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Vacuum tube without silvering still good?

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EEYORE

Tue Mar 09 2010, 11:27PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
I have a chance to buy an RCA 833a tube from someone, but they know nothing about it other than it being in storage for some time. (Belonged to his dad). It looks flawless and the electrodes are all perfect looking. (Looks unused). The one thing that concerns me is that I dont see ANY silvering on it. (from the ghettor, right?).

These observations were made from pictures sent over the internet. I guess my question is, can a tube be good and have no silvering visible? I have one right now that has a lot of silvering. It is also an RCA 833a.

Thanks for the tips!
Matt

Proud Mary

Wed Mar 10 2010, 12:04AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Lots of valves have no silvering you can see, Matt. It entirely depends on the manufacturing process, the type of getter, and so on.

Getters are usually chosen from the alkaline and alkaline earth metals, so if there has been a catastrophic loss of vacuum, whitish oxides of these metals form at once on the inside of the glass. Aside from certain highly expensive thermionic devices, such as big rotating anode X-ray tubes, huge klystrons, and so on, which are often re-manufactured,
the only place for a valve with lost vacuum is the dustbin.

EEYORE

Wed Mar 10 2010, 04:31AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Proud Mary wrote ...

Lots of valves have no silvering you can see, Matt. It entirely depends on the manufacturing process, the type of getter, and so on.

Getters are usually chosen from the alkaline and alkaline earth metals, so if there has been a catastrophic loss of vacuum, whitish oxides of these metals form at once on the inside of the glass. Aside from certain highly expensive thermionic devices, such as big rotating anode X-ray tubes, huge klystrons, and so on, which are often re-manufactured,
the only place for a valve with lost vacuum is the dustbin.

Hello,
Thank you very much!
Matt

Steve Conner

Wed Mar 10 2010, 09:45AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I bought a 4-400 without any silvering, and it works fine.

High voltage, high power tubes can't have getter material splattered all over the inside of the envelope, because it conducts electricity and would break the tube down. So they have to put it somewhere else.

I believe on some high powered tubes, the getter material is deposited on the plate. When the plate gets red-hot in operation, it activates the getter and helps it to suck up gas. Mine is one of these, a 4-400C with a graphite plate coated in some silvery stuff.

On some other tubes the getter is on the filament structure, and you're supposed to run the filament for several hours before applying plate voltage, if they've been in storage for a long time.

Conundrum

Wed Mar 10 2010, 01:09PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

oh, now they tell me... no wonder my vfd's won't work

read on the itron site that they recommend running these for a day or so with no applied voltage apart from the heater to "un-poison" them., they do mention that the effect of not doing so can cause serious loss of brightness and uniformity to the point of failure.

i have some nixies here and those expensive russian 8 digit displays, might explain the lack of light emission.

regards, -A
"Bother" said Pooh, as his Bluetooth headset started smoking...

IamSmooth

Wed Mar 10 2010, 07:08PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Although I am not familiar with all tubes, many of the ones I've used and tested had barium oxide, which is the GETTER (not ghetter).

The getter absorbs the oxygen, which will shorten the life of the elements. The tube can function without the getter, but the life-span will be significantly shortened.

I've built a tube tester to match 211 and 845 triode tubes, and any tube without a good amount of getter had poorer electrical characteristics than those that did. This does not mean it won't function for your application. The tubes I was testing were for monoblock amplifiers.

Proud Mary

Wed Mar 10 2010, 08:14PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IamSmooth wrote ...

Although I am not familiar with all tubes, many of the ones I've used and tested had barium oxide, which is the GETTER (not ghetter).

I'm sure you meant to say that the getters were barium, which turned into its oxide in the process of scavanging.

Corey

Thu Mar 11 2010, 05:55PM

Registered Member #1902 Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75

Would these tubes happen to be from ebay user "conn2"? If so I was also planning on buying 833a from him tell me how they turn out.

-Corey

EEYORE

Thu Mar 18 2010, 12:26AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Yep, they are from Conn2...Got them today. One has a large spot on the plate on both sides. I dont know if this is a sign of damage or not. Here is a picture. Any ideas what this is?
Thanks,
Matt

Arcstarter

Thu Mar 18 2010, 12:53AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Weird, i have never seen an 833 plate that had that flat part! Did not know they where ever like that.

It looks like it maybe got too hot and discolored. That is where they typically heat up most at. But, i am sure it is still fine, i hope.

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