Rick ) writes:
I never could quite figure out exactly what a "Q Multiplier" does.

There is a Heathkit model QF-1 Q Multiplier on ebay and that got me to
wondering.

I know what "Q" is regarding inductors and capacitors ("Quality factor",
reactance divided by resistance) but I don't know exactly what a Q
Multiplier does or how it connects (I assume it connects to a receiver but
am not even sure about that). Obviously it "multiplies the Q" but of what
component(s) or stage(s)?

Any of you boat anchor enthusiasts know about this?

Well this isn't the boatanchor newsgroup, though you'd often be
confused by the recent equipment that gets posted about over there.

A Q-multiplier is a regenerative stage.

You might be familiar with the regenerative receiver, where positive feed
back is introduced to a stage so the same signal is amplified, fed back to
the input, and amplified again. This re-amplification allows for far
more gain than if the stage only amplified the signal once, and it
increases the selectivity of the tuned circuit involved. There is a means of
controlling regneration, so you can limit the amplification, or kick it up
so the receiver goes into oscillation, providing a beat note for reception
of CW signals.

A Q-multiplier is the same thing, though it doesn't bother to demodulate
the signal. Generally, the point is to increase selectivity in the IF
section of the receiver. It connects to the plate of the mixer with
a single connection, in parallel with that stage. INcrease the regeneration
of the Q-Multiplier, and it increases the selectivity. It gives a nice
peak, though does little to improve the skirt selectivity.

It's prime was in the days of the 455KHz IF, where the improved selectivity
would actually make a difference. At a higher frequency, it would still
increase the selectivity of the tuned circuit, but not enough to narrow
the passband for voice or CW.

It was often an add-on for low end receivers, because for the cost of
a single stage, you'd boost selectivity. Sometimes it was included
in the receiver (and sometimes the effect was done by adding some positive
feedback to the IF stage, so it could go into regeneration).

It was sometimes seen in more expensive receivers, where a variant
was often the reason. A rearrangement of the circuit, and it could
be used as a notch filter, to knock out a nearby signal, which was often
more useful in receivers with good IF selectivity where the peaking was
less of an improvement.

One early use for the Q-Multiplier, which never saw that much use,
was for boosting the received carrier in reference to the sidebands of
an AM signal. This peaking of the carrier made sure it was strong
relative to the sidebands, and made for better AM demodulation. The
Q-multiplier was good for this, since it peaked up the carrier but
the bad skirt selectivity didn't attenuate the sidebands too much.
The same basic notion that is the point of synchronous detectors
for better AM reception today.

For a brief while in the sixties, there'd be construction articles
with Q-Multipliers at the front-end of receivers. Someone described
an early single conversion receiver with a 9MHz IF, and put a front
end Q-Multiplier in to improve the selectivity of the single tuned
circuit at the front end, to improve image rejection. (Though, it
never fully made sense, since image rejection was much easier with
the high frequency IF.) The scheme was continued in a few other
receivers, basically because it had been done. And that was the
end of that.

Q-Multipliers faded as IF frequencies increased, and good filters
such as mechanical and crystal came along to replace a string of
IF transformers.

MIchael VE2BVW