>
>> ..... // Another example
>> is Maxwell's conjugate matching theorem. If it were 100% true, amplifiers
>> could be no more than 50% efficient. .
>
>And the logic behind that statement is???
>
? Power transfer is maximal when the resistance of the load equals the
internal of the generator. At this point, efficiency is 50%. However,
in practice amplifiers are c.60% efficient.
>> >For example, Collins had a spotless reputation of good designs,
>> >yet they somehow came up with a goofy idea that lifting the grids
>> >of a GG triode class AB2 PA with high reactance capacitors added
>> >"negative" feedback.
>>
>> ? To find out who's goofy, take a dipmeter and measure the 811A grid
>> resonance in a Collins 30L-1 amplifier. Short the grid to gnd capacitor
>> by soldering a strip of copper foil from the grid terminal to ground.
>> Remeasure the dip frequency. If the dip frequency decreases with the
>> short in place, Collins did not goof.
>
>Or you could measure with the correct instrument
? Translation: A dipmeter indicates that the grid-resonance in the
30L-1 Decreased in freq. when the small grid-gnd cap. was shorted by the
copper strap.
>and measure
>something meaningful like feedthrough power, and see feedthrough
>power increases when the capacitors are installed.
>
? Natcherly, this is a complex, replete with self-delusive pitfalls,
procedure that requires special test equipment which few of us are likely
to have. . . . Tom's continuing battle with the dipmeter invites
comparison to the Thirty Years War. .
>It's a bad idea, for a multiband amplifier, to increase unwanted
>feedthrough.
? The hat trick is to move the grid's parallel-resonance (where
feedback/feedthrough is maximal) away from the anode-resonace, where
damped wave VHF ringing takes place whenever the anode current changes.
>Especially when the tubes have too much feedback to
>be stable without neutralization when the grids are grounded.
>
? In the world of VHF, a grid can not be absolutely, positively brought
to ground potential.
>
- later, Tom
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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