[Amps] Question on installing new Amp tube

[Mike Tubby]

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
>>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps