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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Modulating the grid is called "efficiency modulation". The reason is that, in order to get the proper operating conditions at modulation peaks, you have to run the tube in an inefficient condition at zero modulation.
You can think of it like plate modulation with a single-ended Class-A modulation amplifier, except the modulating and RF amplifying tubes are the same tube, running as a linear amplifier instead of a saturated switch. The efficiency calculations are the same.
By this argument, you can see that plate modulation with a push-pull Class-B modulating amplifier gives you a big jump in efficiency. The RF tube wastes less power because it's running as a saturated switch the whole time, and the modulation amplifier wastes less power because it's Class-B.
As Tesla coilers we might not care about efficiency, but it equates to more RF power output for a given size of RF amplifying tube. Assuming of course the power output is limited by plate dissipation, which may not be the case in VTTC work with short runtimes and monstrous Soviet military vacuum tubes.
Richie Burnett was making Class-E SSTCs with high level modulation by a Class-D amplifier years ago, and many hams are now using this setup for AM on 160m and 80m.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve- The tank circuit of a VTTC tuned the usual way (the lower pole) behaves close to a constant voltage sink, so a little change in voltage swing means a big change in power output. Because of this, the grid modulation of a VTTC is much more efficient than it would seem on a first look...
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