[Boatanchors] Drake preselector construction?

[Ian Wilson]

Thanks, Garey. That doesn't sound like something simple that could
be easily replicated!

73, ian K3IMW

On Tue, Sep 13, 2011 at 1:30 PM, Garey Barrell <k4oah at mindspring.com> wrote:

> Ian -
>
> The 2-B switches in parallel coils to 'pad' the 'main' coils for each band.
>  There is a second slug tuned coil that is switched in for 40M, and then
> 'incremental' coils between BAND switch terminals for the three higher
> bands.  See attached schematic and photo (reduced resolution) from my CD on
> the 2-B.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
> and TR-4/C Service Supplement CDs
> <www.k4oah.com>
>
>
>
> Ian Wilson wrote:
>
>> I'm interested to know how the Drake 2B preselector is able to
>> tune 3.5 - 30MHz. From the schematic, it consists of two
>> similar LC sections. Each section has a 234pF variable capacitor
>> and a ganged variable inductor.
>>
>> To cover the 8.6:1 frequency range requires the square root of the LC
>> product to be also 8.6:1 also. Let's say 9:1 to allow for production
>> tolerances.
>>
>> A 234pF variable capacitor will have a minimum capacitance of
>> say 5pF, with tube and stray capacitances bringing that to a total
>> of perhaps 15pF. This is a capacitance range of say 16:1.
>>
>> An inductance range of (9 x 9) / 16 will be required to realize the
>> LC product needed. This comes to slightly more than 5:1.
>>
>> So .. what did Drake use to implement a small variable inductor with
>> a 5:1 inductance range?
>>
>> Also, this has to be something that can be driven from a common
>> shaft with the pair of capacitors. Or is there some mechanical linkage?
>> The preselector box is fairly small if I recall correctly (not near the
>> set
>> at the moment).
>>
>> 73, ian K3IMW
>>
>>
>>