Most probable reason for screen resistor burning

[catalin gramada]

Hello,
I recorded a issue to one of my power tube from a new 6CA7 matched set. At start up one of the screen resistor it start to burn. It is happen just at first startup after the amp stayed off for a while. Changing the resistor and started again it running well. The episode was happen twice just for same tube. I didn't notice what is happen to the plate as I didn't saw it ( there are on other side of chassis and I was focused on screen resistor to cut the supply off). It was happen just at that tube. The quad is jj 6ca7, there are matched pretty well, 1.9k OT , 490V, individual 1k/5w screen grid resistor, shared 220k grid leaks bias resistors, biased at -44V , 60% arround. After changed the screen resistor and started again I measured the tube at iddle and full throtle and compared with the rest of tubes in circuit it work the same, nothing drift. But the episode is happen again later. Starting burning, changed screen resistor and next startup works normally. I didn't do nothing as swapping tube or whatever to see if the tube is bad. I have to check but no possibility just now. So I ask for You opinion what do You thing can be? Bad tube? Heavy osscilation perhaps? I just thinking, the tube with problem sitting close to OT transformer, first in row, if is matter.
As You see is not a thing I can control as the problem is not manifest constantly is very hard to see what can be bad if I do the measurements into a circuit which is working..Any thoughts ?. Thanks

[Helmholtz]

As you have 2 tubes wired in parallel, most reasons I can think of would affect both tubes.
Especially oscillation of a single tube only is not possible.
Leaves bad socket contact or bad tube with an intermittent internal problem activated by temperature and causing its screen current to increase.

[catalin gramada]

Thanks, I will swap it on other position and hope to confirm the tube is bad.

[J M Fahey]

Only way to be certain.

[catalin gramada]

..and Yes, it was confirmed. It is a bad tube. I swapped it to the other side of the rail. After some successful starts up yesterday, today it was time to smoke its 1k/ 5w screen grid resistor into it's new position. It didn't measure different than the other. It is a new matched quad JJ 6CA7. Some physical issue goes wrong inside the tube I think. I wonder if it was bad crafted or if was damaged during the shipping. As time it was mached in the shop it was lucky enough to pass.
Thanks guys.

....or maybe 490v into the screen grids it is a really a dielectric issue and mostly all other tubes I used (hundreds) goes over specs. I didn't saw nothing arcing but who knows...so I can judge all other power tubes I used till now goes over specs but I just founded one tube stucked with the norm...?

And more confused as found on its sticker on box it was tested with 487V into grid...

Anyhow, switched off/ on, is start up well...

[alexradium]

anywhere over 400V on screens is too much and won't give more power,plus you have even more differential between pin 4 and 5.

[catalin gramada]

Those screen draw (sometimes) current at startup...a lot, why ?
I asked just to share opinions about possible scenarios by the other techs experience as for me no one power tubes I used till now didn't manifest this kind of issue, even with grids supplied at 500V arround. Thanks

Of course this tube will be replaced as I have no intention to use it or to fix somehow.