Have a JCM 800 2203 (100 watt 4 EL34) and just finished some work on it. New filter caps, bias supply caps and set of matched JJ EL34. When I setup the amps bias on idle it has 25 milliamps on each tube with 530VDC on plates and 525 on the screens. When I run a test signal I get one half of the OT (brown lead) doing 80ma on each tube and the other half (red) doing 50ma on each tube. If I swap the tubes the current draw stays with the OT leads. If I measure the resistance across the plates (tubes out) to OT CT I get 14.5 ohms Brown and 16 ohms Red.
I could swap bias supplys to each half of the OT which will prove the last two resistors in the bias supply (one for each half but they measure 220k each so OK) and the bias is good at idle -44vdc on each side, under load it becomes -41vdc Brown -43vdc Red.
Is it possible the OT has an imbalance or fault that is causing this issue? Thanks in Advance Joe
http://www.drtube.com/schematics/marshall/jcm800pw.gif
I could swap bias supplys to each half of the OT which will prove the last two resistors in the bias supply (one for each half but they measure 220k each so OK) and the bias is good at idle -44vdc on each side, under load it becomes -41vdc Brown -43vdc Red.
Is it possible the OT has an imbalance or fault that is causing this issue? Thanks in Advance Joe
http://www.drtube.com/schematics/marshall/jcm800pw.gif
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