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Message: 1
Date: Tue, 13 Sep 2022 07:13:53 +0200 (CEST)
From: "sm0aom@telia.com" <sm0aom@telia.com>
Having worked with really big transmitters; Telefunken 100 kW ISB and
Thomson-CSF 500 kW carrier AM,
tune-up works in very similar ways for all practical purposes.
First the low-power drive stages are peaked up for maximum drive voltage to the
penultimate
driver grid. After this, the procedures differ somewhat between linear
amplifiers for ISB and AM transmitters that are
Class C in the drivers and PA stages.
A servo-tuned ISB transmitter uses the phase relations between the grid and
plate circuits to establish resonance,
and then is the plate circuits and their coupling circuits adjusted to present
the proper load-lines to the tubes.
Finally, the drive level is adjusted so the proper output power and linearity
is reached.
In order for this to work properly, the tuned circuits need to be pre-set quite
closely to their final positions.
This was mostly accomplished by measuring the input frequency and use this
information to access tables of stored
tuning information, either established after the last successful tuning
operation, or at the design stage of the amplifier.
"Before computers" this was done by dividing the tuning range of the amplifier into many
sub-ranges and then use the measured frequency for choosing one of the ranges with its preset
information. After this, "Auto-tune" mechanisms with servo motors were first brought to
their initial positions, and after this phase and load-line discriminators took over control.
Class-C transmitters used grid and plate currents to establish resonance
conditions, and were tuned progressively from the drivers to the final stages
at reduced power. As the final step, the drive was increased to reach the
proper carrier output level before high-level modulation could be applied.
The Thomson-CSF transmitters did not servo-tune in the proper sense of the
word, but used stored settings for each frequency. Instead, the servo-motors
brought the tuning elements to their pre-set positions without any room for
adjustments. This worked well because the tuning settings become quite broad
when operating into a broad-band antenna, and there are no critical linearity
specifications for a Class-C AM transmitter.
73/
Karl-Arne
SM0AOM
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Message: 3
Date: Tue, 13 Sep 2022 09:56:44 -0500
From:donroden@hiwaay.net
I spent 40 years maintaining Broadcast transmitters. It was always my
preference to get the final tube(s) close to their operating points
before working my way back to the lowest power drivers. Keeping the
screen voltage low, or keeping the grid voltage near or beyond cutoff
while using a sensitive voltage or power meter to find the matched
output network settings resulted in a much reduced stress level..... for
both me and the tubes. Cathode over-current relays were set at 1/3rd to
1/4th typical operating conditions until relatively stable operations
were achieved. We always worked from the highest powered stages
backwards to give the lower powered stages a load to work into.
Lowering filament voltages during tuneups to below operating levels also
helped to keep the "bang" factor low. W4DNR