ST-120 tube redplate. Not a bad cathode resistor. what to check?

Last night I noticed the right channel drop on my VTA ST-120. Looked over at the amp and saw the front right Sovtek 6550we redplating.
Something similar happened once around last year, except that time I found a bad cathode resistor.
This time all cathode resistors are measuring fine at 10 ohms.
What else should I check for? What else would cause this? I know it could be a bad tube but it seems really suspicious that it’s the exact same tube location that red plated previously, front right.
I plan to open it up and look for any loose connections, bad soldering, or what have you. But while I’m in there, is there anything else I can check?
I’d really love to take steps that prevent this from happening again. Tubes aren’t cheap! I hear stories about amps like this running for decades without incident. How do I get mine to be bulletproof like that?

Here’s a link to my previous post from when a similar thing happened: https://dynacotubeaudio.forumotion.com/t4888-vta-st-120-power-tube-redplate-bias-reading-25v

you can swap that tube with another one to see if the problem follows the tube (which is then obviously bad) or if the problem stays with the socket, which then indicates a bad connection somewhere on that socket.

tubes4hifi wrote:you can swap that tube with another one to see if the problem follows the tube (which is then obviously bad) or if the problem stays with the socket, which then indicates a bad connection somewhere on that socket.

Good idea. I am trying that now. So far, the amp is working without incident and all bias readings are stable.

ChetJettison wrote:

tubes4hifi wrote:you can swap that tube with another one to see if the problem follows the tube (which is then obviously bad) or if the problem stays with the socket, which then indicates a bad connection somewhere on that socket.

Good idea. I am trying that now. So far, the amp is working without incident and all bias readings are stable.

A tube that has "redplated" is no longer reliable. Throw it away !
Get a matched set from another vendor ( one that offers warranty )

peterh wrote:

A tube that has "redplated" is no longer reliable. Throw it away !
Get a matched set from another vendor ( one that offers warranty )

I don’t intend on using it long term, I have two sets of quad matched tubes as replacement, but I thought it might be useful for diagnostic purposes as suggested by @tubes4hifi above. Would you agree?

ChetJettison wrote:

peterh wrote:

A tube that has "redplated" is no longer reliable. Throw it away !
Get a matched set from another vendor ( one that offers warranty )

I don’t intend on using it long term, I have two sets of quad matched tubes as replacement, but I thought it might be useful for diagnostic purposes as suggested by @tubes4hifi above. Would you agree?

No.
Install new tubes. The problem is very much a tube problem. Don't risk the amp
by using marginal tubes.