BIAS problem on one tube (KT90)

My ST-120 has been unused since april 2023..... I started it up a few weeks ago and it sounded perfect ....HOWEVER, when I checked the bias, 3 tubes (KT90) had perfect bias of 0,5 VDC....BUT, the right back tube maxed my MM totally out.... even at 10VDC scale....I didn't have the nerve to try the 50 VDC scale.... Where do I start ?

MarcvBelgium

Had the same issue today with a set of 6550’s. 3 out of 4 were at .476. The right rear was at 375.1mv. Then my 5AR4 melted down before I could swap tubes to see if it was the 6550 or something else. I posted about it today. I’m guessing the quad cap went, then the 6550, then the rectifier. Going to order the dual rectifier board. Hopefully the will stop this from happening again.

check to make sure the 10 ohm bias resistor has a good connection and still reads 10 ohms

Update :    Finally I got to it..... Cleaned all sockets (including the front ones)+ tube pins with DEOX.....changed tubes front/rear to exclude tube failure. Same result... 0.5 VDC for 3 tubes but the right channel rear tube measures no less than 8.2 VDC (!!) with the bias pot completely closed..... Since I am unable (due to handicap) to take the amp out of its cabinet I have not yet checked the 10 Ohm resistor. My tech (50+ years experience with tube amps) visits on wednesday....first thing, we will check the resistor, if that's ok he will take the amp to his "lab" and repair it..... I'll give an update when I know more.....

BTW, it is NOT that there is NO bias, there is way too much (8.2 VDC)

Following

Any thoughts or advice ?

Thanks for the suggestion Bob, I'll have my tech look into that.

My tech's comment :
Even before I checked the amplifier and took a good look at the plan, I already saw what was wrong, i.e. the cathode resistor has failed. After replacing it with a 2 Watt Rosethal resistor, it plays like new again. Total power wow....2 x 75Watt!!!! Those tubes pull that off without any effort. (KT90 EH)

All is well again