This
unit was purchased by me for $15 at the Rosebud Hamfest in 2013.
It is a piece of world war two heritage. I generally do not
collect or want WW2 relics because my shed is already too full of junk
and any further junk addition must perform a usefull function for
me, as I do not run a museum. It did not know when I bought this
that it was constructed in 1944 according to the General Radio Catalogue. The active ingredients in this unit are the butterfly resonator
and an instance of the famous Western Electric Door Knob
triode, manufactured originally in the early 1930s for the microwave
research community. Mine is a slightly more recent General Electric war version.
So after a long sleep of 70 years , would this antique work again ? Yes !
It
was possible to download the GR catalogues and a service note from IET
laboratories that maintain the GenRad heritage. The filament of this
antique triode is a thoriated tungsten directly heated cathode that
wants 2.2 volts at 3.5 Amperes, an odd voltage requirement
to be sure. I had to hand wind 30 turns through a surplus
toroidal mains transformer ( a great way to light up old
Transmitter Bottles with odd filament voltage ). The original
unit was meant to be operated with the GenRad custom power supply which
included a grid leak resistor and metering of grid current with a magic
eye tube, this being used due to war shortages rather than a more
usefull moving coil meter. This could make the instrument behave
like a grid dip meter.
It tunes from 90 Mhz up to the dizzying
heights of 515 Mhz which was considered microwaves in
those days. The anode on the triode is made from molybdenum
and can dissipate about 5 watts of power. The oscillator was
rated to produce up to 1 watt of output power. At this power
level the anode will glow dark red. The real gem in this unit was
the resonator that permits 2 octaves of tuning range without band
switching ! Thats why the resonator design was patented !
Just try tuning 100-500 Mhz with a variable capacitor or varactor...it cannot be done.
The
oscillator despite its single triode has remarkable frequency stability
due to its solid cast metal construction and the extremely high Q of
the butterfly resonator. After 1/2 hour of warmup this can be
netted on a narrow UHF chanel and it just stays there. Absolutely
amazing ! Even better with a stabilized power supply. Frequency
stability is still a function of output coupling as you would expect.
Powering up for the first time in 70 years.
The
filament is meant to run at bright yellow heat and emits a lovely
bright warm glow in operation. Original cost in 1944 was $250 US
dollars, a small fortune at that time. I let the filament
condition itself. Really antique thoriated filaments should be
run at their rated current for a few hours without applying high
tension. This gives time for the thorium to migrate to the
filament surface and permit outgassing. It is the thorium
atom layer that supplies the electrons, not the tungsten, which
is an electron emitter only at white heat. The initial current
draw at 250Volts was 70mA which is way too high. This
stabilized by itself in about 30 minutes. I expect that this was
due to filament outgassing and the tramp gas before making its
way to the getter. Do not permit anode hight tension to remain if
the oscillator stops. Otherwise ,Substantial and destructive
anode current will flow because grid leak bias will not appear to limit
anode current. The anode is Molybdenum and will tolerate a some
abuse. The output coupling link can be rotated to vary the
output power. It should not be rotated all the way in, this will
result is too much power being removed from the resonator quenching the
oscillation, particularly at the upper frequency limit of the
unit. Frequency stability after a long warm up period is superb.
The General Radio instructions specify 350V for the anode high tension, mine works fine at 250V.
I
used carburetter cleaner to dissolve and remove the ancient,
completely dried out grease on the gearing and put a little dab
of modern teflon grease on the worm. The sealed ball bearings
were still serviceable.
This oscillator would have
been used for deci-metric radar development, it would have been
too expensive for day to day field alignment work. I wonder what it was
doing in Australia ? It will find a second life in the shack for
tuning my UHF and VHF antennas.
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