100 pf is only enough capacitance for about 8 meters! Not even enough for 10
meter operation. The recommended amount is in the neighborhood of 13 pf /
meter. On 80 meters that would be around 1040 pf of capacitance that should
be tied directly to the cathode.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of George badger
> Sent: Tuesday, August 08, 2006 6:21 PM
> To: Tom W8JI; amps@contesting.com
> Subject: Re: [Amps] Tuned Input - IMD and efficiency
>
> 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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> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
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