[Amps] Tuned Input

Steve Thompson g8gsq at eltac.co.uk
Sat Jan 21 02:48:31 EST 2006



George badger wrote:
> I have been using an antenna tuner as an input circuit
> for my HB 8877 lineart for years. When I was at EIMAC
> I learned that when linear amplifier OEMs complained
> of low efficiency it was often caused by too much
> inductance in the input cathode drive cicuit. For
> example, a T input circuit (LCL} was a disaster on 40
> and 80. Similarly, if the lead from the input pi
> network  to the cathode is too long,  a problem is
> created on 10M. The reason is that the cathode current
> is not sinusoidal so harmonic current must have an
> easy path to ground. Unless there is sufficint
> capacitance to ground, efficiency suffers.            

That's always been my understanding of what's best for the configuration.

>         To avoid this on my linear,I experimented with
> a variable capacitor to determine the largest fixed
> capacitor I could get away with and still be within
> the range of my drake tuner on all bands The
> capacitance turned out to be 50 pfd. It is connected
> directly from cathode to ground with short leads.
That's likely to have the side benefit of improving vhf stability.

Steve


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