[Drake] preselector

[Paul Blumhardt]

Most (all?) of the tube receivers made prior to the 2A had a very simple
impedance matching circuit. Typically these were an L network that had a
very low Q. Ergo no real band pass/band reject characteristics at all. In
fact many of those circuits were called "Antenna" or "Peak" or something of
that ilk.

Its important to note that many receivers of the pre-2A era were not "Ham
Band only". Most had one stage of RF amplification and while the plate side
of the RF amp was tuned, but the grid side was basically untuned except for
the impedance matching network.

The advent of "ham band only" receivers ushered in high-Q front ends
(Pre-Selectors).

A Preselector by definition is a band pass filter with a fairly high Q that
is located before the RF amp in the receiver. Its main purpose is to
minimize the effects of strong out of band signals on the RF amp which would
manifest as unwanted IP3 products. A good example of how a preselector is
used in this sense is the Galaxy R-530/1530. That preselector has (as I
recall) 4 tuned circuits in the filter and employs ganged variable
inductors - very sophisticated!

If you fast forward to the late 1970's, we even went as far as using crystal
bandpass filters in front of our R4C's. These were very limited bandwidth.
As I recall, the ones we used passed 7000 to 7050 kHz and cut off VERY
sharply. (you read that right, those were crystal lattice filters) However,
this was the hot set up for contesting on 40 CW from the East Coast pointing
at Europe with stacked 3 element fullsized yagis. Them Foreign Broadcast
signals (and some of the Italian hams) got really strong!

One last thing about "Preselectors". If you look at how that function is
typically implemented, its not preselector in the true sense. Take the R4C
for example. When you peak the Preselector, you are peaking the front end
and mixer stages as well. Finally, the Preselector function can be done
either with ganged capacitors or ganged inductors. Ganged inductors were
found in your S-Lines, C-Lines and such. This method provides tuned circuits
with higher Q and more linear tracking.

Just one more example of how the science of engineering receivers evolved
over time.

VY Sennytree

Paul

> -----Original Message-----
> From: drake-bounces at mailman.qth.net
> [mailto:drake-bounces at mailman.qth.net]On Behalf Of Ed Berbari
> Sent: Wednesday, May 03, 2006 7:34 PM
> To: Dan Cotsirilos K9DTC; Drake Listserver ( QTH.NET )
> Subject: Re: [Drake] preselector
>
>
> Dan,
>
> I am not sure how the preselector is defined.  Many older receivers (I am
> thinking of my Hammarlunds - back to the HQ-120)  They had antenna tuning
> circuits at the very front end.  I believe they were quite common
> in other
> brands as well.
>
> Ed, W9EJB
>
>
> ----- Original Message -----
> From: "Dan Cotsirilos K9DTC" <dcsfree at comcast.net>
> To: "Drake Listserver ( QTH.NET )" <drake at mailman.qth.net>
> Sent: Tuesday, May 02, 2006 11:27 PM
> Subject: [Drake] preselector
>
>
> Did Drake with the 2A invent the preselector. Was that the first receiver
> that had one? Does anyone know?  thanks
>
>
> Dan K9DTC
>
>
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