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Old March 5th 07, 06:08 AM posted to
[email protected] is offline
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First recorded activity by RadioBanter: Feb 2007
Posts: 18
Default Tube equipment question

On Mar 4, 11:21 pm, "
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.

Sure it was.

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.

A number of amateur transmitters/transceivers have used a quick
peaking of the drive/preselector control followed by adjustment of
tune and load controls for maximum output, keeping readings within
operating parameters. I've never seen an amateur transmitter with a
front panel neutralizing capacitor. Neutralization is normally a set
and forget procedure which one needn't worry about until the final
tubes are replaced.

Tuning for maximum output for a given amount of drive has become the
norm in tuning high power, vacuum tube linear amplifiers. All one
needs do is make certain that the bottles don't aren't drawing too
much grid current. A check of linearity can be made with the station
monitor 'scope.

The load capacitor of a pi-net has
the least effect on tuning to a new frequency.

That would depend upon the antenna being used and the amount of
frequency change as well as the type of equipment being used. Some
manufacturers switch in some fixed capacitance on various bands or
portions of bands.

The pi-network output circuit was a favorite among
amateur homebrewers for decades due to its
simplicity and better ability to attenuate harmonics...

....but the Pi-L did a much better job of attenuating harmonics with
only a little more circuit complexity. Quite a number of Novice
licensees found themselves in receipt of OO notices or letters from
the FCC when using a simple pi-net output tank with a multiband

Dave K8MN