When I used to work at E.I.DuPont Co., they continued to suffer over
loosing the tradename nylon years ago, for their fiber. Teflon also,
but they continue to fight unapproved use of that tradename.
Back to ham radio, you can parallel variacs without ballasting, but
Superior offers a set of balancing chokes to use in extremely large
installations. But if the setscrews work loose on the wiper on the
shaft, a lot of current will flow between the parallel stacks, and
cause problems. They have to be closely synchronized, so that only a
few turns maximum are left between parallel windings. Otherwise the
arcing and burning begins. Bring out the marshmellows.
At 11:47 PM -0500 11/5/99, Amps Digest wrote:
>Coming back down to amateur size, I'm currently using a fixed 100-ohm
>resistor in the 230V primary of a 15V transformer, to power the 12.6V
>heater of a GS35b. That is slightly too high a resistance, and in the
>process of selecting a suitable resistor of about 1k to connect in
>parallel, I noticed that the 2nd and 3rd places of decimals on the
>heater voltage continued to vary for some time. Presumably the heater
>was acting as its own resistance thermometer, while the whole
>heater/cathode mass was coming up to temperature.
Interesting observation!
As for using SCR regulators and AC on filaments, in big tubes one has
to be aware of mechanical resonances in the large filaments, nearly
audible. If the wires begin to ring, they can short out to the
adjacent grid. Eimac and others address this barely in their
datasheets. Smaller tubes (with hand sized handles or none) have a
much higher mechanical resonance and are quite stiff internally, and
there is little chance that a ripple harmonic will set it into
dangerous vibration. The big tubes have springs and other
compensators to account for filament sag as it heats up.
73
John
K5PRO
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