Lee,
I think you may be confusing two unrelated issues ...
The 'Reactivation' that PA0FRI describes is a process of making a tube
work again. The 3-500Z is a directly heated cathode, i.e. the filament
is the cathode - it comprises of Thoriated Tungsten. As I understand it
when a tube sits, unused, for an extended period of time the surface of
the filament looses its ability to emit electrons - you could say it
'oxidizes' (but their isn't any Oxygen to speak of). The re-activation
process brings an otherwise 'dead' or 'partially working' tube back to
life by rejuvenating it and restoring its ability to emit electrons.
What you're doing is keeping the filament hot for an extended period
until some of the Thorium atoms migrate back to the surface renewing the
emissive layer.
The reactivation process needs only modest voltage (40-ish volts). Note
that in PA0FRI's diagram the control grid is tied up to the anode. With
40V and 400mA the tube only dissipates 16W and this is really just an
indication of whether your tube has sufficient emission to be able to
conduct with 400mA of anode current.
What you were talking about in your previous post is "gettering" the
tube which is something entirely different. The majority of electron
tubes have a method of "gettering" them - or "sucking up the free oxygen
atoms" or "hardening" the tube vacuum. In the case of the 3-500Z the
anode has a coating of Zirconium - when this heats up (ideally to around
1000C) it mops up free Oxygen atoms - presumably by converting them to
Zirconium-Oxide.
Some tubes with an indirectly heated cathode can be gettered by just
turning on the heater. In the case of the 3-500Z you have to get the
tube (very) hot by running it up to near full dissipation with HT and
anode current.
Mike G8TIC
On 08/08/2015 17:52, Lee wrote:
This scheme makes no sense to me. It is known a 3-500Z tube has it's
getter on the plate. So if you place 45 volts on the plate and draw
400 ma, you have a plate dissipation of 18 watts. 18 watts is not
even close to getting the plate cherry red. In fact it won't get the
plate to glow at all. Since the plate is the getter and it can't
getter unless the plate is glowing, how in the world can this scheme
work? There is something wrong here. This scheme is telling us the
getter works without a red hot plate. This is news to me as every
other gettering scheme I have seen insists on getting the plate to glow.
Lee, w0vt
On 8/8/2015 10:59 AM, Mike Tubby wrote:
If the 3-500Z has stood for a long time unused I suggest you use the
'Reactivation 3-500Z' procedure by PA0FRI, here:
http://pa0fri.home.xs4all.nl/Lineairs/TL922/tl-922eng.htm
When I did this with a new-old-stock tube I used 40V and had no
significant current indication for about 1.5 days, then the current
meter started to 'flick' a few milli-amps and back to nothing and
then after a few more hours it started a constant reading of around
50-60mA which then rose until it hit my current limit.
I used a pair of 0-20DC bench power supplies in series to get 40V and
set the current limit at 400mA.
After the reactivation I put the tube in a PA and it worked fine.
Mike G8TIC
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