On Fri, 23 Oct 98 21:34:11 PDT "doshhan" <doshhan@vaxxine.com> writes:
>
>----------
To: <amps@contesting.com>
>> Date: Sunday, October 18, 1998 07:29:42
>> From: km1h@juno.com
>> To: amps@contesting.com
>> Subject: Re: [AMPS] Revitalising 'tired' tetrodes
>>
>>
>>
>>
>> On Sun, 18 Oct 1998 11:18:40 +0100 Chris W Tran
><chris.tran@zetnet.co.uk>
>> writes:
>> >
>> >Sunday 18th October 1998
>> >
>> >Hello all
>> >
>> >I've got a few tired 4CX1000A ex-broadcast 'pulls', and
>> >want to revitalise them a bit, if it's possible.
>> >
>> >Speaking to my friend Bob Kerr GM4FDT, he quoted from the 1925
>> >Admiralty Handbook about how to revitalise Thoriated-tungsten
>> >cathodes/filaments (as used in the larger Eimac tetrodes)
>> >- run the filament for a few hours/days with all other voltages
>> >disconnected - extra Thorium migrates to the surface and the
>> >emission will increase. Also, as the filament reaches the end
>> >of its life, the filament voltage can be increased by say 5% to
>> >improve emission.
>> >
>> >Does anyone know of a similar procedure which would give new
>> >life to the 'Oxide-coated unipotential' cathodes of the
>> >4CX1000A/4CX1500B series ?
>> >
>> >73 Chris GM3WOJ/GM8V
>> ><chris.tran@zetnet.co.uk>
>>
>> Back in the 60's there was an article on rejuevaniting indirectly
>heated
>> TX tubes. If memory serves it required filament voltage and
>momentary
>> applications of a positive voltage between cathode and grid. In a
>6146
>> that was about 250V and it brought many of my old duds back to life.
>>
>> I may be able to find that article and I'm sure there are others out
>here
>> that have experience.
>>
>> 73 Carl KM1H
>
>I have several procedures for this in my collection of old electrical
>engineering textbooks from the '30s. The procedures were practiced
>widely to rejeuvenate old and hard-to-find receiving tubes with
>thoriated filaments...tubes such as 01-A'a, UX199, 27's etc. However,
> it's my understanding that these procedures are useless for oxide
>coated cathodes. The oxide COATING is a fixed and finite quantity of
>material that is gradually used-up or consumed during the life of the
>tube. Once it's gone, it's gone. By comparison, the thorium used
>in thoriated tungsten filaments is not a coating, but an 'emission
>catalyst' (my phrase...can't remember the correct terminology)
>thoroughly dispersed throughout the cross-section of the tungsten
>filament. Gradual 'consumption' of thorium molecules near the surface
>of the filament leads to reduced emission. By temporarily and
>carefully overheating the filament by running higher voltage and
>current, fresh and yet unused thorium molec!
>ules below the surface of the filament are mobilized and allowed to
>flux up to the surface, restoring emission. The procedure must be
>done with NO voltage on any of the other tube elements, and it isn't
>always successful. There is always the risk of burning the
>filament-out completely. But if the tube is near useless anyway,
>you're not risking much.
>
>If anyone is interested, I'll post a copy of the procedure. The
>procedures I have are for specific tubes. But a generic procedure
>could be extrapolated 'by inspection' of the procedures.
>
>Phil VE3OZZ
There have been many articles on rejuvenating thoriated tungsten tubes,
both in engineering and amateur publications. One of the most concise was
in HRM August 1978, Page 80.
However the original posting was a request for information on oxide
coated filaments.
My earlier reply mentioned rejeuvenating oxide coated filaments....and
YES it can be done. It just took awhile for me to find the reference
which appeared in January 1962 CQ Magazine.
The circuit is very simple and does not require plate or screen voltage.
Filament voltage is required and so is filament seal air flow for big
tubes.
A voltage is applied between the control grid (+) and the cathode (-).
For a 6146, which was the tube in the article, the voltage is 250 to
400VDC. A momentary pushbutton switch is in series with the grid lead and
bypassed by .1mF.
The procedure is to apply 8 to 10 momentary applications of voltage, and
periodically test for emission in a functional circuit.
The circuit worked fine for me back in those years and I saved many 2E26,
6146, 807, 6550 and similar tubes from the trash.
I have never tried it on a large tube and would encourage some
experimenting. After all, if the tube is already pretty well shot, it
wont hurt to try. It may offer some hope for all those soft 8875's
sitting in boxes.
If Rich or someone else wants to include the article on a web page, I'll
mail it to them.
73 Carl KM1H
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