On Sun, 2006-10-08 at 18:42 -0400, RB wrote:
> The 3-500Z type tubes were designed with tungsten-thoriated filament
> material. This is a material much different than standard tungsten,
> although, grant it, any material heated and cooled will exhibit a finite
> life from expansion and contraction related to temperature.
The thoriated tungsten filament isn't all that much different from the
filament in a light bulb except it doesn't get run quite as hot and has
thorium mixed in with the tungsten before the filament is drawn. I don't
think there is an alloying, just a mixing of crystals. Its the thorium
that supplies most of the electrons for the vacuum tube operation. And
one way a tube gets to end of life is that all the exposed thorium has
supplied all the electrons it can. Sometimes an excursion to higher
filament temperature while checking filament electron emission will
restore that tube to operation, sometimes that burns out the filament. A
> I've never seen
> a filament burn out in a 3-500Z tube, although did encounter one the
> wouldn't "light" and measured no resistance between the filament pins. I had
> heated those pins with a soldering iron until the solder flowed, added a bit
> of 2% silver solder and BINGO. That tube provided a decade more of reliable
> service for the ham I did that. He figured he had nothing to lose and as he
> was prepared to buy a new tube, I tried this method that I had read about
> years back, as I would have if it were mine.
>
> The resistors that you refer to are surge suppressor resistors, typically
> installed on the primary of the filament transformer, to "step-start" the
> filament voltage for a few seconds to aid in prevention of the large current
> surge from distorting the filament by delaying full power from the filament
> transformer until the filament gets warmed up slowly with partial power
> first upon turning the amp on initially.
Since the thoriated tungsten filament operates at a noticeably lower
temperature than a lamp filament, the resistance change from cold is
probably less severe, just 10 or 12 times higher when hot, not 15 or 16
time higher.
> There are various thoughts on this
> mythology as When Eimac developed the 3-500Z as an instant ON tube, the
> design criteria was that the transformer impedance was quite adequate in
> itself to prevent the calculated high surge current from reaching the
> filament/s as if someone was switching on a supply already powered on with
> core saturation and impedance of the windings not included in those
> calculations.
Larger tubes often have current inrush limits as part of their data
sheet. Cheap ham tubes often do not, though I think its a good idea. Now
I question whether the transformer as limit (which is definitely NOT
acceptable on larger tubes) is as effective when the filament winding is
on the same core as the plate winding. The output voltage regulation and
current limiting from transformer impedance comes from both the primary
and secondary windings. In the single purpose filament transformer the
percent impedance due to the secondary and that due to the primary will
be close to the same, but in the multipurpose plate and filament
transformer the primary impedance will be chosen to match the major
load, the plate circuit and so will be much smaller than it would have
been in a dedicated filament transformer. Seems to me that to protect
the tubes, that plate/filament transformer won't do well and so the
filament winding should have extra turns for extra voltage that that
voltage be dropped in a resistor outside the transformer.
> My take is that while the step start circuitry may not be
> necessary, it certainly doesn't do any harm either. Many early commercial
> manufacturers of amplifiers using these tungsten thoriated filament tubes
> did not include any filament surge suppression and they provided good
> service life. In recent times, many manufacturers of 3-500Z tube type
> amplifiers provide for the step start approach. The addition of this
> band-aid makes many consumers feel like it is an added benefit and will pay
> more for it. Seems to me that in time, all manufacturers jumped on the
> bandwagon and now have step-start as an added feature and benefit. I came
> from the old school of Eimac engineering thinking of keeping it simple. Of
> course, Your opinions and your mileage may vary.
There's been lots of step start circuits used in big transmitters, at
least at Collins where I once worked.
>
> In all probability, this conversation would be best served on the AMPS
> reflector. Although it started out as a TenTec Centurion Amp discussion, it
> really now refers to all amps using tungsten-thoriated tubes.
>
> The AMPS reflector was one of the first that I unsubscribed from a half
> dozen years or so back as I got quickly tired attempting to educate folks
> that did not really want an education, but more so a debate. I have no
> desire or the time to waste my spare time on any reflector debating heresy.
> If folks would just spend a bit of their time searching the internet and
> paging through books, the truth can easily be found on most any subject. Not
> that everything ever published is correct, but with enough research you can
> make your own educated decision.
>
> 73,
> ÂÂÂÂ)) -:Â:-
> Â.ÂÂ.ÂÂÂÂ))
> ((ÂÂ.ÂÂ ..ÂÂ -:Â:- Rog-K9RB
> -:Â:- ((ÂÂ.ÂÂ*
>
> FCC First Class Radiotelephone licensee (obtained in 1968 at age 18).
I beat you. Got my first phone at 17 in 1958, the same year I got my
second telegraphy with ships radar endorsement and amateur extra.
Then I took up engineering studies at Washington U (St. Louis) and Iowa
State.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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