>
>Hi Wynand,
>
>> I know I have to look at the capacitance of the four valves in parallel as
>> this will impact on efficiency especially at 10 meters. I AM JUST NOT
>> SURE WHAT I AM SUPPOSED TO BE LOOKING AT.
>
>Remember the input system can also greatly affect efficiency. As a
>general rule, the tank Q can change from anything from just over
>the square root of the ratio of anode to load impedance, to very
>high Q's, with little change in efficiency. The efficiency actually is
>maximum with the lowest possible loaded Q, and gradually
>decreases.
>
>Say the tube combination requires a load impedance of 2000
>ohms, and the load is 50 ohms. The maximum efficiency would
>occur with a tank Q of 2000/40=50....then take the square root of
>that number (7) and just add a tiny bit to it to be sure the network
>behaves like a true pi-network (phase shift more than 90 degrees in
>the network).
>
>With decent tank components, even Q's of 40 or more produce
>excellent efficiency, so you have (with good components) a wide
>range of latitude. Many VHF and UHF amplifiers give excellent
>efficiency with tank Q's in the hundreds, because the components
>are good.
>
>An error in input system design can have monumental effects on
>efficiency, if the input circuit at the cathode-driven tube presents a
>high impedance at harmonics of the operating frequency. This
>allows the cathode to "move around" at the harmonics, much like
>you are driving the tube with off frequency signals. It doesn't matter
>of those harmonics are created in the tubes grid-cathode and
>anode-cathode currents, or if they are from a harmonic-rich exciter.
>The results are the same. You have harmonic energy driving the
>tube, and that can greatly reduce efficiency.
>
>So by all means, besides watching the layout as suggested, watch
>the INPUT circuit. Make sure it is near the cathode, the leads are
>short, and it is a LOW-PASS type of network that provides a LOW
>shunt impedance at the cathode for all harmonics up to perhaps
>the 5th harmonic of the maximum operating frequency.
>
>Never use a L-C-L type of input, or a C-L-C "T" type. Use a parallel
>tuned network, or a C-L-C network in pi configuration, and keep it
>as close to the tube as possible.
>
>The Q of the network on the TUBE side of the network should be at
>least two...
Bzzzt.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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