[Amps] Outgassing tubes.

[Ian White G/GM3SEK]

Bill Fuqua wrote:
>     OK, you apply HV to a tube and it arcs a couple of times and then 
>it is OK. Does that mean that you have remove gas from it or gettered 
>the gas somehow? No!  You may have conditioned the tube. There is a 
>procedure called conditioning. This is often done in vacuum devices 
>that have high voltage applied to their internal elements, not just 
>vacuum tubes.  You start at a lower voltage and raise it until you get 
>an arc. The current is limited so that you don't cause serious damage 
>to the elements.  The arc discharge removes sharp pieces on the 
>elements blast away tiny pieces of stuff on the elements that produce 
>field ionization at lower  voltages. You continue this until you get to 
>the operating voltage on the elements that you desire.  I believe there 
>is an EIMAC application note referring to this in some of the higher 
>power tubes.  Sometimes little bits of stuff in the tube get where it 
>should not be or sharp edges are formed during the manufacturing of the tube.

The whole process of "conditioning" includes both removing any small 
internal spikes *and* giving the getter a chance to do its work. Surface 
spikes provide the high potential gradient, but a poor vacuum provides 
the current carriers that allow an arc to strike.

Surface spikes are more characteristic of a new metal surface, but once 
the worst of them have been blasted away (preferably under controlled 
conditions like Bill describes), I'm not aware of any way they grow 
back... a bit like teenage acne, really.

What we see more commonly is arcing inside a tube that is either NOS or 
has not been used for a long time. In some cases the tube may have 
sprung a leak, but don't automatically assume that. More often the arc 
will be due to the slow out-gassing of the tube materials into the 
"vacuum" space, and in that case the tube will benefit from being 
conditioned hot, so the getter can do its work.

In practical terms it doesn't matter why the tube is arcing. If it is 
conditioned in a protected circuit as Bill describes, that will give it 
the best possible chance of getting safely back into service.


What hasn't been mentioned much in this thread is the damage to the rest 
of the amp when there is a large current surge in a tube. Sometime in 
their careers, most amps WILL suffer these current surges, for one 
reason or another - but it doesn't have to wreck the meters or anything 
else.

There is good information on the use of meter protection diodes and 
glitch resistors on both Rich's website and my own. Combine those 
precautions with a fast, low-rated mains fuse [*] and the amp is pretty 
safe against arcing.

Taking it a step further, a fast HV current trip allows you to condition 
older or NOS tubes in the amp. Normally I just throw 'em in and switch 
on, because the amp will take care of itself if there's any problem. If 
the tube doesn't arc, then well and good. If it does, then fall back to 
a 24h burn-in with low or no HV, and run the tube up gently from there. 
(But don't try that unless you know that the amp really can take care of 
itself!)



[*] One of the less promoted advantages of a step-start is that it 
allows you to reduce the rating of the mains fuse, which protects the 
whole amplifier much better.


-- 
73 from Ian G/GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek