Marshall 6100 Bias Drift

[MS6100]

Hej Guys,
i got a problem on my Marshall 6100LM from '95. I replaced the tubes with 6L6GC-STR and tried to get the bias correct. I measrue with a bias-master. In standby everything is fine i meassure -50V. If I turn on the standby the current is high 47mA and the voltages drops slowly after 15-20 minutes with volume down I read -49V Volts and 58mA. So my bias drifts away. Any suggestions?

[jeffsmusic4919]

First you should read this;
http://home.myfairpoint.net/vze1u14s/pe ... /id18.html

Secondly, I am not a amp tech. Please be careful in there. If you aren't sure what you are doing take it somewhere. If I were you, I would wait for someone more qualified. From what I have learned from the good people here, I will give some guidance though. After 15 minutes the tubes should be at optimal operating temps by then. That is when you measure the current draw. If the tubes still drift after the initial warm up, you then have a problem. That is just the inherent nature of power tubes, they will drift till warm up.. Just watch for red plates while you warm up. Be sure to base your bias settings on plate voltage. I have the same tubes, as far as I know they handle the voltage this amp has just fine. I hope this helps.