Marshall JCM800 4010 Needs Sound Help

[misterc57]

This one uses (3) 12AX7 preamp and (2) 6550 output tubes.

I just replaced the three large can caps that were all bulging on my JCM800 combo.

The amp does not sound right. Notes sound a bit crackly. Seems like it sounded better with the old caps.

I checked for DC at the output with the amp at idle, there was none.

I checked all tube voltages. Output tubes seem right but on the preamp tubes I question the plate voltages (seem high), when I compare to a reference chart.

V1 Pin 1 = 275 vdc
V1 Pin 2 = 0 vdc
V1 Pin 3 = 3 vdc
V1 Pin 6 = 230 vdc
V1 Pin 7 = 0 vdc

V2 Pin 1 = 126 vdc
V2 Pin 2 = 1.4 vdc
V2 Pin 3 = 1.5 vdc
V2 Pin 6 = 320 vdc
V2 Pin 7 = 126 vdc

V3 Pin 1 = 251 vdc
V3 Pin 2 = 30 vdc
V3 Pin 3 = 50 vdc
V3 Pin 6 = 239 vdc
V3 Pin 7 = 50 vdc

V4 Pin 3 = 433 vdc
V4 Pin 4 = 431.5 vdc
V4 Pin 5 = -52 vdc
V4 Pin 6 = 434 vdc

V5 Pin 3 = 433 vdc
V5 Pin 4 = 431 vdc
V5 Pin 5 = -52 vdc
V5 Pin 6 = 433 vdc

Are my preamp plates to high? Thoughts on what to try next?

Where can I find a board reference (that shows which component is which compared to the schematic)?

I am attaching the schematic that I have (this one shows EL34 output tubes)

Thank you.

Marshall-JCM800-50W-4010-Schematic.pdf

[alexradium]

This one uses (3) 12AX7 preamp and (2) 6550 output tubes.

I just replaced the three large can caps that were all bulging on my JCM800 combo.

The amp does not sound right. Notes sound a bit crackly. Seems like it sounded better with the old caps.

I checked for DC at the output with the amp at idle, there was none.

I checked all tube voltages. Output tubes seem right but on the preamp tubes I question the plate voltages (seem high), when I compare to a reference chart.

V1 Pin 1 = 275 vdc
V1 Pin 2 = 0 vdc
V1 Pin 3 = 3 vdc
V1 Pin 6 = 230 vdc
V1 Pin 7 = 0 vdc

V2 Pin 1 = 126 vdc
V2 Pin 2 = 1.4 vdc
V2 Pin 3 = 1.5 vdc
V2 Pin 6 = 320 vdc
V2 Pin 7 = 126 vdc

V3 Pin 1 = 251 vdc
V3 Pin 2 = 30 vdc
V3 Pin 3 = 50 vdc
V3 Pin 6 = 239 vdc
V3 Pin 7 = 50 vdc

V4 Pin 3 = 433 vdc
V4 Pin 4 = 431.5 vdc
V4 Pin 5 = -52 vdc
V4 Pin 6 = 434 vdc

V5 Pin 3 = 433 vdc
V5 Pin 4 = 431 vdc
V5 Pin 5 = -52 vdc
V5 Pin 6 = 433 vdc

Are my preamp plates to high? Thoughts on what to try next?

Where can I find a board reference (that shows which component is which compared to the schematic)?

I am attaching the schematic that I have (this one shows EL34 output tubes)

Thank you.

[ATTACH]45550[/ATTACH]

pin2 of V2 shouldn't have voltage,either c7 is leaking or V2 has grid emission/defect
report all the pin 8 voltages

[misterc57]

Woops, updated voltages;

pins 4&5 on V1, V2, V3 are 3.15 AC to ground

V1 Pin 1 = 275 vdc
V1 Pin 2 = 0 vdc
V1 Pin 3 = 3 vdc
V1 Pin 6 = 230 vdc
V1 Pin 7 = 0 vdc
V1 Pin 8 = 2.07 vdc

V2 Pin 1 = 126 vdc
V2 Pin 2 = 1.4 vdc
V2 Pin 3 = 1.5 vdc
V2 Pin 6 = 320 vdc
V2 Pin 7 = 126 vdc
V2 Pin 8 = 129 vdc

V3 Pin 1 = 251 vdc
V3 Pin 2 = 30 vdc
V3 Pin 3 = 50 vdc
V3 Pin 6 = 239 vdc
V3 Pin 7 = 50 vdc
V3 Pin 8 = 50 vdc

V4 Pin 3 = 433 vdc
V4 Pin 4 = 431.5 vdc
V4 Pin 5 = -52 vdc
V4 Pin 6 = 434 vdc

V5 Pin 3 = 433 vdc
V5 Pin 4 = 431 vdc
V5 Pin 5 = -52 vdc
V5 Pin 6 = 433 vdc

[misterc57]

I swapped tubes at V1 and V2. No more voltage at V2 pin 2, and there was no voltage at V1 pin 2. Now the amp sounds fine!

[g1]

Clean the sockets. Swap the tubes back. If it's all fine you are good, unless the tube or coupling cap has an intermittent fault.