Et all:
I second John's sentiments here. This is more like the kind of discussion I
envision for a reflector. A question is posed, and those who have the
background, share their knowledge, which educates those of us who are not
experienced about the given topic.
Thanks to everyone who weighed in.
An interesting part of the WLW history, is that Adolph Hitler was quite aware
of the station. and referred to it and Cincinnati as "liars", because WLW
presented information to folks in/around his country/worldwide, that did not
support his agenda.
It brings a smile to my face to know that a local station was a somewhat of a
PIA for him, during that terrible time in world history.
Don
On Tuesday, September 13, 2022, 11:22:26 AM EDT, John Lyles
<jtml@losalamos.com> wrote:
Thanks Karl-Arne and Don for sharing insights in tuning up high power
communications xmtrs!
John K5PRO
> ----------------------------------------------------------------------
>
> 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
> ---------
> 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
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