km1h @ juno.com wrote:
>
>On Sat, 14 Mar 98 05:44:02 -0800 Rich Measures <measures@vc.net> writes:
>One of the "no hassle" untuned input circuits floating
>>about recommends putting an inductor in series with the grid
>>termination
>>R to improve the 12m and 10m input SWR. Doing so is an invitation to
>>vhf
>>oscillation.
>
>Agreed
>
>
> If Emtron, QRO, and Alpha do not use such a circuit,
>>Good.
>> . However, one does not know until one sees the circuit.
>
>I do not know what they or Alpha use.
The 91B "absorbs" the input capacitance of the 4CX800As by making it act
as C2 in a pi-network. The tube input capacitance is about 100pF total,
and the fixed capacitors used for C1 are about the same value. Thus the
transceiver sees a 1:1 impedance transformation into the 50-ohm
terminating resistor.
At all HF/VHF frequencies, the grids see the 50-ohm resistor connected
directly to ground.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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