[Amps] Question on installing new Amp tube

[Lee]

Mike,

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.
Lee, w0vt

On 8/8/2015 1:26 PM, Mike Tubby wrote:
> 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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