Bill, you mean for this to apply to grid driven operation, don't you?
In cathode driven service it is more like peak cathode voltage divided by
peak cathode current. Note peak cathode current in linear classes is on the
order of 3X average current.
73,
Gerald K5GW
In a message dated 8/26/2009 1:49:55 P.M. Central Daylight Time,
wlfuqu00@uky.edu writes:
Grid to Cathode impedance of a tube
is the grid to cathode voltage divided by the
grid current!!!
73
Bill wa4lav
At 02:41 PM 8/26/2009 -0400, DF3KV wrote:
>But the cathode choke still keeps the impedance low
>
>73
>Peter
>
>-----Original Message-----
>From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
>Behalf Of Gary Schafer
>Sent: Mittwoch, 26. August 2009 19:54
>To: dezrat1242@yahoo.com; amps@contesting.com
>Subject: Re: [Amps] RF parts and parasitics!!
>
>If drawing a conclusion from a test you must first have the proper test
>setup with the proper equipment. Using an SWR bridge to determine if the
>input impedance of the tube is constant is not the right equipment.
>
>The average input impedance may be rather low but that is just what it is,
>average. When the cathode is driven positive the grid to cathode becomes
>negative. At some point in the RF cycle the tube is cutoff and no grid
>current flows and no plate current flows. Class B, AB2 etc.
>
>73
>Gary K4FMX
>
> > -----Original Message-----
> > From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
On
> > Behalf Of Bill, W6WRT
> > Sent: Wednesday, August 26, 2009 11:58 AM
> > To: amps@contesting.com
> > Subject: Re: [Amps] RF parts and parasitics!!
> >
> > ORIGINAL MESSAGE:
> >
> > On Wed, 26 Aug 2009 07:20:56 EDT, Gudguyham@aol.com wrote:
> >
> > >Interestingly enough right smack at the top of the 3-500z heading
for
> > >these tubes there is a link to the W8JI web site which he explains
> > >parasitic's.
> >
> > REPLY:
> >
> > While I generally agree with W8JI's conclusions, he makes a serious
> > mistake
> > about halfway through the article when he says " When grid current is
> > absent,
> > the grid-to-cathode impedance is nearly an open circuit". In a
grounded-
> > grid
> > linear amplifier (the kind under discussion) this is simply not true.
> >
> > The input impedance of a GG amplifier is always relatively low,
regardless
> > of
> > whether the grid is drawing current or not. The reason is that the
input
> > signal
> > is "modulating" the cathode current, regardless whether the drive is
100
> > watts
> > or a fraction of a watt. The cathode current represents a low impedance
> > load and
> > the drive signal "sees" that always. It is true that when the drive
signal
> > increases to the point where grid current is drawn, the impedance
> > decreases, but
> > the amount of decrease is relatively small.
> >
> > You can prove this yourself. With an SWR meter between your TX and the
> > amp, turn
> > the drive down low enough so no grid current is drawn. Note the SWR.
> > Gradually
> > increase the drive all the way up to maximum while monitoring SWR. Note
> > the SWR
> > changes very little. If W8JI's statement were true the SWR would be
very
> > high
> > with low drive where there is no grid current and he claims there is
an
> > "open
> > circuit". It isn't.
> >
> > The rest of his article is very well done. His idea of inserting a
small
> > capacitor in series with the parasitic suppressor resistor is
intriguing.
> > Please
> > note that "small" means only in the sense of value. This capacitor will
> > carry
> > significant current at the higher operating frequencies and needs to
be
> > rated
> > accordingly. Also note that this capacitor now forms a parallel
resonant
> > circuit
> > with he suppressor inductor, although it is a low-Q circuit due to the
> > presence
> > of the resistor. Just something else to keep in mind.
> >
> > Those pesky resonances are everywhere. :-)
> >
> > His article is at: http://www.w8ji.com/vhf_stability.htm
> >
> > Comments welcome.
> >
> > 73, Bill W6WRT
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
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