Marshall jcm800 bias issues

[eddiep1966]

Hi all new to the forum,
I have a Marshall JCM800 1959SL on the bench. The plain was a retube, bias, cleaning, MV mod and maybe some gain mods but it wasn’t going to let me get away with that. I noticed one of the tubes was red plating. Check the bias circuit and voltages all checked out. Checked the screen, grid resistors and they were in spec and seem fine. I then began to think well maybe its the old tube. Replace the tubes with the new ones and set the bias trimmer to the cold side and check the current through the tubes. Three of the tubes were running around 25ma and the other was around 36ma. Rechecked Bias circuit, voltages and still all checked out. Moved the high tube to another socket and it ran at 25ma and stable. But this time I noticed the one tube socket that was high the current was climbing very slowly and would not stabilize. The only other thing I can think of would be causing this is a bad tube socket. So the new sockets are order and on there way. Just wondering if any one had seen this or any one have any thoughts or suggestion.
Thanks

[pdf64]

Schematic for this model? There’s a lot of versions.
Maybe its pin 5 control grid stopper, or connection, has gone wonky?
Have you checked for oscillation?

[Stevem]

Since you have swapped tubes and the issue stays with that socket, and if you test and find that the bias voltage is stable on this socket and checking like the other 3 pin 5s do then it can only be the gripper for the tube pin on 5.

Once you get things back to normal I would shoot for a idle bias setting of 33 ma.

[Stevem]

Since you have swapped tubes and the issue stays with that socket, and if you test and find that the bias voltage is stable on this socket and checking like the other 3 pin 5s do then it can only be the gripper for the tube pin on 5.

[eddiep1966]

Thanks for the suggestion Steven and Pdf64. No I don't have a schematic for this amp but the only thing I have seen component deference so far is it has 2.2k screen resistors. All the others I have seen were 1k. Don't know if these were stock or were placed in by someone else. But I agree if not the socket and it must be the grid stopper.

[pdf64]

…it has 2.2k screen resistors. All the others I have seen were 1k. Don't know if these were stock or were placed in by someone else...

In the 90s, Marshall EL34 replacement valve packaging instructed that for pre JCM800 models, 2k2 screen grid resistors should be used.
So it may have been modded in accordance with that, eg as a safety measure.

[eddiep1966]

Well so that explains it then pdf64 that goes with the other info were the amp was retubed by another, dated 96 so I'm sure he added the mod. Thanks the parts will be here today or tomorrow I'll post an update.

[eddiep1966]

Well got the sockets replaced and monitor each one for a few hours. The tubes are running stable. So now maybe move ahead on this thing.
Thanks