JCM800 1959SL Bias Issue
Posted: Mon Aug 02, 2021 11:43 pm
Hey, new to the forum,
I have a JCM800 1959SL by the tag on the chassis made around 10 month of 88 that's having some bias issues. I replace the tubes and one of the power tubes will not settle down. With the bias resistor set completely cold three of the tubes will idle around 25ma and stable the other will be around 38ma. The one odd ball tube will then start to climb slowly and continue to run away if I don't kill the power to it.. At first I was thinking bad tube until I move it to another socket and it then became stable at around 25ma. I have tested all components on the socket and it all checks out. Guess my next move will be replace the tube sockets and give her a try again. I'll go on and replace them all if 1 socket has fail then sure the rest are not far be hide. Just wondering if any one has seen this before and many thanks to any comments or suggestions.
I have a JCM800 1959SL by the tag on the chassis made around 10 month of 88 that's having some bias issues. I replace the tubes and one of the power tubes will not settle down. With the bias resistor set completely cold three of the tubes will idle around 25ma and stable the other will be around 38ma. The one odd ball tube will then start to climb slowly and continue to run away if I don't kill the power to it.. At first I was thinking bad tube until I move it to another socket and it then became stable at around 25ma. I have tested all components on the socket and it all checks out. Guess my next move will be replace the tube sockets and give her a try again. I'll go on and replace them all if 1 socket has fail then sure the rest are not far be hide. Just wondering if any one has seen this before and many thanks to any comments or suggestions.