Date: Sun, 20 Dec 2015 23:17:49 -0700
From: John Lyles <jtml@losalamos.com>
To: amps@contesting.com
Subject: [Amps] Tube wear out phenomena
Thorium is added at the 1% level to pure tungsten, to improve efficiency
of electron emission from the filament of direct heated tubes. It allows
a reduction of the temperature of a filament and reduced heating power,
which all contribute to longer life over tungsten filaments. It is not
added to the other metal parts of tubes.
It is true that the number of turn-on events is a factor in the lifetime
of large tubes. Without overloading or misusing the tube,
filament emission lifetime should be the primary end of life event. When
the filament is energized and deenergized repeatedly, even with careful
current limiting and ramping of voltage, mechanical changes occur to the
'basket' and lead to sagging and growth in diameter at the bottom. Given
enough of these events, there is a finite possibility of shorting
filament to control grid in large modern high gain tubes (where the
spacing is already very close). Hence, it is better to leave them
operating, and reduce the voltage when going to standby, but keeping
some heating, sometimes called black heat mode.
Operating a high power tube below it's rating doesn't have an effect on
lifetime, unless the filament is operated far below design which can
result in damage. On the contrary, operating below rating usually does
contribute to lifetime extension, especially if the filament voltage is
managed so that emission is not excessive, far beyond the peak plate and
cathode current desired in the operating point.
I have some amplifiers that run over 100 kW at HF, using modern
tetrodes, and they routinely exceed 30,000 hours life before
replacement. They are used for peak current, but the average power is
quite lower than their rating.
73
John
K5PRO
## folks who have used fil xfmrs that are rated for much greater current than
required tell me
they have ended up with grid to fil shorts. IE: using a 30A fil xfmr for a
single 3-500Z etc.
These were cases where no fil step start was used. Typ fil xfmrs will
usually, but not always,
limit the peak inrush current to a value equal to double the normal
operating current.
## some will use a small rheostat in one leg of the fil xfmr primary.... to
reduce the fil V a bit...and
it also offers some inrush current protection. Some will use a high Z type fil
xfmr to limit max inrush.
Other methods include manually operating a varaiac. or motor driven variac.
Other schemes
include a SS ramp up of the fil voltage. Another method... a buddy used on
his hb 3CX-6000A7
was a step start resistor in the fil xfmr pri and a variac. Variac used to
fine tweak the fil V. Once set,
it was not touched. Then the step start resistor + timer was used to do the
step start. Direct heated
tubes for the above.
## You would be surprised at how low a fil V you can use for normal operation
on a new or rebuilt tube.
This is all for direct heated GG triodes. Both eimac and svetlana say to
run the fil v at normal rated voltage,
measured at the socket, for the 1st 150-200 hrs. During that time, the
emission increases a bunch, then reaches
a peak, then level off. At that point, ( we use 200 hrs total fil on time),
the fil V can be decreased. The procedure
we used was to reduce the fil V till the PO just started to drop off by a few
watts. Then the fil v is increased by .1 volt.
## If just ssb / cw is used, we just pulse tuned the amp...and reduced fil
V till the pep watt meter dropped off a few watts.
If rtty /FM used, then the full cxr is used, but same procedure used. Its
all checked every few months. So far after several
years of use, and several thousand hrs of fil on time, the fil V has not had
to be increased. The 3x6 requires 7.0 vac..and
its been sitting at 6.1 volts for years now. Since the getters are fil heat
activated, the fil V is brought up to normal 7.0 vac
twice a year for a day or two..then reduced back down to 6.1 vac.
## Fil V is measured by installing small 100 uh chokes in each leg with some
test leads..with chokes at the socket end.
Other end of the test leads is wired to the panel to a pair of banana jacks.
Small value resistor, like 100 ohms, is wired into each
leg of the test leads, just prior to the pair of test jacks. This ensures that
in the case of an accidental short at the test jack end, nothing
bad will happen. The pair of resistors will not affect the external meter
reading. A fluke 87 is plugged into the pair of test jacks.
The pair of chokes keeps drive RF out of the DVM. A .01 uf is wired form
each test jack to chassis.
## A surplus sola constant V xfmr is used to feed a steady 120 /240 to
the input of the variac / step start resistor.
Output of that mess feeds the pri of the fil xfmr. A 110-120 vac @ 60 hz,
hr meter is wired b4 the variac, usually
between one hot leg and neutral.
## b4 the tube was put into operation, it was run at rated 7.0 vac fil V for
a full 48 hrs..with just the blower on..and no
B+ and no drive. That was to fully getter the tube, b4 B+ and drive applied.
After 48 hrs, tube put into normal operation.
After 200 hrs, fil V was reduced. Same concept was also used on a 3CX-3000A7
tube. I don’t bother with the 3-500Z tubes,
except to use step start, fil V is left alone.
## I investigated the black heat concept, and came to the conclusion for typ
ham use, even if the tube was used for
2-8 hrs per day, its probably not worth the effort to used black heat. Which
would involve running the fil V at aprx
3-5 volts during the times it was not in use. The blower would still be
required, albeit its speed reduced.
I can see the merit for the use of black heat for large SW broadcast tubes
etc...where the total off time isnt very much,
and or when the broadcast tube is used for say 4 hrs on, 2 hrs off, then on
again for 3 hrs, then off for a few more hrs etc.
At that point you may as well leave the fil on in the black heat mode during
the off periods.
## You can read detailed reports of the black heat results for SW broadcast
use in the various literature, esp from the UK.
Jim VE7RF
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