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Old 14th May 2011, 03:32 PM   #1
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Default Resurrecting weak tubes

I have a few WE 429A pentodes that appear to be weak. The getter flash on the glass is smaller and darker than the good ones and they also draw very little current. Here is a link to the datasheet www.westernelectric.com/spec_sheets/429A.pdf

Putting 400v on the plate, 150v on g2 and then 0v on g1 the tubes draw 25mA! Not even close to where they should be. I found that if g2 goes over 150v it really starts to glow brightly. G2 is being supplied by a simple pot and over time I did notice the voltage creeping up VERY slowly.

So the question is do they have the potential to be resurrected or should I send them to George for further "testing"
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Old 14th May 2011, 07:28 PM   #2
DF96 is offline DF96  England
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Low current means the cathode is either worn out or poisoned. I believe you can sometimes get an improvement by running the heater very hot for a short time. The idea is that emissive atoms migrate from the body of the cathode coating to the surface, where they boost emission. It doesn't always work, and it can ruin the grid by coating that too. Only worth trying if the valve is rare, expensive and would otherwise be thrown away as being life-expired.
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Old 14th May 2011, 10:48 PM   #3
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I'm looking for some ingenious Yankee to show us how. A vacuum pump, some glass tube, some twisted wire, a little cesium. Voila! new cathode heater, in a new envelope. Old plate and grids. The cathode wears out, it is a twisted wire with a coating on it. Pittsburg ought to be just the place, plenty of natural gas there. If this unemployment stays on, I'm going to start cutting up the 7199's and 6CA7's in the basement.
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Old 14th May 2011, 10:57 PM   #4
forr is offline forr  France
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I read somewhere a trick which consists in putting old valves in an hot oven for sometimes. Some valve people may know more.
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Old 14th May 2011, 11:11 PM   #5
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I've heard of the oven trick before, activating the getter to absorb the gas inside. May have to be too hot for octal bakelite base tubes, but compactrons should be fine.

I was able to remove a kink in the V/I curves (seen on a curve tracer) of some damper tubes, by overheating the filament grossly for a few 10s of seconds. The old CRT rejuvenators worked by increasing filament voltage indefinitely by an auto-xfmr.

There was a CRT rebuilder in NJ at one time I think, could fix old Tek scopes. Maybe not around now, but if they are, I'm sure for a price they could fix any old tube. Might as well ask for a new Titanium-oxide no-heater cathode nowadays, last forever.
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Old 14th May 2011, 11:38 PM   #6
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Is this overheating trick viable for directly heated tubes as well?
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Old 15th May 2011, 12:09 AM   #7
Doz is offline Doz  United Kingdom
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CRT rejuvenators would cook the heater for the duration of the process and briefly "flash" a suitable HT between the cathode and grid... sometimes with disastrous results ! Like smoking amp says, some dealers added another heater transformer in the heater line with typically a 10% boost to the 6.3volts.

If the CRT or tube is knackered anyway , and you can get some more out of it ... what have you got to lose?

I occasionally re-build 1950's TVs ... quite often the CRT will have fallen "asleep" after 30+ years of inactivity (the monochrome system A service was decommissioned here in 1983) ... more often than not, the CRT will "wake" up after a period of use. I've had some 1950's Brimar 12AX7's (NOS) that were the same... useless for about 2 hours, then they gradually came alive.
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Old 15th May 2011, 02:46 AM   #8
jono1 is offline jono1  United States
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CRT rejuvenation is a subtractive process.

Eventually you will deplete the cathode until the emission falls off to unusable.

Some CRTs could be restored with a rejuvenator, if the cathodes were poisoned. The energy (stored in capacitors, typically) boiled off the impurities which "went elsewhere". Given the massive real estate in a CRT, it is probably less of a problem than in a relatively small vacuum tube.

In any event, the process typically gave a year or less of continued operation. In many cases the tubes were killed by the attempt.

I suspect (but do not know) that a CRT rejuvenator, applying a relatively short burst of high energy, is safer and more effective than "cooking" a tube by raising the cathode's voltage.

I'm sure this topic has been covered in 50s - 70's literature somewhere. I've always been interested myself.

I'm sorry that I sold the CRT rejuvenator. But with CRTs going the way of the dodo bird, they might be available as cheap surplus.
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Old 15th May 2011, 04:32 AM   #9
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The 429A has a 20 volt heater so I tried burning in the tube at higher voltages, like 26-30. The tubes run much closer to the datasheet at higher heater voltages but blue glow is visible between the grids. Guess they really are duds.

They do seem to be conservatively rated, at double the plate dissipation there are no signs of redness. Although the screen grids tend to glow brightly.
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Old 15th May 2011, 01:26 PM   #10
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Blue glow and "consumed" flashing indicates possibility of gas. I think an extended period of leaving the filament burning at normal voltage without a HT applied should be able to cause the gas to be absorbed. Give that a try before giving up on these tubes.
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