[Amps] Miller-Larson effect on thoriated tungsten filaments

[Jim Thomson]

Date: Tue, 25 Jun 2013 09:14:39 -0400
From: "Carl" <km1h at jeremy.mv.com>
To: "Mike Waters" <mikewate at gmail.com>, <amps at contesting.com>
Subject: Re: [Amps] Miller-Larson effect on thoriated tungsten
filaments


I doubt if that has much effect on our low power tubes Mike but the "boys" 
with the 10-20KW rigs might be interested.

I also never had to replace any of the WW2 era TX tubes while in the USN and 
those were cycled on only when necessary.

Much ado about nothing for this forum. When was the last time you saw a 
3-500 with a bad filament?

##  Those devices that rf parts sells... that go into one or both legs of the fil xfmr have to be
in stagnant air.  They normally run hot.  IF there is forced air on em, their resistance will increase. 

##  The triac ramp up circuit will also work, but then you have no way to tweak the fil voltage.
A switching supply will work, but the inrush current on the tube will cause the switcher to shut itself off. 
The switcher has to be sized a lot bigger. 

##  W8JI claims a lot of the chinese  3-500Z tubes wont  produce low imd, unless the fil V is at least 5.1  to 5.2 V. 
A simple step start  will step start the B+  and also the fil..and both at the same time...plenty good enough for amps
like L4B, etc. 

##  On big metal tubes like 3x3 and 3x6 etc, a variac  + step start, or variac alone will do the job...and allow for precise
fil V control.   A simple cam on the variac shaft + microswitch + relay will become a fail safe circuit.... in the event you lose commercial
AC power..and variac is cranked up.  When AC power returns, you have no V from variac.... until its turned fully CCW, which engages
the microswitch  + relay.   This ensures the fil never gets hit with a full bore surge. 

##  for folks like myself, that have lousy line regulation off the street...and or the 240 line sags on TX,  a Sola  constant V xfmr is used
to feed a constant V  to the variac.     Then the fil is not only regulated, its also fully adjustable. 

##  In normal operation  on a new or rebuilt tube,   fil is operated at rated fil V for the 1st  48 hrs  with no drive and no B+.   Once
gettered,  drive and B + applied.   After a total of 200 hrs of the fil hr meter, the fil V is turned way down, until the PO just begins to drop off
by a few watts... then fil V increased by .1 volt.  Knee effect.     During the 1st 200 hrs the emission is increasing.  After 200 hrs,  
THEN the fil V can be reduced. 

##  In most cases, the fil v can be reduced by 15%.   I not only ramp up the  fil V.... I also ramp it down.  Blower is run for a few mins with no fil V,
blower over-run.   The above procedure  will  extend the life of the bigger metal tubes  by a huge amount.   As the emission slowly drops off, the fil
V can be increased.   It can be increased  from –15%   to   + 5 %.    Then the fil is toast..and needs a rebuild.   But to get  from –15%  to + 5% takes 
a looooong time. 

##  I would still minimize the on off fil cycles if possible.... and still use ramp up-down  each time.    IE:  turn it on after dinner...and leave it all on till bed time.

Later........  Jim  VE7RF