The tricky part of tuning any transmitter is, of course, the Final Amplifier. We learned in basic transmitters 201 (prerequisite black magic 101 or voodoo 102) that the proper procedure for tuning a tube amplifier is "peak the grid, dip the plate, adjust the load for maximum output". You probably memorized this till you say it in your sleep.
When we get to any broadband transmitter, such as a TV transmitter, we aren't trying for a nice clean, narrow bandwidth peak anymore. Instead, we are looking for a flat, linear peak over a broad spectrum (6 MHz for Television). This poses some particularly peculiar problems (say that 10 times fast)! The entire tuning procedure must now be changed to afford us a comprimise between Q and Power. (Q being the sharpness of the peak). We want a minimum of Q with a maximum of power. Problem is, most tubes want to increase Q and power simultaneousely.
When we tune a tetrode for maximum Q (the normal for a transmitter in narrow bandwidth applications) we are looking at a minimum of grid current (Ig) A maximum of Screen Current (Is) and a minimum of plate current (Ip). As we adjust the coupling of the tube, we find that Q begins to deteriorate, and bandwidth begins to broaden. We also notice that the overall power of the tube also declines. At the same time, the SWR rises, so we must adjust the loading to compensate for this. adjusting the loading also has an effect on the linearity of the overall broadband signal.
So what exactly are the procedures that we need to go through to accomplish all this magic?
Here they are, as best as I can describe them:
Pre-Tuning Procedures :
Get a copy of your tube's manufacture specs and get familiar with the tube -
don't exceed the tube's maximums if possible.
Make sure that all interlocks and temp sensors are functional
Set the filament voltage to factory reccommended setting (usually 6 - 7 VDC)
Reduce the drive power to about 25% (standard practice for tuning ANY transmitter)
If they aren't already close - adjust the grid, screen, and plate voltages to factory specified settings.
Make sure that your VSWR is low - if it is high with the drive down - you REALLY have a problem! (Check your antenna and transmission lines).
Now for the MEAT! How to tune the blasted thing
If you memorize this memory aid - it will help you along:
Presidential Suite, Lucky Couple!
How will this help? Because that is the tuning order for a tetrode in wide band usage. PSLC.
Reduce the drive power to about 25% (standard practice for tuning ANY transmitter)
Connect a network analyzer to the input of the tube, and examine the reflected signal coming from the directional coupler.
Inject a wide band swept signal from your network analyzer that is at least as wide as the bandwidth of your desired signal (For 6 MHz, use about a 7 - 12 MHz signal). Keep in mind that the wider the signal you inject, the more "understanding" of the tubes signal you will have. But for final tweaking, narrow things down so you can pay particular attention to details. Note - in lieu of a network analyzer, a sweep generator or tracking generator can be used to inject the signal, with a spectrum analyzer used to monitor the output/return.
Tune the plate to Fc (Carrier Frequency)
Tune the secondary tuning to close to the band edge (Fc + 4MHz for video). This should widen out the signal. Don't expect it to look flat at this point. Flatness (linearity) isn't the object just yet. As a matter of fact, many tubes will never be completely flat, but will look like a "saddle" when you are done. Keep in mind though, that you want the "seat" of the "saddle" to remain flat for as wide as possible with the top of the shoulders barely exceeding your max desired bandwidth (5 MHz for Video). Now comes the tricky part.
Adjust the loading for the LOWEST Q over about 6 MHz bandwidth (for video) of the entire band (make the saddle FLAT). Note that it may not be perfectly flat, even when finished, but "flatness" is the goal here. Note that Loading SPECIFICALLY adjusts for the "height" of the saddle.
Adjust the coupling control for the LOWEST possible Q over the same 5MHz bandwidth (again - make the saddle FLAT)over about 5 MHz bandwidth of the entire band (make the saddle FLAT). Note that for the most part, coupling adjusts the "width" of the saddle.
Remember that a couple can rent the PresidentialSuite once, and be a lucky couple several times a night! So you may have a "Presidential Suite, Lucky Couple Lucky Couple Lucky Couple" scenerio. Addjust loading and coupling several times to make things look right. If the saddle isn't flat - go back to steps 5 & 6 and - make the saddle FLAT !!!
If your tube and cavity are working correctly, you should only have to tune the plate and secondary once. If after doing loading and coupling several times, it still doesn't quite look right, try starting over.
Post Tuning Checklist
Double check the following:
Filament. grid, and screen voltages should not have changed drastically.
You should now have Just Enough headroom
Just Enough bandwidth
Low VSWR
A minimal plate current at ZERO signal condition
LOW temperatures on the tube seals (if you hear this sucking sound - you probably don't need to check this, as your tube has just gone gassy and destroyed itself.)
If all this is right, your tube is probably working correctly. You can now begin to raise the drive to the tube. Remember to keep an eye on the Post Tuning Checklist and shut 'er down and start over if anything starts looking quirky.
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