How To Tune A

Tetrode Transmitter

By: Ray Dall

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 :

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.

Plate (Primary) Tuning, Secondary Tuning, Loading, Coupling.

Post Tuning Checklist

Double check the following: 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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This Course was written by Ray Dall © All Rights Reserved.
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And for what it's worth... this page was last updated HexDate 01-11--7D1

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