Marshall jcm 900 SL-X 2100 Preamp Problem

[hadley101]

Got a personal Marshall JCM 900 SLX 2100 with Grid and Cathode Down
on V1, V2 & V3

Replaced BR3 when purchased few months ago as it was cooked
and all Electrolytic & Filter Capacitors

It's been running beautifully for the past few months as just started
to sound poor

Ran some voltages and checked all the Gird and Cathode resistors to the
best of my limited knowledge

Really do appreciate any and all help

First the tubes

V1

1 197V
2 0
3 1.9V
4 7.1V
5 7.1V
6 284V
7 197V
8 188V

V2

1 299V
2 0
3 2.9V
4 7V
5 7V
6 183V
7 0
8 1.9V

V3

1 198V
2 0
3 1.8V
4 7V
5 7V
6 319V
7 198V
8 188V

R3 & R6 23.2 Ohms

Chassis Ground to Chassis 1 ohm

This amp has a ground setup I'm not familiar with nor quite
understand

Have a quad of TAD 6L6GC Biased

http://www.drtube.com/schematics/mar...slxa-61-02.gif

http://www.drtube.com/schematics/mar...2100-60-02.gif

http://www.drtube.com/schematics/mar...2100-60-58.gif

[Pedro Vecino]

Voltages are good. Rather high compared to a 50 watt version I have here but does not indicate a problem. I have 6.0V in heathers and 430V in the plates with the amp biased (32mA/tube).
You have not described the voltages of the fourth 12AX7 (driver). They should be something like this (somewhat higher probably, but maintaining the relationship):

226
33
36
-
-
201
22
36
-

You need to explain in more detail what is "poor". In an amplifier like this (without clean sound) it can be more difficult to interpret.

[hadley101]

Last night using some wooden chopsticks found one of the
plates for the PI(V4) needed to be replaced, R23

R23 100k, the outside pair of tubes V7&V8 was flashing
a bit brighter blue than first pair V5&V6

The best way to describe the tone problem was the notes
wouldn't sustain, dead flat and the Bass was breaking up.

Guess could say the Bass frequencies were flubby.

Sounded very anemic,weak and thin

Sorry, haven't gotten to purchasing a oscilloscope yet,
plan on for this kind of work. Know it's needed very much
and can't give a proper description of what the signal
is doing.

Haven't had a chance to test it since changing R23, it was
split in half.

Changed all the preamp 12AX7's with known quality
before changing R23

Here is voltages for

V4
1 297V
2 20V
3 41V
4 3.9V
5 3.9V
6 288v
7 14V
8 41V
9 3.9V

Thanks for helping very much!

Thought the voltages might be off as this problem
was posted @ Marshall forum and someone there
was helping me, didn't tell me those were in the range.

Will post back results later today after work

[hadley101]

Pedro,

Seems replacing the PI plate resistor corrected
the amp and it sounds good again.

Thank you so much for letting me know the
preamp voltages were normal

[Pedro Vecino]

Good news. I'm glad.
Greetings.