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Old 27th November 2008, 07:25 PM   #1
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Default Bad vacuum?

Just wondering if my suspicions about this tube having a bad vacuum are correct. Its a small power pentode and the heater will light up for a second or two and then go out. The heater is still drawing some current and the tube gets warmer than the others with no plate voltage applied. There are no visible cracks on the glass and the getter is still silver.

Sound like a dud?
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Old 27th November 2008, 07:33 PM   #2
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Sounds pretty odd actually, but possible. Perhaps see if applying plate voltage results in the tell-tale gassy glow seen in some pictures that tubelab has posted in the past.. (Don't do this in an amp obviously..)
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Old 28th November 2008, 02:32 AM   #3
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Ok heres what happened. I wired up a 350 ohm cathode resistor and 4.7k on the screen grid, 100k from g1 to ground. Nothing happened until about 300 volts. Then brilliant blue/purple flashes and the current meter pegged on my IP32 power supply Guess its fate is sealed.

That will teach me to not buy spare tubes for projects.

Guess its time for a
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Old 28th November 2008, 08:45 AM   #4
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Quote:
Originally posted by astouffer
Then brilliant blue/purple flashes ...
Bad on power supplies, but absolutely beautiful to watch

Maybe a current limited supply and make a "lightning globe" with it?

Cheers!
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Old 28th November 2008, 11:55 AM   #5
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Default Re: Bad vacuum?

Quote:
Originally posted by astouffer
Just wondering if my suspicions about this tube having a bad vacuum are correct...etc. ...There are no visible cracks on the glass and the getter is still silver.

Sound like a dud?

Bit of a contradiction here.... Some NOS heaters did flare up on switch-on but that's normal.
Bad vacuum usually coincides with a fouled getter gone/ing white. You may not see a crack in the tube base near pins but it needs only a slightest sleek for air entry.
My guess is that the screen grid is gone duff or that g3 is nearly s/c on anode..The Ei KT90's were very prone to this.
I always buy spare tubes but from acquainted reputable sources.

Buying a quickie from ex-ebay has it's risks and as times are getting harder.... I always keep a good stock of the ones used....(the way things are going we have to buy up to keep dealers in business !)

richj
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