[AMRadio] Amp / Tube question

Patrick Jankowiak recycler at swbell.net
Wed Nov 23 19:58:14 EST 2005

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.

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 


> From: John Lawson <jpl15 at panix.com>
> Subject: [AMRadio] Amp / Tube question
>    Shut it off, put on the plate cap, hit the power sw - BAM! as Emeril 
> might say. A large blue flash went off in the tube and I welded the line 
> fuse to it's cap.  And smoked the meter dropping resistor - though it's 
> been replaced before, I see.

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