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Re: My VTTC fun begins...

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 low-emission tubes: http://www.antiqueradios.com/chrs/journal/rejuvenation.html

Here is a relatively recent paper describing low-emission problems with high power RF tubes used in the CERN nuclear particle accelerator, and how filament power monitoring and control led to greatly increased tube life.


Scott Hanson

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, January 28, 2007 3:20 PM
Subject: Re: My VTTC fun begins...

Original poster: Brett Miller <brmtesla2@xxxxxxxxx>


> Brett,
> If a particular tube has weak emission, you can
> often
> raise the filament voltage some and run the tube
> that way.
> For example for a 10V filament use up to 11, 12, or
> 13
> volts as needed.  It will shorten the filament life,
> but for
> intermittent VTTC it's OK, and otherwise the tube is
> not
> of much use anyway except for making a wimpy coil.

Well that is great information.  I know I have read
past posts where you and possibly other tube coilers
mentioned this technique (running several volts over
to improve a filament with weak emissions), but I had
not thought about it in a while.

When I claimed I had tested my tubes and found their
filaments "good" I was simply referring to the fact
that they were not found to be open circuit and many
were tested under the proper voltage with my rewound
MOT secondary filament transformer.  However, I do not
yet know if any of my tubes are weak.  Is there anyway
to test for this condition outside of actual operation
in a VTTC (class C armstrong osc.) circuit?  A lot of
my tubes are surplus and salvaged used from the
Amateur Radio community.  Some are new old stock, such
as my pair of 4-65A Eimac still in the box with bubble
wrap...the box is stamped 12/62, but the two tubes
appear to be in pristene condition, and the filaments
light up bright at their proper voltage and current

> It's been said that one can run the filament at a
> higher
> than normal voltage for awhile and sometimes full
> emission will be restored permitting one to then
> lower
> the filament to normal again.  I haven't had any
> luck
> with such rejuvenation however.

What would be the actual mechanism of this
rejuvenation?  Any source you could refer me to?

> I'm looking forward
> to
> seeing your VTTC results.

That is exciting in and of itself.  I look forward to
viewing some of your VHS tapes which document your
VTTC work.

> It all looks fine so far
> in
> your mock up photo.

...still just trying to walk the path laid down by
yourself, Steve Ward, and Cameron Prince, et. all

> I like to keep very good notes of all my tests,
> showing
> tune points, cap values, current input, grid coil
> turns,
> grid coil height, coupling height, spark length,
> spark
> appearance, coil sound, etc.  Certain
> adjustments will give a raspy sound which is
> generally
> a bad thing.  Some adjustments give a sizzling sound
> at low power.  In some cases the coil may even sound
> like it's oscillating somewhat erratically.

I am sure experience will eventually be my guide here.
 The only reference I have for VTTC sound are some of
the videos I've watched from people on the list, all
of whom were using staccato at the time.  For the
first while (not sure how long) I will be running
without it.  I have two used 833A's now, so perhaps if
the worst happens and I fry one, it will be my cue to
start building my staccato board (or if the breaker
will hold up for that matter).
Nevertheless, hopefully intuition (and reading your
posts a lot) will be my guide with gauging efficiency
through auditory analysis.

I plan on taking copious notes as well, hopefully
excitement of first light won't break my attention
span.  Then again, I may just take verbal notes at the
beginning and end of video clips and create a digital

> When
> tuning the
> coil, the goal is to
> simultaneously obtain; longest sparks, lowest input
> current, and minimal tube redness.

I will place my Fluke amp clamp over one side of the
mains input to the plate transformer (large MOT,
shunts knocked out).

> Coupling, grid
> feedback level, grid resistance, primary tune point,
> and
> tank impedance all have an effect.  This can make
> tuning
> a VTTC a slow process unless you happen to hit all
> the
> right parameters by chance early.  It's much easier
> to get
> long sparks than it is to get minimal tube redness.
> Some designs seem to be more finicky than others.
> If you're copying an existing design, that makes it
> easier.

I plan on breaking down the goal process for this coil
into steps:

1. Achieve oscillation
2. Some sparks (any at all)
3. longer sparks
4. longest sparks
5. fine tune for efficiency
6. go to 4

In the end I don't really have to have 30+ inches or
anything like record breaking sparks, I just want to
learn the techniques you outline above and get some
real experience optimizing and tuning tube coils.
After that I would like to test these skills by
substituting the 833A with my 304TL, 813, and some
others which might stand up to the plate voltages
generated by this system.

> I'm sure you're familiar with these issues anyway
> but it
> may benefit other new VTTC'ers in any case.

I am familiar with most of them, but not yet to an
instinctual level, since I haven't had the chance to
apply them in a real world coil yet.  I am anxious to
start gaining real experience.  Every day I read
certain texts several times over again.  It is easy to
forget certain details while caught up in the
emotional anticipation of a new project.

> An interesting tube to try is the 3-1000A (4-1000A?)
>  This has
> a plate voltage rating of 6kV I think.  It's a tube
> that I never
> tried yet.  I think someone on this list tried one
> but I can't
> remember the results.

I believe it was Ross Overstreet.  It's construction
is detailed on his site.

> John


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