[Amps] Tuned input

[Gary Schafer]

Thanks Karl-Arne, very informative!

Someone mentioned putting a capacitor directly at the cathode as part of the
tuned input network to alleviate the inductance problem on 10 meters. It
seems that would be fine on 10 meters but would not help the situation on
other bands if a remote tuned (antenna tuner) input circuit was used.

The auto tuner being discussed for an input tuner, mounted near the tube,
would suffer a similar problem even if a fixed capacitor was used for 10
meters. If the auto tuner indeed is an L network then there would be
insufficient capacitance on the lower bands.

It would seem that the tuned input circuit is more important from the
standpoint of supplying a clean series path for the plate circuit signal
than it is for supplying a constant load to the exciter.

73
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of sm0aom at telia.com
> Sent: Tuesday, January 24, 2006 3:17 AM
> To: garyschafer at comcast.net; amps at contesting.com
> Subject: [Amps] Tuned input
> 
> The reason for short-circuiting the harmonic currents in the cathode
> circuits
> is that they would cause voltage drops over the cathode circuit if not
> bypassed.
> 
> As the cathode and plate circuits effectively are in series, these
> voltage drops
> may influence the slope and shape of the plate load line,
> depending on their phase and amplitude relationships with the
> fundamental current.
> 
> This can cause decreased power output and efficiency, in a similar way
> as insufficient Q in the plate circuit.
> 
> If the impedance of the cathode circuit becomes real at some harmonic
> frequencies,
> power will also be dissipated in the circuit.
> 
> Bill Orr's reommendation of capacitance in the cathode circuit comes
> from the
> recommendation to have a circuit Q of about 3, and for a 100 ohms
> input impedance
> the resultant Q in a resonant circuit has this value with a
> capacitance of
> 13 pF/m. Values of around 20 pF/m were used in circuits with lower
> input impedances.
> 
> 73/
> 
> Karl-Arne
> SM0AOM
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