My amp did the same thing so dropped in the GE 5AR4 from my Heathkit AA-100 (Probably an 50 year old tube) and the system came back up. I did find that the bias changed some so I dialed it back in but where all the pots used to be at the 12:00 position now the pots for V7 and V3 were a little past the 12:00 position. The amp ran fine for about another week but then it blew the fuse again a few days ago after about 10 minutes of listening at a pretty low level. I replaced the fuse and powered it back up but the fuse popped as soon as the TDR kicked over and applied power the high voltage to the rectifier (the rectifier also looked to have some arcing inside as it happened). I placed an order for a WZ68 as well as a couple new Ruby 5AR4's then today, on a whim, I put the GE rectifier back into the Heathkit and the amp played fine. I then put the Ruby 5AR4 in the Heathkit and it also played fine. I guess my question is does the VTA-120 push the limits of the 5AR4 and is that why the WZ68 is the recommended solid state replacement of might I have other issues (bad output tube possibly?)
I plan on powering it up with the Ruby and the driver tubes sometime today and seeing if it stays on, then adding the left channel output tubes or the right channel output tubes just to see what happens but I was curious if others had seen this issue using the 5AR4.