sm5ki wrote:
>
>In the latest issue of the Brittish ELECTRONICS WORLD (11/2000) there is an
>interesting article called " NEW LIFE FOR OLD VALVES" where the author
>actually accelerated the gettering process by a factor of five by putting
>small tubes ( valves,hi ) in an 120C oven for 12hours. As I understand it
>he could reduce the gas grid current for lower input noise in small signal
>tubes used at low signal levels such as microphone input.
>
>So, could this be applied in a 3-500Z by increasing the temperature even
>further? Very nice methode then in the Swedish climate where the
>temperature sometimes goes down to very low levels. My flat needs that
>additional heat then, hi!
Small-signal tubes are gettered by a chemically reactive metal film
(usually barium, mostly) on the inside of the glass. You can see the
"silver" film inside, usually at the top. This operates at low
temperatures, and may be helped a bit by baking the tube (it probably
gets close to 120C in normal operation anyway).
If the guy was really serious about re-activating small tubes, he should
use an induction heater to try and throw a new getter film on top of the
old one. That's how the film was originally deposited - if you look at
the tube, there is always a small ring of some kind which carried the
getter material, and could be selectively heated by an induction coil
after the tube had been pumped and sealed.
Tubes like the 3-500Z are gettered in a different way, by coating the
outside of the anode with zirconium or similar (the matt grey finish).
This is chemically activated by the heating of the anode. In the last
stage of manufacture, after the whole tube has been baked, pumped and
sealed, the vacuum is cleaned up further by induction heating the anode
to activate the getter.
In normal operation this process continues at the operating temperature
of the anode. If the tube is being used, it is also keeping its own
vacuum clean.
When the tube is cold, gases are very very slowly released into the
vacuum space. This is why tubes that have not been used for a long time
can "re-gas", and may have to be heat treated to clean up the vacuum
again.
On the other hand, if there has been a very slow leak in one of the
vacuum seals, the tube is dead. Even if it had been in operation rather
than sitting in a box, it would still be dead by the same time.
>
>Prosit de Hans SM5KI
>----------
>>Från: Pete Smith <n4zr@contesting.com>
>>Till: amps@contesting.com
>>Ämne: Re: [AMPS] advice on an old SB-220
>>Datum: fre 10 nov 2000 23.16
>>
>
>>
>>At 04:54 PM 11/10/00 -0500, Chuck Counselman wrote:
>>...
>>>This is as much a question as an answer, but FWIW:
>>>
>>>I know nothing about 3-500Z's, but a close friend and co-worker
>>>successfully degassed quite a few 50-year-old (WW II vintage) 807's and
>>>other relatively small tubes by letting them run with (controlled) plate
>>>current. First he ran them with just their cathodes heated; but nothing
>>>happened. Then he applied B+ (a few hundred volts) and adjusted the grid
>>>bias to set the plate power dissipation to a healthy but safe level -- and
>>>the tubes degassed themselves within a day or two IIRC. Whether this
>>>technique will work with a 3-500Z, I don't know. Perhaps in an 807 the
>>>gettering material is applied to the plate, so it works when the plate is
>>>hot. I have no idea how a 3-500Z is gettered. Is its plate metal? Is the
>>>outside of the plate gray? If so, that might be a getter.
>>
>>It's been recently said on this reflector (and not contradicted, which may
>>be almost a first!), that the gettering material in 3-500s is on the plate,
>>and that the plate must show slight color for the gettering process to
>>work. If these tubes are severely gassy, it could be a delicate process of
>>ramping up the plate voltage, adjusting the bias for just a bit of color,
>>waiting a while, ramping up a little more, and so on. surely adding a
>>glitch resistor in series with the +B would be a good idea regardless, to
>>limit current through any arc that occurs.
>>
>>
>>73, Pete Smith N4ZR
>>
>>Contesting is ... Extreme Radio
>>
>>The World Contest Station Database
>>is back up and running at
>>http://www.qsl.net/n4zr
>>
>>
>>
>>--
>>FAQ on WWW: http://www.contesting.com/FAQ/amps
>>Submissions: amps@contesting.com
>>Administrative requests: amps-REQUEST@contesting.com
>>Problems: owner-amps@contesting.com
>>
>>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|