I use an antenna tuner between my transceiver and my
linear and eliminated the input pi networks. I did
this to cover the WARC bands. Because the length of
the coax is not defined and the impedance for
harmonics looking back into the tuner is not defined,
I added a capacitor with short leads directly from
cathode to ground at the tube socket.This forms a
direct path on all bands for harmonic current
necessary for good performance.
The value of the capacitor should be the largest your
tuner can handle on all bands. I determined the value
experimentally, temporarily substituting a variable
capacitor making sure the tuner can match the cathode
input on all bands. This will work for manual tuners,
automatic tuners and tuners in the transceiver. In my
case the value turned out to be 100 pFd.
73
George W6TC
--- Tom W8JI <w8ji@w8ji.com> wrote:
> > 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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