Well there's a good possibility that the socket is worn, what with the "tube rolling" mentality***. But it may not be. Can you please report on whether all tubes tried cause the screech. I'm thinking it could be oxide on the tube pins themselves since some of the tubes you've tried pop and others do not. It doesn't come up often, but I've seen it. I had a small bucket full of 12au7 tubes pulled from a Conn organ that sat out in the rain. I had to use one of those brass wire toothbrushes from the hardware store to clean off the pins on most of them to get good contact in any socket. Also, what is the nature of the other tubes you are trying in those sockets? Are they new? NOS? Used but test good? Unknown? Also, it may even be V4 that's microphonic. Granted you are only detecting this issue on the trem channel right now, but I wouldn't rule out the possibility. The screech is very much like some of the worst microphonioc tubes I've heard. You really do need to plug in some known good preamp tubes.
***Tube rolling is an internet term used to describe the practice of trying multiple tubes in any given circuit position to find the best sounding combination and/or order for the tubes available. Many sockets take an unnecessary beating with this practice because time and frustration can lead to aggressive pulling and plugging. This is especially bad for amps that have PCB mounted sockets because it increases the risk of failed pads and traces.
***Tube rolling is an internet term used to describe the practice of trying multiple tubes in any given circuit position to find the best sounding combination and/or order for the tubes available. Many sockets take an unnecessary beating with this practice because time and frustration can lead to aggressive pulling and plugging. This is especially bad for amps that have PCB mounted sockets because it increases the risk of failed pads and traces.
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