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RE: Vacuum tube rejuvination
Original poster: "Rich & DJ" <rdj@xxxxxxxxxxxx>
Brett , Scott.
You might want to think about a pressure box to run the tube in if you
are going to run it for 24 to 48 hours. I made mine out of plywood , no
RF shielding is needed. This let me run a boxer fan on the box and exit
at the base of the tube, keeping the seals cool. Big glass bottles like
mine run at 7.5V 21 amps, on CX tubes air flow is a must.
Rich, in the middle of Missouri
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Monday, January 29, 2007 10:21 PM
Subject: Vacuum tube rejuvination
Original poster: Brett Miller <brmtesla2@xxxxxxxxx>
Thank you for that excellent description of thorium
oxide depletion in tube filaments. I will also read
the articles you have linked. This is exactly what I
wanted to know.
--- Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "huil888" <huil888@xxxxxxxxxxxx>
> In a vacuum-tube with a directly-heated
> thoriated-tungsten filament,
> the actual source of electrons is thorium oxide,
> which is alloyed
> with the tungsten filament. The thorium is brought
> to the surface of
> the filament during special processing when the tube
> is built. As
> this surface layer of thorium is slowly depleted
> during tube
> operation, electron emission gradually drops. Very
> slight increases
> in temperature greatly increase the rate of thorium
> depletion, which
> is why tube manufacturer's specify VERY close
> control of filament
> voltage (= filament temperature).
> With a tube that is already showing reduced
> emission, a way to
> temporarily increase emission is to increase
> filament temperature.
> However, this will relatively quickly deplete the
> remaining thorium.
> Nevertheless, given the total run time of a typical
> VTTC, this may
> yield acceptable results.
> There are a number of papers available on the web on
> increasing the
> filament emission of "tired" vacuum tubes, and also
> guidelines on
> first firing-up high power vacuum tubes that have
> sat in storage for
> many years. Search on "vacuum tube rejuvenation" or
> similar topics. A
> general recommendation seems to be to run the tube
> for some period of
> time with filament power applied, but with no B+
> applied. 24-48 hours
> of operation in this mode allegedly helps to de-gas
> the tube, and
> reduce the potential of internal arcing.
> Here is an article describing the "rejuvenation" of
> Here is a relatively recent paper describing
> low-emission problems
> with high power RF tubes used in the CERN nuclear
> accelerator, and how filament power monitoring and
> control led to
> greatly increased tube life.
> Scott Hanson
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