Marshall 9100 One side out

[Mr_bibbles]

Hello all,

https://www.thetubestore.com/lib/the...-Schematic.pdf

I am having a lot of trouble with the Marshall 9100 dual 50W 6L6 power amplifier. One side ("A") is producing no sound. I have traced the signal with an audio probe to V2A grid (Phase inverter), where it disappears.

v2A
1. 270VDC
2. 0VDC
3. 3VDC

6. 279VDC
7. 0VDC
8. 5VDC

Here are the voltages from the WORKING side as a comparison

1. 115VDC
2. 0VDC
3. 1VDC

6. 115VDC
7. 0 VDC
8. .6VDC

I have read online through all the 9100 threads, it seems like there is an issue with the fuses that cause a loss of bias. I have bias voltage on the power tubes, and have swapped TR2 with a known good transistor, no change. I've checked the dual diodes as well and the surrounding resistors, and have metered over the little black fuses (they check OK)

Anyone have any idea what's going on?

[Helmholtz]

As V2 pins 3 and 8 (cathodes) are jumpered, it's not possible to have different voltages here.

Otherwise it seems V2 is hardly conducting. No wonder with a bias of 3V or more.

Check TR2 and R15.

[Mr_bibbles]

Helmholtz, Thank you for responding! You are correct, cathode reads 5v on both 3 and 8, sorry about the mis-read.

R15 is within spec, (275 OHMS) I swapped TR2 with a known good transistor from the working side B, there is no change, side B works fine with the swapped piece so I can confirm both TR2's are good.

TR2 Voltages in circuit;

SIDE A (not working)

C. 0
B. -.2 VDC
E. 5 VDC


SIDE B (working)

C. .4 1VDC
B. 1.1 VDC
E 1.2 VDC


So because we know TR2 is a working part, this points us to the TR2's supply? IF tr2 is not passing cathode ---> ground then the 12ax7 won't conduct? I will try following those supplies back through r28, does this make c23 a prime suspect?

[Helmholtz]

Seems you mixed up collector and emitter. Collector is connected to tube cathode, so same voltage.
Base must be around 1.2V. If not check/replace 1N4007 diodes.

Your voltage readings are pretty inconsistent. Meter battery good?

[Mr_bibbles]

Seems you mixed up collector and emitter. Collector is connected to tube cathode, so same voltage.
Base must be around 1.2V. If not check/replace 1N4007 diodes.

You voltage readings are pretty inconsistent. Meter battery good?

Whoops! looks like I was reading it off the board, I had a feeling it was wrong from looking at the schematic but trusted the PC board. I'll check the batteries but you actually helped me solve it!

R28 was OL, I now have sound and think I can get the rest. Thank you so much for your help, I have been spinning my wheels with this strange PI but it was a very simple issue of transistor supply.

I am very grateful!