In a message dated 96-11-20 17:51:11 EST, you write:
>9. However, on page 35 in Eimac's *Care and Feeding of Power Grid
>Tubes*, Figure 21 shows tank circuit Q versus relative harmonic output.
>With a Q of 6, the third-harmonic content is over 1.5%. This means that
>with 1000w output at 1.9MHz, the amplifier will be delivering 15w at
>5.7MHz. This level of out-of-band emission is clearly unacceptable to
>the FCC.
> Eimac recommends using a Q of 15 to obtain satisfactory suppression of
>harmonics. According to Figure 21, lowering the tank Q from 15 to 10
>triples the second-harmonic level. .
> Any comments, Mr. Rauch?
Hi Rich / W2CYA,
You are using data from a class C non-feedback 4-65A tube PA and trying to
apply that data to a class AB application that has considrable negative
feedback.
You have also misread the graph.
A grounded grid AB triode has negative feedback that reduces harmonics, as
well as a very large harmonic reduction occuring from linear operation. Even
if we tried intentionally to conjugately match the harmonic to the load (by
tuning the tank to the third harmonic) the power available would likely be
less than you postulated.
Both the class of operation and inherent negative feedback reduce the
harmonic content, and the tank polishes-off unwanted energy further.
The relative harmonic content shown in the graph is NOT expressed as a
percentage of anything, let alone the fundamental signal. The change in
RELATIVE level is from 1/2 at a Q of 15, to 2 at a Q of 5. That's a six dB
decrease in relative third harmonic suppression when Q is reduced from 15 to
5, and that is a sensible change.
DOUBLING the loaded Q results in about a 3 dB change in suppression, unless
the Q is initially very low.
If your speculation was was correct, a Q of 30 would result in a second
harmonic level of -24 dB or so, and a Q of 12 would give us all harmonic
levels of -14 dB!!! We'd all be in big harmonic trouble, and would be using
cavities on our HF PA's.
73 Tom
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