Electro Harmonix 12 at's are particularly bad for being microphonic right out of the box.
I needed a replacement for the phase inverter for a customer..I went through 8 or 10 that I had in stock before I gave up and put a 40 year old RCA 12 at in as a temporary replacement.
Four years later, the owner is still running that tube and is happy leaving it as is.
As mentioned, warm the amp up, volume up..guitar plugged in but volume off.
Hold the base of each tube and give them a gentle flick with your finger..the output tubes can be noisy (more of a rattle than chime) just as much as any of the pre amp tubes and again as mentioned, the first tube in the gain stage is prone to being the loudest for microphonic chime.
