[Bazz4CQJ]

In previous threads we’ve discussed the issue of selectivity in vintage (valve) receivers. Most often the problem is a simple lack of selectivity, or in the case of the HRO, a choice between the eye of a needle or wide open barn door.

Continuing to delve in to the internet on this, I discovered the crystal filter system which was patented in 1938 by the Hammarlund Corporation and described in great detail in the magazine “QST” December 1938 (page 35). It’s 13.4MB so not attached. Looking at a number of the Hammarlund Rx’s made after ’38, the design appears to have become a “trade mark” feature of all their sets.

It’s quite a simple design, using a single 455kHz crystal which is subjected to various degrees of load impedance by a circuit which switches in resistors of values which set the range of the bandwidth and then a capacitor which fine tunes the bandwidth within that range. This “two knob” control was reported to be very effective and required no skill to use. The claim is that “The higher the impedance load, the wider the filter”.

Shown below are the circuit of the filter and its frequency response at different settings, as taken from the ops manual for the Hammarlund HQ129 –X receiver.

Now, it has to be said that in the QST article, it is implied that the crystal necessary for this circuit is something special, but I’m not sure to what extent any crystal made at Hammarlund in 1938 was very distinctive from a crystal made by any other company in 1938, but I could be wrong.

The only other slightly quirky feature of the circuit is that it needs a differential tuning capacitor to make it work. Such capacitors are not common in the UK. I’ve been wondering if the use of two varicap diodes, suitably configured, might be substituted for the differential.

Curiously, we rarely seem to discuss Hammarlund receivers on the forum. I wouldn’t claim to have the expertise to look at this circuit and make deeply informed comment, but Hammarlund were flogging this for some decades.
Thoughts?

B