[Amps] 10 kw CCS on 6M - High Power Amplifier Installations

Sat Sep 17 04:44:39 EDT 2022

The Aspidistra transmitter was built by RCA, but I have never seen any detailed description or block diagram over it.
It may however be suspected that it used the "tuning drawer" or "tuning waggon" concepts, which were pioneered by Marconi in the 1930s, were used. 

Here, pre-tuned circuits could be manually exchanged in a very short time to establish proper conditions on the new frequency.
The Aspidistra transmitter operated on a quite limited number of frequencies used by the German Home Service broadcast transmitters, "Deutschlandsender" or "Reichssender", which were known in advance so they could be prepared.

The medium-frequency AM transmitters used here were tuned into quite low-Q antenna systems that did not change much with weather or ice.  Only one high-power MF transmitter was installed  (1984) during my time at Swedish Telecom Radio, the 600 kW Telefunken S4006 on 1179 kHz which used a phased-tower directional array. 

It was specified to handle a VSWR of 1.3:1, but could be adjusted by servo-motors to keep it below 1.2:1 if the need would arise.
This was the only broadcast transmitter in recent times which operated into some form of tuned antenna.

The Thomson-CSF HF transmitters operated into either curtain antennas from TCI or log-periodics. It was the flash-over risks in the feedlines and rotary joints that ultimately limited the allowable VSWR, if I recall correctly the limits were set to 1.5:1 for the log-periodics.

All HF broadcast transmitters here operated at only day-time staffed sites, and the frequency changes were entirely automatic.
Due to energy costs, the transmitters were switched off during the idle-time, and a stored time-table at the central broadcast control site started and stopped the transmitters via remote control.

The 500 kW Thomson transmitters at the Karlsborg site that I am most familiar with had a local programming table where frequencies and antenna directions were stored. 

When the "start" command for a given frequency was received from Stockholm via remote control,
the frequency was entered into the exciter, the antenna started to rotate to the direction associated with the frequency, and the main power switched on. 

After about 5 minutes, the warm-up cycle was completed, and the servo motors in the transmitter had reached their preset positions, and reduced power, about 100 kW, was ordered and the output power checked. If nothing dangerous happened, full power became applied and the programme feed was connected.

When the programme for a frequency and direction had ended, the transmitter was ordered to stand-by, a new frequency entered into the exciter and the antenna rotated. After this, the procedure was repeated. 

The transmitters were remarkably reliable, after some teething problems were solved after installation in 1974, they could operate unattended for months on end. Regular scheduled maintenance was cleaning of air filters and checking the tube operating points bi-weekly and monthly the frequency accuracy, modulation depths and audio distortion were checked. 

Once a year, the transmitters were taken off the air for a week, cleaned internally, inspected for wear of moving parts and power tubes exchanged if needed. 

In 1993, the high-level modulated Thomson-CSF AM transmitters were taken off the air due to maintenance and energy cost reasons, and replaced with new ABB (Brown-Boveri) transmitters that also permitted SSB and DRM transmissions at the Hörby site only. 

The Karlsborg site was then de-manned after the remaining Telefunken transmitters for HF air-ground services were replaced with solid-state amplifiers. 

All staff members were retired.

My own part in this was that I worked with engineering management at the Head Office of Swedish Telecom Radio, who owned and operated the HF transmitting sites, during the 1980s and 90s.

73/
Karl-Arne
SM0AOM

----Ursprungligt meddelande----
Från : cw_de_n5op at sbcglobal.net
Datum : 2022-09-17 - 01:54 ()
Till : amps at contesting.com
Ämne : Re: [Amps] 10 kw CCS on 6M - High Power Amplifier Installations

I want to thank everyone for sharing their wonderful experiences with 
very high power transmitters.

My curiosity was piqued especially when I read about the 600 kW British 
Aspidistra MW transmitter used for "black propaganda" in Geoffrey 
Pidgeon's "The Secret Wireless War." What struck me about it was that it 
had frequency agility and could be tuned to different MW frequencies to 
avoid jamming, etc. I have no idea if it was channelized but that seems 
likely. Still, the antenna matching network had to also be adjusted when 
the frequency was changed and I have to wonder how the match was kept 
acceptable as the wind blew or when the antenna accumulated ice.

I can only imagine that AM BC stations must have to deal with that. How 
much of a mismatch can big transmitters tolerate? And I've always 
wondered how the huge shortwave BC stations were switched back on based 
on their schedules. With this much power, it's a delicate business!

73,

Kim N5OP

On 9/13/2022 3:04 PM, sm0aom--- via Amps wrote:
> Thanks,
> I can also add that the Telefunken transmitters used Wye-Delta switching for reducing plate and screen voltages during tune-up.
>
> Still remember the tune-up sequence from a "cold start" by heart:
>
> 1. Heat the cathodes for two minutes. Seemed like an eternity for the operators...
> 2. Apply RF and wait a few seconds for the band-switch and tuning servos to reach their preset positions
> 3. Apply plate voltage in Wye connection and wait for the servos to converge, starting with the servos in the driver stages
> 4. Apply plate voltage in Delta connection and wait for the fine-tuning to converge
> 5. Adjust the drive control until full power is reached and the "ready" condition is back-signalled
> 6. If reduced power (1/3) is selected, set the plate voltage control to "Wye"
> 7. Remove drive and wait for "key" command from the remote control
>
> A complete cycle took about 30 seconds after warm-up, and the procedures were almost identical between
> both the 30 kW and 100 kW transmitters.
>
> 73/
> Karl-Arne
> SM0AOM
>
>
>
>
>
> ----Ursprungligt meddelande----
> Från :jtml at losalamos.com
> Datum : 2022-09-13 - 17:22 ()
> Till :amps at contesting.com
> Ämne : Re: [Amps] 10 kw CCS on 6M - High Power Amplifier Installations
>
> 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 at telia.com"  <sm0aom at 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 at 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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