[AMPS] Design of Plate RF Choke

[i4jmy@iol.it]

> ---------- 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