[Heath] Gettering 572B's for a Heathkit SB-200

[Kenneth G. Gordon]

On 10 Aug 2015 at 21:56, Wilson  Lamb wrote:

> I'm very aware that I have a limited view on this and am happy to learn. 
> So if there are sites that explain what is going on and can verify that the
> treatment helps, they would be most helpful and interesting. And how do we know
>  a failed tube would have been helped by gettering??

My understanding and experience tells me that gettering will NOT help a 
completely failed tube: that process is really only useful for a good tube which 
has not been used for a long time.

Tubes with plates made of the same material as the 572B (carbon?) getter 
continually while being used. When such a tube has not been used in a long 
time, usually a couple of years at least, they sometimes must be "gettered" 
before use in order to prevent arc-overs when HV is applied due to the 
residual gas that has built up in them.

I am ambivalent about re-gettering unused tubes: in my experience 
sometimes it helps, and sometimes it does not appear to do any good 
whatever. In my opinion, since the PLATE is what does the gettering, the 
proper procedure would be to apply some plate voltage to the tubes as they 
are "cooking", but not enough plate voltage to arc over. I.e., connect them as 
a diode.

It is sometimes possible to "reactivate" thoriated tungsten filamented tubes 
which have gone "soft" or in which the filament-emission has fallen by 
depeltion of the thorium layer, by operating them with filament voltage only for 
a while.

That filament voltage, however, must be noticably higher than the NORMAL 
filament voltage in order for any residual, unavailable thorium to be "boiled" to 
the surface of the filament.

That thorium layer, by the way, is only one molecule "deep".

For instance, in an old handbook I had at one time, the suggested procedure 
for reactivating thoriated-tungsten filamented transmitting tubes was to apply 
2.5 times the normal filament voltage to the tube (with no other voltages 
applied) for 1 minute, then reduce that voltage to 1.5 times the normal voltage 
and hold it for 1 hour, then test the tube. If the tube had not regained its 
normal plate current capability by this procedure, one repeats the procedure, 
then re-tests. If the tube is not back up to normal by then, then discard it.

Using this procedure, I successfully reactivated about a dozen 304TLs I was 
given which had been used as shunt regulators in an NMR machine. These 
were so flat that they wouldn't draw even 10 mA with 2500 VDC on the plate 
and NO grid bias.

After reactivation, they performed as new in amateur service for many years 
afterwards.

HOWEVER, when I tried this same procedure with flat 572Bs, it was only 
marginally successful and the effect didn't last long.

I then asked a manufacturer of tubes about this process, and it was 
suggested to me that raising the filament voltage of 572Bs to only 1.5 times 
the normal filament voltage for 1 minute would probably help redistribute the 
thorium, at least for a while.

Also, FYI, I spent quite a bit of money and at least two years of part-time 
investigation to learn all of the above while trying to figure out how to get 
572Bs which would work longer than a few months in my own SB-200s.

The only solution which worked was to buy those new 572Bs which RF Parts 
is now selling after THEY did over a years worth of work to find a maker who 
would make them at least as well-made as the old Cetrons.

As far as I am concerned, RF Parts succeeded admirably. I have a pair now, 
and they are really superb, and the cost is much lower than I had anticipated.

I intend to buy another pair as soon as I can get in the queue.

YMMV.

Ken W7EKB