> Here's my take on why harmonics are important. The current
> pulse the
> tube passes every half cycle is not a pure sinewave
> (unless you run
> class A). It's a spike of current so it contains harmonics
> and the
> harder you drive the tube towards saturation/flat topping,
> the higher
> the harmonic content goes. If the impedance in the cathode
> circuit won't
> pass the harmonics then it's going to mess up the way the
> tube works.
If the cathode sees a high impedance on even harmonics,
looking back at the input circuit, it rounds the transition
between off and on.
I can easily hurt efficiency by making the cathode see a
high impedance at the second harmonic, but it depends on the
amplifer an tube how much it hurts the system.
IMD can also be related to what the exciter sees. You don't
want those harmonics reaching the exciter. The problems it
causes also depends on the phase of the harmoics, so even
cable lengths matter.
As you point out, placing the low pass C-L-C tuned input or
parallel L-C network (bandpass) at the cathode makes the
system independent of changes in cable length and exciters.
This assumes the network cuts off and looks like a low
impedance well below the 2nd harmonmic of the drive
frequency.
> As an aside, adding some capacitance with low inductance
> leads from
> cathode to grid won't only help linearity/efficiency, it
> might improve
> vhf stability too.
I can't recall seeing any systems where the cathode is
involved in VHF parasitics in an HF cathode driven
amplifier, although that doesn't mean it can't ever happen.
Every case I have seen has been a grid-anode problem.
73 Tom
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