Reprinted from the AMPS archives... yet again...
THIS INFORMATION APPLIES ONLY TO GLASS TUBES like the 3-500Z.
For ceramic-metal tubes, see the final paragraph of this post.
The following procedure came directly from a former Eimac tube designer,
who was with the company when they were still manufacturing glass tubes.
<quote>
*************************
If a tube was last used years ago, or was manufactured years ago, or is
new, it's safest to assume it needs gettering. And the suggested routine
is this:
Cooling is of course required throughout.
a. Bring the filament up to rated voltage slowly. For example, run
the primary of the filament transformer through a light bulb, or
similar to limit the shock of turning it on.
b. Then after some time, bring the voltage to the specified value,
and run it for about 100 hours.
c. Then, apply the anode voltage through some current limiting means
for a while, and, finally,
d. Run the tube at a dissipation level near the specified limit for
a few hours.
The reason for this is that the getter needs to experience a spectrum
of temperatures, mopping up different species at different
temperatures. Furthermore, the lowly 3-500Z for example, has getters
at the base of the grid and the base of the filament as well as
sprayed on the anode.
While this conditioning schedule is seemingly over-complicated and
ultra-conservative, it should result in no surprises when putting the
tube into service.
Terms like "a few hours" are vague because different tubes require
different treatment. If we're "burning in" a monster tube that will
cost us $50,000 to replace, we're going to be more conservative than
with a $50 tube.
***************************
<end quote>
CERAMIC-METAL TUBES do not have a getter on the anode, because the anode
runs far too cool. Instead the getter is located on the cathode (for
indirectly heated tubes) or on the filament (for directly heated tubes).
Therefore ceramic-metal tubes can only be gettered by applying
heater/filament voltage.
73 from Ian GM3SEK
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Mike
>Tubby
>Sent: 08 August 2015 19:26
>To: amps@contesting.com
>Subject: Re: [Amps] Question on installing new Amp tube
>
>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@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|