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Old March 4th 07, 09:53 PM posted to
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First recorded activity by RadioBanter: Feb 2007
Posts: 18
Default Tube equipment question

On Mar 4, 5:48 pm, Mike Coslo wrote:
Solid state transmitters are notoriously finicky about matching to the
antenna. Tube equipment is not, so I am told, and early experience seems
to bear that out.

I don't know if "finicky" is a good term, Mike. "Different" might be
the best word to use. Transmitters with vacuum tube finals can match
a wider range as a rule, but there is a limit to what they can
handle. Some of the older Johnson and Globe/WRL rigs matched a wider
range than some of the other brands.

Certainly I can see one of my newer rigs start to fold back at 2:1.

Most Japanese gear is designed to do just what you're experiencing.
That's a form of protection for the final transistors. Ten-Tec does
it differently. Ten-Tec rigs do not start to reduce power. They
depend upon the supply to fault and trip if too much current is
drawn. That's why it is important to use a Ten-Tec supply with them
or to use a fast breaker rated to trip near the maximum current draw
expected of the transmitter.

What are the practical limitations of the Tube finals apparent

That depends entirely upon the design specifications set by the
If the manufacturer's manual says "45-90 ohms", that's the practical
Some of those old Globe transmitters used to have ranges of up to 1000
or 2000 ohms as I recall. At the other end of the spectrum,
Hallicrafters produced some transmitters without even a loading
control. These were designed to be used with an antenna presenting
something very close to 50 or 75 ohms.

Is it safe to compare the load, plate, and drive controls to some of the
functions of a tuner? (possible real dumb question)

Not the "drive" control, but certainly the other controls of a tuner
could be considered comparable to the "tune" and "load" controls.

If the only antennas we have are trapped verticals, tribanders or
dipoles/inverted vees cut for the band to be used, we might get away
with not having to use a tuner at all. Still there are likely to be
frequencies significantly removed from the antenna's resonant
frequency where one might need a tuner to enable the transmitter to
make full power. We aren't likely to need a wide range tuner for
those times. A simple T-match will likely enable us to find a
combination of settings which will present a low VSWR to the
transmitter. The main alternative is to have a variety of antennas
which present a VSWR of under 2:1 to a transmitter.

Dave K8MN