Mike
You have a good memory. Thanks for the kind words
about the baluns. They are so simple and clean. They
are voltage baluns. W7EL and others [later] pointed
out that current baluns are better for loads that may
be unbalanced. In fact, voltage baluns can sometimes
create more problems than they were supposed to solve.
I am now trying to make current baluns based on
similar construction.
In my comment on grounded grid amplifier input
capacitance, I was referring to all types of power
tubes, directly heated, indirectly heated with
isolated heater and indirectly heated with connected
heater like the YC156. In all cases the cathode must
be isolated from ground for RF drive.
Someone mentioned recommended high capacitance values
for pi network input circuits. The input circuit must
provide adequate Q, matching, and a path to ground for
harmonic current. The tuner provides the Q and the
matching . The capacitor provides the path to ground
for high harmonics.
73
George W6TC
--- Mike <hinrgdj@midsouth.rr.com> wrote:
> Hello, George, can I assume were talking about tubes
> where the cathode is
> not internally connected to one side of the filament
> as in the gs35/yc156
> external anode type tubes??
>
> OBTW please contribute more to the AMPS reflector, I
> certainly do appreciate
> the wonderful Ham Radio articles on your balun
> construction from years back!
>
> 73, Mike WD4EFI
>
> -----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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> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
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