stated voltages seem plausible..
But how on earth are you getting 3.3volt on the heaters? Thats about half of what the HV should be.

please re-measure and report back.

I’m sorry I wasn’t clear in saying that at all— it’s 3.3vAC on each winding. 6.6vAC total on the supply

[nandrewjackson]

I would say all of those HT voltages are about 100VDC too low.


I was always under the impression that Marshall's are at about 500 VDC at the diode rectifier. 


I've never actually had a DMM down inside one, I'm going by what I've heard.