[Amps] RF choke switching

[Vic K2VCO]

Most ham amplifiers use 200 uh chokes for all bands. It's hard to make a 200 uh choke that 
won't have resonances near a ham band when WARC bands are included. But this choke only 
has 2.26K of reactance on 1.8 mHz, which is comparable to the load impedance of the tube. 
As a result, the choke becomes part of the tank circuit -- a low-Q part that dissipates 
power. I suspect that this is one of the reasons efficiency on 160 meters is often lower 
than other bands (all of my amplifiers seem to put out the most power on 40 meters).

Even a 100 uh choke has 8.8K reactance on 20 meters. And it's much easier to make one 
without resonances below 30 mHz.

On 6/9/2011 7:10 PM, Eddy Swynar wrote:
> Hi Vic,
>
> For some time now I've mulling over a very similar set-up here...only in my instance, I
> was thinking of somehow shorting the bigger inductor whenever I wanted to use the
> higher frequency bands, and letting the two chokes stay in series for operation on
> 160-meters...
>
> I was hoping to use a 200-uh. solenoid choke for the high frequencies, and a multi-pi
> National choke (3.0 mh.?) for Topband.
>
> I wasn't planning on using anything more fancy here for a switching medium than a
> manually-inserted (by way of a wooden dowel) shorting banana plug. That, or a mercury
> switch that might be somehow made to tilt "on command". I s'pose it'd be a requirement,
> too, to somehow mount the coils at right angles to one another...?
>
> Interesting stuff, indeed...
>
> I sure would be appreciative if you might keep us all informed as to your progress in
> this regard, Vic, with my sincerest of thanks in advance...
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
>
> **********************************************************************************************************************
>
>
>
> On 2011-06-09, at 8:43 PM, Vic K2VCO wrote:
>
>> I'm planning another amplifier (2 x 4-400A, 1.8-28 mHz) and I was thinking that I
>> would deal with the "RF choke problem" by having two chokes in series. The one
>> nearest the plates will have enough inductance for 20-15-10 meters with no series
>> resonances below 30 mHz. The second will be 1 or 2 mh to provide enough inductance
>> for the 1.8-7 mHz bands. I will use a vacuum relay to connect a bypass capacitor from
>> the junction of the two chokes to ground on the high bands. Of course there will be
>> further bypassing at the cold end of the big choke.
>>
>> That way, both chokes will always be in the circuit to help keep RF out of the power
>> supply but the resonances and distributed capacity of the large one won't upset the
>> higher bands.
>>
>> Is this a good idea or am I missing something? -- Vic, K2VCO Fresno CA
>> http://www.qsl.net/k2vco/ _______________________________________________ Amps
>> mailing list Amps at contesting.com http://lists.contesting.com/mailman/listinfo/amps
>

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/