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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