SV: [AMPS] Design of Plate RF Choke

[sm5ki]

This may sound crazy? You use a small step-up transformer with just a few
primary turns in series with the plate lead.  The secondary load is a
parasitic resistor series tuned to the parasitic frequency with a trimmer
cap. What happens then?

By the way - did we not have tuned parasitic chokes in them old ham days?

Prosit de Hans
----------
>Från: "i4jmy@iol.it"<i4jmy@iol.it>
>Till: k7fm@teleport.com
>Kopia: amps@contesting.com
>Ämne: Re: [AMPS] Design of Plate RF Choke
>Datum: mån 6 mar 2000 12.30
>

>
>> ---------- Initial message -----------
>> 
>> From    : owner-amps@contesting.com
>> To      : "AMPS" <amps@contesting.com>
>> Cc      : 
>> Date    : Sun, 5 Mar 2000 15:16:00 -0800
>> Subject : [AMPS] Design of Plate RF Choke
>> 
>> 
>> Question -
>> 
>> Assuming we can control all of the parameters of the plate rf choke (I
>> realize that is not possible), what are the ideal characteristics?
>> 
>> Primary goals are sufficient impedance throughout the range of the 
>amplifier
>> and no unwanted resonances.  But, can we ask for more?
>
>Be satisfied if you have enough reactance, no resonances within 
>operational frequency range, and if this choke is still rated for the 
>current you need.
>
>>  The impedance
>> decreases below the design frequency - which would reduce gain below 
>the
>> design frequency.  That is good and easy to attain.
>
>Under my interpretation, the assumption the choke determines the gain 
>is not correct.
>The choke must have a considerably larger impedance than the amplifier 
>itself and its "swamping" effect at lower frequencies (or when its 
>impedance is anyway too low) can't be inherently considered the 
>parameter for gain lowering.
>The plate choke is dynamically in parallel with the tube output.
> 
>
>>The next question is
>> how do we want the rf choke to look above the design frequency?  If 
>it has
>> low impedance, that would reduce vhf gain and therefore reduce 
>parasitics.
>> This characteristic ought to be desirable - but perhaps not 
>attainable.
>
>In my experience, plate choke/s do not create problems if their 
>resonances are far away the operating range.
>
>
>> Perhaps the design of a rf choke that has no resonances and sufficient
>> impedance throughout the design frequency is all one can hope for.  
>Perhaps
>> the inefficiency and stray inductance of the plate bypass capacitor 
>would
>> render the low vhf-uhf impedance of no real value.
>
>Internal electrodes and lead wires have generally enough inductive 
>reactance to give problems at VHF. 
>No way....
>
>> 
>l tuned circuit?  Even 
>though
>> this circuit, in theory. is resonant at one point, we recall that 
>parasitics
>> were common even when parallel tuned circuits were used rather than
>> pi-network developed by Collins in the 1930's.  Buddy Alvarez 
>constructed a
>> 4CX1000 amplifier using a home built rf choke wound with Manganin 
>wire (a
>> resistance wire).  It had no resonance.  He used no parasitic 
>suppressor in
>> the plate circuit and it ran with no problem.
>
>I used no manganine (or other resistive) wire for a plate choke and no 
>suppressor in an amplifier using the same tube without having stability 
>problems (but the 4cx1000 socket was instead modified).
>
>
>> 
>> Another question is:
>> 
>> Has anyone tried to "short out" parasitics rather than "arrest" 
>them.  By
>> installing a band-pass type filter for all frequencies above the 
>normal
>> operating range between plate and ground, any undesired oscillations 
>would
>> be dampened without the loss of a parasitic suppressor.
>
>The series reactance to reach the filter would prevent the idea to work.
>
>> 
>> Comments would be appreciated.
>> 
>> Colin  K7FM
>
>73,
>Mauri I4JMY
>
>
>
>
>
>--
>FAQ on WWW:               http://www.contesting.com/ampsfaq.html
>Submissions:              amps@contesting.com
>Administrative requests:  amps-REQUEST@contesting.com
>Problems:                 owner-amps@contesting.com
>Search:                   http://www.contesting.com/km9p/search.htm
>
>

--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm