thumping, hum, and distortion from one tube

[denti alligator]

I've  been running my Stereomour II without problems for a couple-few years now. It's stock + shunt regular upgrade. Today I began hearing a horrible humming that also began thumping. I switched the 2A3 and it switched channels. It's also distorting the sound, as if there were some kind of bad interference. I assume I have a bad tube. Could it be something else? Any other trouble shooting I should do before getting a new tube?

If it is the tube (likely): do I purchase a matching pair to replace them or is it OK to get just one? Can I buy one from you guys at BH?

 

[denti alligator]

This morning the channel is totally dead. Switching 2A3 tubes and the dead channel follows. So I have a dead tube.

I ordered a matched pair of Shuguang 2A3Cs. We'll see how those sound.