[AMRadio] 4-1000A Grounded Grid Class-C


Donald Chester k4kyv at charter.net
Thu Oct 3 14:21:46 EDT 2013


> From: Rob Atkinson <ranchorobbo at gmail.com>

> So is a grid driven class C final different from the cathode driven
> example (as pertains to feed through issues)
> because grid driven takes less drive power for a given output?  I mean
> do you escape the feed through power problems with grid driven because
> the drive power is a lot less?


No, the amount of driving power required has nothing to do with it. The 
feed-through is inherent to the g-g circuit.  With grounded grid, the 
output appears from the plate of the tube to ground at the plate tuning 
network just as with a grid driven amplifier, but the grid drive is 
inserted between the cathode and ground. The actual output delivered by 
the tube still runs from plate to cathode, effectively placing the input 
(rf drive) in series with the output load as far as the tube is 
concerned, "ground" merely being a reference point in the series circuit 
between cathode and plate. Thus with a steady carrier driving the 
cathode with respect to ground, when the actual plate output from the 
tube reaches zero at 100% negative modulation peaks, the steady carrier 
feeding the input remains in series with the load, allowing most of that 
driving power to appear at the plate tuning network (total driving power 
minus the power consumed in the grid circuit).

Driving the negative modulation peaks past cut-off during overmodulation 
still results in splatter and distortion, whether the rf that appears in 
the transmitter output at cut-off is zero at baseline, or at some fixed 
minimum determined by the amount of feed-through. In the latter case, 
the scope may not show modulation in excess of 100%, but you are still 
attempting to exceed the modulation capability of the transmitter.

Don k4kyv


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