I lost a couple of good TH347 tubes due excess filament current at turn on.
I thought there was sufficient turn on current limiting but apparently
not.
A circuit was devised from readily available parts that slowly turns the
filament on over a 2 minute period of time.
The filament transformer is a 50A unit; the tube needs 34A and that makes
the problem worse.
The circuit has a 100 ohm power resistor in series with the 120VAC
transformer primary. A Dayton timer is used to operate a Radio Shack 120vac
relay
after 60 seconds. This relay switches a pair of series connected Keystone
surge limiters (from Mouser) across the 100 ohm resistor. Two were used
because one did not take enough time to reach a low resistance.
A second timer switches on a second relay after another 60 seconds. This
relay shorts both the 100 ohm resistor and the current limiter. With the
resistor and limiter shorted, full voltage is applied to the filament
transformer. Additionally, the 100 ohm resistor and surge limiter will begin a
cool
down cycle making them ready for another limiting cycle if needed.
The maximum filament current happens when the second relay closes and
everything is fine as the filament is up to temperature by then.
In series with this circuit is a 10 ohm rheostat that is used to adjust the
filament voltage to the needed 5.9 VAC.
73,
Gerald K5GW
In a message dated 8/28/2009 10:53:35 A.M. Central Daylight Time,
ka4inm@tampabay.rr.com writes:
> The ramping is a lot easier to do
> with DC supply than AC (which might use variacs, saturable reactors, or
> other ancient technologies). I use an SCR regulated mains supply with
> good filtering.
Filaments can just as easily be ramped up on ac, this way with no
fuss. I have seen ferro-resonant regulator feeding the filament
transformer, the size was carefully chosen so the low resistance of the
cold filament "bogged down" the regulator to a few % out, as the heater
warmed, the resistance rose as did the output of the regulator until
opium temperature, current and solid regulation were achieved.
I first saw this on 8.5 Volt 25 Amp Varian klystrons.
--
Ron KA4INM - The problem with socialism is that eventually you run
out of other people's money.
[Margaret Thatcher]
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