Date: Tue, 25 Jun 2013 09:14:39 -0400
From: "Carl" <km1h@jeremy.mv.com>
To: "Mike Waters" <mikewate@gmail.com>, <amps@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
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