On 04/11/2011 04:05, R Johnson wrote:
> I have just been given an old SB-220. It has been sitting in an attic for 20+
> years.
>
> I have removed the tubes and have been bringing the voltage up slowly
> to reform the filter caps. When I get to 120V in and the caps aren't steaming
> and/or blowing smoke I'll be ready to check it with the tubes installed.
>
> I know that the need 3-500's will need to be "Gettered" to remove any gas
> diffusion
> thru the metal to glass seals on the tube.
>
> I read a procedure on (probably) on this list on how to do it.
> Now that I need it, I can't find it.
>
> Does anyone remember this thread within the last 3 or 4 months ???
> If not,can any recommend how to cook these tubes in ???
>
> TNX es 73
> Bob, K1VU
>
Bob,
Before you worry too much about gettering the tubes I suggest that you
check them for emission and reactivating them if necessary... I found
how to do this on PA0FRI's site, here:
http://www.xs4all.nl/~pa0fri/Lineairs/TL922/tl-922eng.htm
<http://www.xs4all.nl/%7Epa0fri/Lineairs/TL922/tl-922eng.htm>
I was sceptical - but it works!
Get a socket, heater transformer, milliammeter, a variable DC supply
0-40V and a cooling fan (a muffin fan will do).
Connect the filament, connect the grid to the anode (yes, the anode) and
apply DC supply (+ve) to the anode+grid and -ve to filament (either side
will do). Put the fan on to stop the tube seals from over heating.
Set the DC voltage to around 35V and see if any current flows - there's
a good chance that you'll see no significant currently flow (perhaps a
few tens of uA)...
Now, leave your fixture on for several hours to 'cook'. This can take
several tens of hours... eventually you should see the some signs of
current flow - the meter may flick up to 10-50mA now and then... this is
the Thorium migrating back to the surface of the heater and starting to
emit electrons.
After several more hours it should settle down and you should be able to
get around 400mA of anode current with just 40V DC on the anode
(remember that you've got the grid tied up to the anode, so you really
have a big diode at this point). At this point the power dissipation is
40V DC 400mA => 16 watts.
Now if you adjust the anode voltage up and down the lower voltage at
which you can achieve 400mA anode current the better the tube.
Until you've checked out your tubes and got them emitting electrons
there's not much point in worrying about gettering them ;-)
Now, the getter in the 3-500Z is a coating painted on to the outside of
the anode. The getter only works when the tube's anode is hot (glowing
dull red/orange), for this you need to be dissipating approx. 300-400W.
I gettered my reclaimed tubes with a 1200V DC supply, 1A anode meter (1A
fuse in series) and a DC supply. You need to get the tube to draw
around 250-300mA and to do this you need positive DC on the grid - I
used a 20V DC bench supply.
You need the tube in a proper fixture with plenty of cooling and then to
cook it for a period of a couple of hours.
After re-activating and re-gettering several tubes that were given to be
as 'soft' and unsable I have one in my homebrew 6m amp (QST 1971 design)
and two in my TL922. The 6m amp will give over 800W out for 35-40W
drive... not bad for a ''useless'' tube (according to its previous owner).
Mike G8TIC
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