[Boatanchors] Drake preselector construction?

[Garey Barrell]

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
>
>