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Old 25th September 2010, 08:51 PM   #1
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Default Question About Worn Tubes

Hi all. I have a pair of RCA VT 229's that although I was assured that they measure strong (gas, mutualconductance etc.), the black plates appear to be somewhat stripped rather than smooth. Would I be playing with fire installing these in my power amp? I fear a potential breakdown under load in a circuit. Thanks in advance.
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Old 25th September 2010, 10:22 PM   #2
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Don't worry about it. You can't predict failure by the looks of the plates.
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Old 26th September 2010, 12:20 PM   #3
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Thanks Bob. What about performance?

Now I notice that one tube has a halo of colors I can see in flash. Doesn't this usually indicate a gassy condition? The test of this tube for gas was supposedly negative.

I'll probably try them anyway and keep my fingers crossed.
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Old 26th September 2010, 02:41 PM   #4
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Originally Posted by Monjul View Post
Thanks Bob. What about performance?

Now I notice that one tube has a halo of colors I can see in flash. Doesn't this usually indicate a gassy condition?
Coloured rainbowed getters are an indication that the gettering was done at too high temperature. Minute traces of metallic oxide vapours become combined with the getter. There is nothing bad about this, only the resale problem of such tubes of how they look.
A getter sensing air is quite different and will immediately go chalk white and that's finished. With the gettering done on the top, a clue is to look from the inside and see how good the reflective is.

In the pics, #24-25 the view of a side getter of a TungSol New Edit 6550 tube which has only done 300Hrs under thrashed conditions; a getter in this condition indicates the tube is past it's peak. The anode dissipation has been exceeded, the grids strained but looking at the condition of the top getter between the upper mica cage and the inside envelope would indictate nothing was wrong.

Those of us experienced with tube amps, can say with fair certainty what life is left in a tube by carefully examining the getters. SOme tubes don't have side getters i.e original EL34 pencil types, but many tubes have been culled from amps which still had a good life left in them. There is no rule of thumb; it depends on the operating conditions. The cue is often to take tubes out of hard pressed amps and fit them into amps with lower dissipation and power. If the bias/current statistics are stable, one can be suprised how much life is left, sometimes years.

Pics of a KenRad 5U4 from 1955; (#33-35), looking near the neck at the getter low down is good, nothing amiss.
Looking at the getter from above, one gets a different picture, misleading due to reflections. This rectifier is 100% fine.

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Old 26th September 2010, 03:48 PM   #5
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Thanks Rich. Based on your responses I decide to clean the pins and give them a try. They seem stable and sound very good with good detail and a very wide soundstage. Certainly better than the 6SL7's in my current stash of tubes.

So I'll probably keep them (only $25 the pair) and use them until I can afford a new pair which can be quite expensive now a days.

And my apologies for incorrect info. They are not RCA's but Sylvania VT 229's supposedly from the 50's.

Last edited by Monjul; 26th September 2010 at 03:53 PM.
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