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Old March 5th 07, 12:21 AM posted to
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First recorded activity by RadioBanter: Aug 2006
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Default Tube equipment question

On Mar 4, 12:53?pm, wrote:
On Mar 4, 12:48?pm, Mike Coslo wrote:

Some may tell you that all it takes to tune up a tube
transmitter is to "peak the grid and dip the plate", but
that's simply not correct. Tuneup procedures vary
according to rig design and you have to be specific.

Sorry, but that was procedure for pre-WW2 high-
power HF transmitters. While it is a simplistic
phrase, it still applies. A more exact procedure
was to tune up the exciter with reduced drive
power and literally peak the grid current. Plate
current was then observed with the plate tuning
adjusted for a slight, but observable dip in plate
current. Of (perhaps) greater importance was setting
the neutralizing control for minimum grid current;
"dipping" the plate current should produce the least
grid current peak on adjusting the plate tuning.

For best results on setting the load-side capacitor
of the common pi-network without a bidirectional
power meter, a detector way out in the field with
meter next to the transmitter is the simplest way
to "tune" that capacitor. However, with about 34+
other high-power transmitters all in the antenna
field, that is impractical; presets for that control
would suffice. The load capacitor of a pi-net has
the least effect on tuning to a new frequency.

When someone does about two QSYs per shift
on at least 15 different transmitters with pi-network
output circuits (all with vacuum tube PAs), yes,
one "gets used to it" but what I described was the
correct phrase.

The pi-network has been around and used in HF
transmitters since at least the late 1930s and has
survived past the start of the semiconductor era.
However, the convenience of broadband transistor
power amplifiers has pretty much tossed that whole
tube tuning procedure. Used with a Bruene detector
sensor for an automatic antenna tuner, it makes
QSYing a snap, even jumping bands (with a broad-
band antenna, of course).

"Peak the grid and dip the plate" is an old correct
phrase. It will be found mentioned in the current
US amateur radio question pools.

Yes, there are exceptions. I was once involved with
a distributed amplifier design that would cover over
an octave of spectrum using tubes and was NOT
tuned at all in normal operation. Since that one
involved over a dozen vacuum tubes (ceramic-metal
medium-power types), it would not be suitable for
ordinary amateur radio HF transmitter stations. The
vertical amplifier of the old Tektronix 54n series
oscilloscopes used push-pull tube-type (all glass
envelope "receiving" type) distributed vertical deflection

The pi-network output circuit was a favorite among
amateur homebrewers for decades due to its
simplicity and better ability to attenuate harmonics,
that coming to be more and more prominent in
regulations as HF users became more plentiful.