[AMRadio] Amp / Tube question

John Lawson 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 
> 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

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