|[AMRadio] Amp / Tube question|
jpl15 at panix.com
Wed Nov 23 23:35:50 EST 2005
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
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
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!
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