[Amps] Conditioning Spare Tubes After Storage

[Ian White, G3SEK]

SM0EJR 'Joe' wrote:
>I have had some experience in this....
>
>After buying a few different russian tubes....(GU-43B, GS-35B, GU-84B,
>GI-6B/7B).
>I've made some tests:
>
>I have about 10 of each type, except for the GI-7B, for which I have 
>about 20 or so.
>
>Here's the story:
>
>A GS-35B which could handle max 1100 volts before it started to 
>'leak'/spark... After 24 hours of heaters on, the tube can withstand 
>7800 volts on Anode-Cathode potential. It now works in HF-PA without 
>problem...I don't think it would if I didn't do this heating-up!
>
>Another GS-35B had 3kv as upper limit, but after cooking it, a 8 kv 
>limit was reached before a leakage 50 microamps occured!
>
>One of the GU-43's had a 2200 volt limit, but after letting it cook for 
>a
>while,
>it can 'take' 6 kv before leaking.
>
>A copule of GI7Bs had about 1500 v, but after ritual, I think at end, I 
>had approx 3900 volts on both...
>
>
>The gadget for these measurements was a K8CU High voltage breakdown 
>tester.
>
>http://www.realhamradio.com/High_Voltage_Breakdown_Tester.htm
>
>I even tried it on 5CX1500A's ( I have four of them on hand )...all 
>were about slightly below 6 kv before... Not much happened after 
>heating them up...They were already quite healthy!!
>
>So to finalize the whole story:
>
>1) Western tubes with external anode MIGHT be more stable.. But I only 
>tried one type of tube.. I have a 4CX5000A, but no transformer for it's 
>filiament, so I don't know that much about that one!
>
>
>2) Russian tubes with external anode gains, at least if storage/shelf 
>time is long. Some tubes were quite good, but wouldn't bet too much on 
>that!! The methods for gettering might be different from western 
>tubes?. Or just that the sealing for russian tubes is just more leaky, 
>so that you
>have to use
>the mechanism of gettering to solve it  ;o)
>
>So, I WOULD NOT PASS this 'cooking ritual' before kick-starting a new 
>PA with russian tubes!!
>


That all makes very good sense.

I don't believe that these Russian tubes are significantly leaky, in the 
sense of a faulty seal that lets air in from the outside. More likely 
they are made from ceramic and metal materials that are less pure than 
those used in western tubes, and/or they have not been as thoroughly 
de-gassed during manufacture. Such tubes may pass all the tests at the 
factory, but over time they very slowly release some of their gas 
content into the tube envelope.

All "vacuum" tubes actually contain lots of gas molecules. Even a "good" 
tube contains millions to billions of free gas molecules bouncing around 
inside the envelope (depending on the size of the tube and the quality 
of the vacuum). During operation, this gas is continually being removed 
by the getter, which acts like a 'fly-paper' to which the gas molecules 
will stick by chemical reaction.

The getter in a receiving tube is the silvery deposit on the glass, and 
is typically barium metal. It is very chemically reactive and will 
mop-up gas molecules even at room temperature, but it's too volatile to 
be usable at the high temperatures of transmitting tubes. Getters for 
transmitting tubes are made of materials that are less volatile, but 
also less chemically active, so they need high temperatures to make the 
'gettering' reaction go faster. But if the tube just sits in a box and 
never gets hot, the getter doesn't work at all well.

That is why transmitting tubes can accumulate gas if they are not used, 
but will often get better again if they are pre-conditioned by a period 
of pre-heating. (The exception of course is if a seal has actually 
failed.)

The getter is usually located at the hottest part of the tube. In a 
ceramic/metal tube, this location is on the cathode, which is why 
heater-only operation is enough make the getter work - as Joe and many 
others have found. In a glass transmitting tube, the getter is usually 
on the anode, so it needs HV and anode current (anode dissipation) to 
make the getter hot enough to work - but not too much HV, or else the 
tube may arc.

At extremely high temperatures, higher than the tube will ever reach in 
service, the chemical reaction will reverse and the getter is forced to 
release the gas it has trapped. When the tube is being manufactured, it 
is deliberately heated to such extreme temperatures while the tube is 
still being externally pumped. Once the tube is sealed, the getter is 
then responsible for maintaining the quality of the vacuum for the rest 
of the tube's life.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek