Wonder if any study has been given to the life of these lamps when the turn
on current is limited. In other words, would the "about ready to fail" time be
increased by providing some turn on current limiting protection? Maybe the
turn on cycles wears the lamp out more than the actual on hours?
No one would ever do that to help the life of a 50 cent light bulb but how
about a $1000 transmitting tube?
I run a TH347 tetrode on 1296MHz that qualifies (begs) for some filament
protection. The tube manufacturer warns in the data sheet to limit the filament
current to something like 2X for the first a.c. cycle. Taking this a step
further I decided to limit the current to less than 1X at turn on. This was
accomplished by connecting a couple of 100 ohm surge limiters in series with
the
filament transformer primary. With 200 ohms in series, a maximum of .6 amps
flows in the primary and 12 amps in the secondary. The tube is spec'ed for 34
amps filament current so this is very conservative current limiting. About
three seconds after turn on, the filament volt mater begins to rise and reaches
about 5.8 VAC in maybe five or six more seconds. At this point, a Dayton
Solid State Timer connects AC to a small DPST relay. One set of relay contacts
shorts the surge limiters, allowing the full 6 VAC to the tube filament and
the
other set closes a control loop for hv and ptt circuits.
Shorting the surge limiters allows them to cool to cabinet temperature,
making them return to 200 ohms and ready for the next turn on cycle. Also, I
assume that the lifetime of surge limiters is extended by not allowing them
sit
and cook for hours on end.
This is so simple and inexpensive that I intend to add the circuit to some
other 8877 VHF and HF amplifiers that normally don't use filament surge
protection.
If anyone is aware of a downside in using such a circuit I would appreciate
hearing from you.
73,
Gerald K5GW
In a message dated 2/11/2007 3:47:59 P.M. Central Standard Time,
g0fvt@hotmail.com writes:
Hi,
not much experience of this, however I once worked for a lamp manufacturer,
there was a tendency for lamps to fail at switch on ( I believe we have all
seen this).
The cold resistance of a typical tungsten filament is roughly 14 times lower
than the running resistance so if the supply impedance is low the inrush
current is in the region of 14 times higher than the running current. I
think the failure mechanism is related to the uneven rate of temperature
rise whereby some parts of the filament briefly run much hotter than normal
as some other parts are still warming up. I believe I have seen this in
tubes/valves whereby sometimes the filaments go quite bright on switch on
and subsequently appear to go dimmer.
With the life testing of lamps it was acknowledged that lamps that failed on
switch on had little life left anyway, I believe the life testing schedule
called for one interruption to the power source for 30 mins each day to
simulate a reasonably realistic situation.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|