Sorry, I was a bit tired when I replied.... Pi (there are 2 of them, it's a stereo amp) are V2 and V3. I don't know which tube it is on the amp, but you can try this :
The PI are probably the tubes closer to the power tubes. If it's not clear enough, remove one tube (amp of) and test. If only one channel isn't working, it's a PI, if the 2 channels are muted, then it's V1 (input tube for both channels).