I have always heard of long time "idle" 3-500Z tubes needing to be
gettered because the high voltage can flash over and destroy the grid.
I have never heard they have a problem emitting electrons off the
filament from being idle. This is news to me. However, I would be
much more concerned with gettering a gassy tube then initially worrying
about electron emission. Electron emission is a mute point if you have
destroyed the grid from flash over due to a gassy tube.
On 8/8/2015 1:26 PM, Mike Tubby wrote:
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
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.
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:
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.
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