Any technological minded folks around? I've been fighting a redplating EL-34 in the outer socket of my Mark IVb. It began only redplating when I would switch modes (either pentode/ triode, or Class A/ Simulclass), but now it redplates any time the Amp is on. I have tried swapping the outside EL-34s, and the same socket/ position red plated, and also tried putting in all 6L6's, and again the same position red plated, so it isn't a single faulty tube. I've checked voltages in several places in the amp, and nothing is reading abnormal. Pin 3 (the anode/ plate pin) on the redplating socket was at 440V in both pentode and triode, while the same pin on the other socket varied from 450V to 454V in triode/ pentode modes. Inside, no components are obviously broken/fried/burnt, but I'm not trusting any piece.
So my question is: where should I look for faulty pieces? I have a copy of the schematic and a DMM, and have checked voltages safely thus far (using a wooden skewer taped to the test probe to prevent any shocks, and the ground probe clipped to the amp's neutral). Reading a few posts on this site I found by searching, redplating is caused by an overvoltage in the bias supply (?), but I haven't found a source of the overvoltage. Additionally, the tube does not redplate in Tweed power mode. V on pin 3 for that mode is 410 for both EL34 sockets.
Thanks for any help offered. If I can't fix it DIY, I will probably have a local tech look at it, but I feel this repair should be well within my capabilities.
So my question is: where should I look for faulty pieces? I have a copy of the schematic and a DMM, and have checked voltages safely thus far (using a wooden skewer taped to the test probe to prevent any shocks, and the ground probe clipped to the amp's neutral). Reading a few posts on this site I found by searching, redplating is caused by an overvoltage in the bias supply (?), but I haven't found a source of the overvoltage. Additionally, the tube does not redplate in Tweed power mode. V on pin 3 for that mode is 410 for both EL34 sockets.
Thanks for any help offered. If I can't fix it DIY, I will probably have a local tech look at it, but I feel this repair should be well within my capabilities.