JCM800 Bias Help

[jakeru19]

Hi everyone,

I am attempting to bias my 1983 JCM800 4104 combo for the first time. I set my multimeter to DC mA and connected it to a bias probe, which I placed between one of the EL34's and the tube socket. I turned on the meter and powered up the amp. So far so good.

The multimeter reads between 75-80 mA depending on the particular tube and socket. This seems far too high; I am under the impression it should be about 35mA. Furthermore, the bias trim pot seems nonresponsive, i.e. when I turn it nothing happens. What am I doing wrong, or is my bias knob or some other part broken? Why is the bias so high?

Most appreciatively,

Jake

[52 Bill]

I set my multimeter to DC mA and connected it to a bias probe, which I placed between one of the EL34's and the tube socket. I turned on the meter and powered up the amp. So far so good.

Welcome to the place Jake. Most bias probes insert a 1 ohm resistor into the cathode circuit, does yours? If it does then, you need to set your meter to read DC volts. The DC voltage in mV on the meter display will equal DC mA current in the cathode circuit of the power tube.

[Alan0354]

Assume everything is right according to 52 Bill, if you read 75 to 80mV, that's too high.