Unbalanced Bias Question

[dorrisant]

I have a Marshall 2150 PowerCell 100w here on the bench. Electrolytics look to be fine. I changed out the screen grid resistors as well as several in the bias supply. The 220k feed resistors (R24 & 25) were a bit out of balance so I replaced them with a matched set of metal films. I also changed R26. I have all new JJ tubes in it and the bias range seems to be just fine. Problem is this thing takes a good half an hour to creep up to full bias... Meaning - the bias starts out at about 22-23 mA per tube and after nearly a half hour it has crept up to approx 39 mA per tube on one side of the OT but the other side levels off at about 27 mA per tube. It doesn't matter if I swap power tubes into any configuration the bias will end up exactly the same. This is a matched & burned in set of power tubes that will bias up evenly in another amp. So the problem doesn't seem to be with the tubes. I did take out C102 & 103 just to see if it would change anything but it did not. What am I missing? I have removed C14 & 15 for bench testing to see if they are leaky... Will report the results shortly.

[Phil_S]

I don't know much...have you changed the bias supply cap(s)? Have you disconnected the OT to run low voltage on it to see if both sides of the secondar(ies) are putting out the same voltage? I'm thinking this is two separate problems. The cap problem was caused by a bad OT. Curious, how does it sound when it runs up to full bias current?

[tubeswell]

Did you check to see if it was something else upsetting the tube current on the errant socket?

like a leaky coupling cap after one side of the PI plates upsetting the output tube bias?

or a blown screen resistor on one side?