BillinSF
100+ Head-Fier
Sorry to hear about that hum. The WA22 is a fantastic amp, but it is sensitive to poor tubes. It took me quite a few months to find a complete set of tubes that worked perfectly in this amp. Now I have quite a few good sets, and the amp produces great sound with a completely black background. My recommendation is to buy another ECC35 tube. Try and find one that has the same construction as your "good" one.
I don't think it's damaging the amp - just yours and mine OCD.
Leftside, I appreciate that advice and your knowledge and will replace the bad one. More than anything else I'm mad at myself for buying on eBay when there are established enterprises selling the same thing. Yeah, a bit of OCD since the tubes do sound great.