[Amps] RF parts and parasitics!!

[TexasRF at aol.com]

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 at 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 at contesting.com  [mailto:amps-bounces at contesting.com] On
>Behalf Of Gary  Schafer
>Sent: Mittwoch, 26. August 2009 19:54
>To:  dezrat1242 at yahoo.com; amps at 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 at contesting.com [mailto:amps-bounces at contesting.com] 
On
>  > Behalf Of Bill, W6WRT
> > Sent: Wednesday, August 26, 2009 11:58  AM
> > To: amps at contesting.com
> > Subject: Re: [Amps] RF  parts and parasitics!!
> >
> > ORIGINAL MESSAGE:
>  >
> > On Wed, 26 Aug 2009 07:20:56 EDT, Gudguyham at 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
> >
>  >
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>
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