[Premium-Rx] Front end selectivity

[Michael O'Beirne]

Good evening gents,

I am all for old-fashioned techniques where they work, such as front-end selectivity.  I would never suggest that we go back to the HRO (a receiver I never liked at the best of times) but much may be done with a tube or two intelligently applied.  

One of the most bombproof front ends is on an HF receiver Fred O has never seen.  It's the CJA made by UK's GEC for the Royal Navy in the mid-late 1960s.  The front end comprises two RF amps each of a very low gain special quality ruggedised 12AU7 in cascode followed by a another 12AU7 twin triode configured as a double balanced mixer.  There are two Hi-Q tuned circuits in the antenna circuit, two more before the 2nd RF amp and two more before the mixer - yes guys, that's 6 tuned circuits before a fully double balanced tube mixer.

It's similar in concept to the Racal MA197B preselector.

The LO drive is 2 - 3 watts!  The LO looks like a free running valved Hartley with AFC lock applied from a synthesiser via a reactance tube across the LO.  The LO inductors are big (about 20mm diameter) with a Q of not less than 100.  The AFC lock is only about +/- 20kHz so there is not the phase noise of direct synthesiser control.  This is followed by 2 wideband buffer amps to generate 3 watts.  All this lot is contained in a giant 7 section, 8 band turret about 15 inches long.

There is only one IF, at 1.6MHz, direct into a sharp SSB 2.7kHz crystal filter followed by 4 IF amps.  The filters are relay switched to avoid the problems of non-linear diode switching.

The set is said to have a dynamic range of 135dB, though I don't know under what test conditions.  They were used on warships running transmitters up to 1kW with an antenna separation less than 100 feet from the receiver whips (if that).

This is not a radio for most domestic QTH's. They are big and very very heavy, but not in the massive league of some of the 800 lb "fridges" from TMC.  There are very few left

I mention the CJA only because it shows what can be done with a very clean LO, just enough gain in the front end to keep the noise down, and the effect of heroic pre-mixer selectivity.  I suspect a transistor version would be fabulous, but at what cost?  Imagine the AOR 7030 with this sort of front end selectivity, and preferably with proper knobs, a 19 inch rack and a better display such as that of a back-lit RA1972!!!!

Got to get to bed.
73s
Michael
G8MOB
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