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

Steve WA1QIX wa1qix at piesky.com
Thu Oct 3 13:18:16 EDT 2013

It's a little more complex than just the drive power.  The power "return" 
for the tube is the cathode, so the cathode is always in circuit.

The only feed through power you will get with a grid driven arrangement is 
that caused by insufficient neutralization.  Some triodes such as the 304TL 
or the 450TL as examples take a great deal of drive power, but this power 
never appears in the output if the amplifier is properly designed and built 
because a grid driven tiode will require neutralization.

If your amplifier does not yet exist (this is a new construction project?), 
you are seriously better off using a grid driven topology.  I have a 
personal preference for triodes, but they do require more drive and are 
harder to neutralize, and on the higher bands might be harder to work with 
depending on the tube.  If you are staying on 40 meters and lower, you have 
nothing to worry about if you choose a triode, but tetrodes work just fine 
and are easier to drive.  You will need to modulate the screen.  Avoid 
self-modulating.  You will introduce a gradually increasing phase shift at 
low frequencies as well as other deleterious effects.

The modulators should really and absolutely be a triodes for a variety of 
technical reasons, not the least of which is the significantly lower plate 
resistance of the tube, and the triode's ability to deliver a greater plate 
swing and into an imperfect load (such as a modulation transformer!).

Anyway, just thoughts!



At 10:05 AM 10/3/2013 -0500, you wrote:
>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?
>On Thu, Oct 3, 2013 at 9:22 AM, Steve WA1QIX <wa1qix at piesky.com> wrote:
> > At 07:55 PM 10/2/2013 -0700, you wrote:
> >
> >> Is it possible to run a 4-1000A in grounded gridS, cathode driven, biased
> >> for class-C and then plate modulate it and get 100% modulation? Has 
> anybody
> >> tried this?
> >
> >
> > It is possible.  The only important factor is how does the unmodulated feed
> > through power effect the linearity of the RF amplifier as a square law
> > device.
> >
> > As the modulation approaches 100% negative, the feed through power becomes
> > an increasing factor.  It will not behave like a classic negative peak
> > limiter, which has no effect until it comes into conduction.  So, there may
> > be some distortion, but how much is unknown having never actually done it.
> >
> > Do you know how much feed through power you actually get?  This will be an
> > important number to know.  Because AM is a square law function, 1/10th the
> > unmodulated plate voltage (90% negative modulation) is 1/100th the power.
> > If you have, say, 5% feed through power, that is actually a lot with 
> respect
> > to 100% negative modulation.  I didn't figure it out, but it may be 20%
> > modulation... just something to consider.
> >
> > If you do this arrangement, don't use the feed through as a negative peak
> > limiter (of sorts) without also protecting the modulation transformer (if
> > there is one).  Implement a protection diode/resistor/power supply across
> > the transformer secondary to protect the transformer during the periods of
> > the modulation cycle where there is no current.
> >
> > If it were me, I'd give it a try !! :-)
> >
> > Regards,
> >
> > Steve
> >
> >
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