[Amps] Outgassing tubes.

[Will Matney]

See below,

*********** REPLY SEPARATOR  ***********

On 10/3/05 at 8:21 AM Ian White G/GM3SEK wrote:

>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.


In B/W and color CRTs this actually does happen but to be honest, it's been so long I forget the process that causes it. If I can run across my books on it, I'll look it up. However, we're looking at an electron gun here and I don't remember if this makes a difference, most likely it does. A CRT's electron gun(s) can get clogged up with this and is what makes the emmission fall off and sometimes causes shorts where the stuff flakes off. Anyhow the rejuvenator really sets up the CRT first and tests it just like any tube tester. The cleaning or rejuvenation is done by discharging a capacitor bank into the elements of the gun (sounds scary dont it). You can actually watch this in operation as it arcs and stuff falls out of the gun into the neck of the CRT. It's best to have the CRT tilted at a 45 degree angle I found for this. Sometimes you can save one this way and sometimes not. However, I've only lost maybe 2-3 CRT's in several hundred I've probably cleaned. Of course you make the customer aware that they have a bad CRT and this may or may not work and they still might have to buy a new one. I'd imagine this could be done to CRTs for scopes if one had the proper socket. They make a universal adapter too that just has clips for each pin and you hook the CRT up by it's pinout. The process that causes this, and if I recall they likened it to like the cathode growing a beard, or something similar, I cant remember as it's been too long since my TV service days.

>
>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.


We want to keep in mind though that one doing this every day, or several times a day, is most possiby a gassy tube and ought to be changed out, or at least checked with a hipot.


>
>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.

Sure I did. I said it could harm the rectifiers and capacitors where the arc is similar to touching the B+ to ground quickly. It's a good sized current surge with many joules of energy. If it was bad enough, or excessive, it could even cause a failure in a HV winding of the plate transformer. I recommend that any new tube be baked in the way we were talking to prevent any arcing at the get go to save the power supply. One never knows that one may or may not arc. This especially on NOS tubes I'd think.


>
>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
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Best,

Will