Hum/redplating Xits X10

[goldenGeek]

It seem slike everyone drops off amps with weird problems at my place these days

Anyway,I have been trying for a few days to diagnose this Xits X10 (I believe it is loosely based on Vox/Matchless or something similar dual EL84/cathode bias). The owner complained that it had changed sound and sounded a bit flat. I measuried the tubes and the EL84s turned out to be really worn, so I bought two new new ones. When I installed the new tubes I noticed that they we're red-plating (or at least it seemed that way), and I have tried multiple other tubes, they all turn really red after a while. There is also a noticeable loud hum (50/60-Hz) before it gets good and warm, then the hum is attenuated pretty well... but then the red-plating appears after a bit more time. I have tried to replace the cathode bypass cap and resistor to make it run cooler, originally 120R/50uf, with 270R/50uf, but there is no change. Does anybody have a clue where to continue to search? By the way, the plate voltage is about 304V before the resistor, 296V after.

I have fixed a few odd ones with your help in the past so I keep my fingers crossed

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[Stevem]

This is not a weird problem, as if you’ve worked on enough amps it’s common in older amps,especially with paper & foil type caps!
Try this test, with the amp on and both output tubes yanked out do you measure any DC voltage on Pin 2 of each tube, if so then the PI out coupling caps are bad and leaking, if not then I will bet ya that the output transformer is now bad.

Also note that it does not take long for even a minor red plate condition to greatly weaken a tube and if nothing else make the tube far more likely to go micophonic and then go on to short out.

[goldenGeek]

This is not a weird problem, as if you’ve worked on enough amps it’s common in older amps,especially with paper & foil type caps!
Try this test, with the amp on and both output tubes yanked out do you measure any DC voltage on Pin 2 of each tube, if so then the PI out coupling caps are bad and leaking, if not then I will bet ya that the output transformer is now bad.

Also note that it does not take long for even a minor red plate condition to greatly weaken a tube and if nothing else make the tube far more likely to go micophonic and then go on to short out.

Thank you @Stevem I did not measure any DC on pin 2, but I noticed that there was a short between pin 3 and 4 (cathode and heater) when I looked at it closely at one of the tubes... It seems to be working fine when that short is removed.