I bought a couple NOS RCA 12AX7s recently and one is too microphonic to use in my stereo preamp. Was going to put it in the guitar amp just for fun to see what the feedback is like. Is this tube good for anything else or should I junk it or exchange it?

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Junk. I got a NOS 12ax7 from Tube Depot that was too. I sent it back and they sent a solid one that sounded really good. https://www.tubedepot.com/products/jan-ge-12ax7wa (It was on sale for 50 bucks).

The Tube Store has their own line of chinese tubes now and based on admittedly limited experience I think they are good stuff. https://www.thetubestore.com/tube-brands/preferred-series