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The BC-453 was used to sharpen IF selectivity on superheterodyne
receivers by tuning around the passband of the ~455 "kc" IF frequencies. It was known as a "Q-Fiver," and added a second conversion stage down to the 85 kc IF of the command set. Why not play with regeneration at the BC-453 IF level, if you think you need it? (The regeneration scheme works--I used it on several old receivers, and a WW II tank transceiver.) If you are worried about Images with your mixer scheme, you may want to look into some RF selection/amplification before the mixer. Back in the Fifties and Sixties, one-tube PreSelector projects were popular, (several good receivers, such as the Drake 2B, had them built in). The extra tuned circuit should reduce your image. --W9NPI Antonio Vernucci wrote: I would like to build an old-style receiver using a BC-453 stuck at 500-kHz as IF chain. A possibility would be to put a pentode in front of the BC-453 mixing the incoming RF signal with the signal generated by a variable local oscillator (VFO). To improve selectivity, one could adopt a regenerative arrangement whereby part of the pentode plate signal is fed back into the grid (by inductive coupling), and the cathode resistor is then adjusted just before the tube starts oscillating. In other words, a kind of Q-multiplying converter (I think this is called "Q-dyne" receiver). What I am not fully sure about is if the increased selectivity I so obtain turns into a higher rejection of the image frequency, or just into a narrowing of the received bandwidth (which is already narrowed down by the tight BC-453 85-kHz IF transformers). My feeling is that said regenerative scheme would offer no advantage in terms of image rejection, but I would value very much your opinion on that subject. 73 Tony I0JX |
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