On Jan 20, 2006, at 11:48 PM, Steve Thompson wrote:
>
>
> 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.
>
Adding a low-L resistor in series would help to reduce VHF-Q.
> Steve
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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