"Antonio Vernucci" ) writes:
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.

A) There's really no reason to "boost" the selectivity of the BC-453.
They were once seen as the ultimate in selectivy, at a time when
there were few options. If the IF is too wide, then you need to
adjust it.

B) The Q-Multiplier works by adding regeneration, but the prime
point is just before it oscillates. Which means that when you need
it to oscillate to provide a beat note, you lose control of selectivity.
And the proper point is so sharp that external factors will kick
the circuit into oscillation. Which is where the superregen came in,
keeping it at that crucial point of highest gain without the need for
crucial tuning (but incidentally bringing other problems into play).

C) If you have a self-oscillating converter, which seems like you
are talking about, note the regen to the point of oscillation is
at the oscillator's frequency. That's not going to do a thing on
the signal frequency, and it's not going to do a thing on the
IF frequency. A mixer by definition has a different frequency on
the input from the output, which does not make for proper regeneration.

Unless they were modified, most BC-453's were used in multiple conversion
schemes. Either tapped into the 455KHz IF of an existing receiver (and
if the existing receiver was single conversion, leaving its image problem
intact), or as a tuneable receiver with some sort of converter ahead of
it. The better ones used two stages of conversion, to get around the
image problem that arose with a high signal frequency dropping immediately
down to 455KHz.

Michael VE2BVW