[Amps] Choke input filters, design criteria?

[Carl]

----- Original Message ----- 
From: "Bill Turner" <dezrat1242 at yahoo.com>
To: "Amps" <amps at contesting.com>
Sent: Monday, December 17, 2012 10:35 PM
Subject: Re: [Amps] Choke input filters, design criteria?


> ORIGINAL MESSAGE:
> On Mon, 17 Dec 2012 17:36:26 +0000, Chris wrote:
>
>> I am
>>wondering if I should attempt to use it rather than a capacitive input
>>filter?
>
> REPLY:
> Choke input filters are a holdover from the old AM transmitter days
> when current draw was relatively constant and HV capacitors were
> expensive. Today's amps for SSB and CW need a constant voltage source,
> not a constant current one. If you use a choke in a power supply where
> the current draw varies a lot, you will see the output voltage
> swinging widely, just what you do not want.
>
> Toss the choke. Capacitors only.
>
> 73, Bill W6WRT


BCB transmitters often run the Class AB thru B modulators from the same PS 
as the Class C final so there is certainly a wide current variation.
Voltage regulation is helped by the combination of the Class C load and 
value of the input choke.

Hams can use a choke to limit an otherwise too high B+ and/or to be able to 
use a high impedance old style transformer. The DC resistance of those 
transformers is also much higher, often a factor 0f 10-20 or more.

With a sufficient choke value and constant current load to substitute for 
the Class C final such as a healthy bleeder resistor, they can be used on 
ham CW/SSB. Not very efficiently tho. In the old days the HV PS was 
controlled by a toggle switch or PTT activating a relay in the transformers 
primary. This did away with a big part of the resistive load. Certainly not 
very practical for VOX, QSK, etc, but fine for PTT or a foot switch.

Carl
KM1H