Talk me down here...overvoltage in a metal shell

[johnnyreece]

Hey, all! I've got a project going on in another thread, but I thought I'd post here, as it's a more general topic, and I'm hoping to spur a little discussion. I'm thinking about trying something that may be a bad idea: I'm wanting to run a 6J5 above max plate voltage. Like, potentialy around 340 when the max is 300. Here's the thread:

https://tubeamparchive.com/viewtopic.php?t=26380

I'm a touch worried about this, as with the 6J5's metal shell, I can't see if it's redplating, or doing anything else interesting that it shouldn't do. I may just be being a sissy (queue Hans and Franz), but I'd like to get some opinions before going too far into this. I could attempt to drop the B+, but then I'll need to get into dropping string changes (which I'm not too good for, but I'd rather build something as close to the schematic as possible before making changes).

There's tons of anecdotal evidence of people doing this with other tubes (I'm looking at you, EL84), but not much on this particular oddball tube. The Google Machine has failed at providing an answer on this one. Anyone have any thoughts?

[David Root]

Maximum plate dissipation is 2.5W, so theoretically at 340Vp if you don't exceed 7mA you're OK.

I say theoretically because the metal jacketed tubes are known to overheat and fail more often compared with GT glass versions.

I would be inclined to try it at 340V and up to 5ma (Pd=1.7W).

Interestingly the Tung-Sol datasheet shows grid voltage vs plate current curves at up to 400Vp. Scroll down to the last graph.

I've not tried this, just looking at those curves, but I doubt Tung-Sol would have published that graph if they had no confidence in it.

[johnnyreece]

Thanks for the reassurance! I'll just go ahead and bias it up at 70% and see what happens. I think that was what I decided to do originally, but while waiting for back ordered parts, the mind wanders towards "what if". I've got spares, and parts on hand for the MOSFET B+ reduction, so I'll give it a go, and if it blows...well, back to the drawing board!