On Wed, 23 Nov 2005, Patrick Jankowiak wrote:
Probably a bad tube, any 3-500Z should be fine in there. Sometimes, an abused one will have a deformed grid from overdriving, and it's too close to the plate, or one of the grid wires is broken and sticking out towards the plate, and it can arc. Or the tube could have some gas in it, causing an arc. Had this happen with a few old used tubes before. I even blew a fuse on a bias supply with a 304TH that had a grid-filament short once.
That is likely what is happening - it was from the estate of a silent key by way of a friend of mine - I saw it a his place right after he'd unloaded it - I made him an offer he couldn't refuse (traded it for a small debt) - but it was as-is... I've checked the tube for gross shorts, none - the fils work brightly so I don't thing the tube's very gassy, if it is. But as sson as the B+ hits the plate it shorts internally.
Right now I'm scouting for a new 3-500Z - NIB, NOS, or Known Good Pull.
If you are worried, you may try next time to put perhaps a 2 to 5K/100 to 200 watt resistor in series with the plate of the tube and the plate cap of the amplifier, just to check, for a test only. You can lay the resistor on a piece of wood or other insulating material, and use 10KV test probe wire, or just route the leads so they are away from other things. This is very dangerous and must be done with great caution of course, and only for a few moments. -but then the arc in a bad tube will only be pretty, and not destructive (hopefully). -and you can just shut off the amp if bad things happen.
Maybe if I also run a small set of Jacob's Lddder wires up from the tube, when it shorts I can shut the lights off and laugh evilly whilst exclaiming "It's ALIVE!!! It's ALIIIIVE!!!"
Damn electronics anyway. O well - need to get the Ranger and the Valiant working - hopefully this weekend.
Cheers, thanks, and Happy Thanksgiving! John KB6SCO
