Uneven plate voltages? - now a Marshall troubleshooting thread

[Yoda]

So I have a JCM 900 2100 I just retubed and while checking plate voltages of the EL34s three of them were 463v while one was at 446v. What could cause such a discrepancy?

Thanks in advance.

[sluckey]

Bad connection? Two plates are directly connected together for each pair of tubes. The only way for voltage to be different for only one tube would have to be a bad connection to the tube on the other end of the wire.

Or it could be operator error. Check again.

[xtian]

Tubes being equal, it could also be a leaking coupling cap, allowing DC on the grid, don't you think?

[pdf64]

Probing power valve anodes, especially true pentodes, can invoke oscillation, thereby making a nonsense of the meter reading.
Consider why you need to measure anode voltage? eg what is that telling you further to the HT voltage?
The same issue crops up repeatedly, probably because everyone with a meter feels duty bound to measure it.
Whatever, if it really needs doing, then remove the LTP valve first.

[sluckey]

Tubes being equal, it could also be a leaking coupling cap, allowing DC on the grid, don't you think?

No, I don't. Grid voltage is not a factor. The plates are connected together. Probably with a 2 or 3 inch wire. How can you measure 463v at one end of that wire and measure 446v at the other end of that wire?

[xtian]

Tubes being equal, it could also be a leaking coupling cap, allowing DC on the grid, don't you think?

No, I don't. Grid voltage is not a factor. The plates are connected together. Probably with a 2 or 3 inch wire. How can you measure 463v at one end of that wire and measure 446v at the other end of that wire?

I am forced, forced, I tell you, to agree.

[Reeltarded]

WAG: is it the tube next to the PI?

[Yoda]

I will do further investigation such as seeing if the lower plate voltage follows the one tube but for now I can say it wasn’t the tube next to the PI.

[Yoda]

This amp is starting to blow fuses again, only when taken off standby and I start playing it for a minute then volume loss and blown HT fuse.

I’m looking at replacing filter cap cans next. Looks like I need to tackle an over current issue before I can get to those plate voltages.

[Yoda]

Now this thing blows fuses regardless of switch positions so I will try again after re-capping the power supply. The original cap cans are 30 years old so they have to go.

[Yoda]

It is tube V6 (if counting first preamp tube as V1) that shows a lower plate voltage as well as causing the over-current condition. If I remove V6 the light bulb dimmer dims significantly and it is the only tube that affects the current pull so much so I suspect something is awry with the circuitry for V6. Current draw is lowest when I leave in only the outer pair of power tubes (V4 and V7).

[Yoda]

So the full volume of the amp comes back if I pull the inner two power tubes (V5 and V6) and the overcurrent situation stops so I believe I’ve narrowed it down to something affecting the center two output tubes.

I pulled the power amp board for a visual check and I tested the screen resistors and they’re still OK and nothing looks charred.

Any help is appreciated.

[Reeltarded]

Still here still thinking

picture of those sockets from both sides?

[Yoda]

I will post pics of the sockets as soon as I can. I think for now the main culprit is the filter caps being bad and I have new ones on the way. There doesn’t seem to be enough high voltage for any volume and the notes are “fuzzy“ even at clean settings and I’m only measuring 198v on each filter cap section. New filter cap cans should solve a few of these problems.

[pdf64]

Only 198VDC at the reservoir caps, which connect to the rectifier?
Are you sure your meter is ok, eg tried a new battery?
Is this the schematic? https://el34world.com/charts/Schematics ... w_2100.pdf