SV: SV: [AMPS] Design of Plate RF Choke

[sm5ki]

----------
>Från: "sm5ki" <sm5ki@algonet.se>
>Till: "i4jmy@iol.it" <i4jmy@iol.it> , k7fm@teleport.com
>Kopia: amps@contesting.com
>Ämne: SV: [AMPS] Design of Plate RF Choke
>Datum: mån 6 mar 2000 20.26
>

>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
>>
>>
>>
>>
>>
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>>
>>

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