[Amps] Tuned input design / measurement questions

[Vic K2VCO]

On 4/16/2013 8:37 AM, Chris Wilson wrote:
> I am unsure if the antenna matching "stuff" in my Kenwood TS-590
> exciter precludes the need for this, though? I am trying to match 50
> ohms to near 50 ohms,which is the cathode impedance of the 3CX3000A7
> valve, I guess the antenna matching unit in the Kenwood would do this,
> if I just removed the stock Pi input on the Henry, but would I then
> lose the desired "flywheel effect"?
If you don't have a tuned input circuit in the amplifier, then the return path for the RF 
cathode current is through the coax and the transceiver output tank. This causes the 
performance to be dependent on the cable length. It also might damage components in the 
transceiver.

> I am similarly confused by advice I have had regarding*measuring*  the
> stock 27.12 MHz input circuit in the Henry, as a test of my measuring
> methods. I am advised to put a 50 ohm carbon composite resistor from
> the cathode of the 3CX3000A7 to ground and use the AIM on the RF input
> socket to the RF Generator RF deck to measure how it is now. But
> looking at the circuit diagram of the Henry I see there are isolating
> capacitors between the PI input circuit and the actual pins for the
> cathode. Shouldn't the loading resistor be direct to the output side
> of the Pi tuned circuit? If not I now have doubts over whether a dummy
> load DIRECT to the output side of my own Pi circuit, on the bench, is
> correct, or if it needs the de-coupling capacitor before the dummy
> load?
The capacitor is probably large enough that it won't affect the results either way. It 
won't hurt to put it in, though.

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/