EMRFD Message Archive 2350
Message Date From Subject 2350 2008-11-02 06:01:58 bkopski Q MULTIPLIER QUESTION I'm curious about and seeking some understanding of the classic Q-
Multiplier popular long ago. In that regard, here is some text from
an early handbook:
"In one condition it narrows bandwidth and in the other condition it
produces a sharp null or rejection notch."
"Condition" ??? The handbook has nothing more to say about this.
In looking at the schematic of the old Heath Q Multiplier, I
see "peak" and "null" switch positions and associated trimmers and
I'd suppose that, in classic Heath tradition, the manual probably had
full explanation but the circuit dwg. is all I have.
So I ask: is there somehow a choice of peaking or nulling maybe
like switching "mode" - and if so how does the "nulling" part work?
Is the idea to notch out a nearby offender? Putting it another way,
peaking I understand (I've made plenty of regens) but nulling I
don't. Perhaps someone here can suggest further reference or offer a
2351 2008-11-02 11:12:34 bkopski Re: Q MULTIPLIER QUESTION Q MULTIPLIER QUESTION - FOLLOWUP
Earlier today I posted seeking help understanding the "peaking"
and "nulling" operation of the classic Q Multiplier. Since that post
I have found a site that provided a free download of the Heath QF-1 Q
Multiplier. True to Heathkit form, the manual describes the circuit
operation I sought help with. The site is:
and you may find it helpful in your own searches. It appears this
site has at least a zillion manuals of all kinds available!
Anyway, now having some better insight and understanding of Q
Multipliers especially in the "nulling" operation, I set about to
better understand the manual schematic. This is a bit of a challenge
because the Heath design included a multi function rotary switch that
somewhat obscures the circuit flow. But I went through it connection
by connection and came up somewhat stymied, thus:
The circuit utilizes the two triode sections of a 12AXT (a vacuum
tube to you youngsters out there!) and as best I can tell one section
seemingly operates WITH NO PLATE CURRENT because the cathode has NO
DC return path that I can see. I went so far as to trace the wiring
in the (famous) heath wiring pictorial and got the same result. So
unless there is some not-obvious alternative path (plate to
grid ???) this tube section is non-operating. But being a Heath
design I know this is unlikely in the limit Heath stuff works.
So now I seek Q Multiplier info of a different kind namely some
insight or explanati
2352 2008-11-02 12:36:50 Sudipta Ghose Re: Q MULTIPLIER QUESTION As an old SWL very active during 70s and 80s, I built one such
contraption and used it to the hilt! The schematic was circulated at
that time by the Dutch Radio Station Radio Nederland, Hilversum,
Holland. In case of interest, I can mail a scan of the schematic.
de Sudipta Ghose
2353 2008-11-02 12:36:53 cwfingertalker Re: Q MULTIPLIER QUESTION Bob,
I still run my Drake 2B with a Q-multiplier. The peak and null
functions are either series or parallel tuned circuits at the IF
frequency. Q-Multipliers were very affective at low IF. The notch
was used by AM'ers to notch out offending hetrodyne carriers or on CW
a signal stronger than the one you were listening too. Peak was used
for weak CW signals. You could only use one or the other.
For reference see this website. They have a schematic of the Drake 2B
Q-Multiplier. Mine works great after all these years. I guess if an
experimenter wanted to use a low IF frequency system you could build
one. But with DSB circuits you would propably get better results.
2354 2008-11-03 02:10:40 Dupont Francis Re: Q MULTIPLIER QUESTION Hi Bob,
> I set about to better understand the manual schematic. This is a bitYou are right. It is often the case!
> of a challenge because the Heath design included a multi function
> rotary switch that somewhat obscures the circuit flow.
> as best I can tell one section seemingly operates WITH NO PLATEAs I understand this rotary switch, it seems that the 12AX7 cathode is:
> CURRENT – because the cathode has NO DC return path that I can see.
- OFF: not connected
- SHARP: connected to 10k pot (peak adjust)
- NULL: connected to 10k pot (null adjust)
- BROAD: connected to 10k pot (peak adjust)
Have fun with Q-Multiplier!
2355 2008-11-03 16:30:20 Rick Re: Q MULTIPLIER QUESTION Hi Bob,
As usual, you have posted a profoundly interesting question. It is useful to remember that
the classic Q-multiplier circuit became popular in the 1950s, after 40 years of some of the
brightest designers on the planet devoting their careers to radio design using vacuum
tubes. It is an elegant and simple circuit with few components, but "how it works" is not
so easy to grasp. That makes it a useful object for study.
Here are a few steps along that path. First, consider the regeneration circuit. All that does
is remove the losses from the tuned circuit, which in effect, "multiplies the Q." The losses
can be reduced all the way to zero, and beyond--which is why the peaked signal can be
stronger than the signal without the Q-multiplier connected.
Once you accept that regeneration is simply used to increase Q, you can forget about it.
Model the tuned circuit with feedback-one section of the triode- as a simple parallel
tuned circuit with very high Q. Just one L and one C. If you want to play with it in Spice,
give it a Q of 4000. But it is useful to remember that the folks who really understood how
this worked didn't have computers.
In Peak mode, all you do is connect that high Q tuned circuit in parallel with a high
impedance point in the circuit. Q-multipliers were most frequently used at 455 kc, and
often connected with a single wire to the plate of a 6BE6 converter in parallel with the first
IF can. For receiver guys, that makes a lot of sense, as it gets the high selectivity
immediately after the first mixer. I checked three schematics--the Hammarlund HQ-150,
the Hammarlund HQ-110, and the Drake 2A/2B, and all three use an identical connection
to the plate of a 6BE6 converter. To think about that behavior, imagine a parallel LC tuned
circuit across a very high impedance load. Off resonance it looks like a short, and on
resonance is it a very narrow single tuned circuit. Very simple.
In Null mode it gets really interesting. The second triode stage is connected as an
inverting amplifier with feedback--just like an op amp or one of Wes's great little
feedback amps that you have studied in detail. The input to the inverting amplifier is
connected to the narrow tuned circuit, and the output of the inverting amplifier is
connected in parallel with the 6BE6 plate. That's a puzzle. How does the receiver circuit
even know that the high Q tuned circuit is there?
It is useful to think about those feedback amplifiers, and learn a little about them in the
process. The feedback resistor is connected from the output back to the input, and
provides a path. The reverse isolation (S12 for microwave engineers) is not particularly
good for feedback amplifiers. That means the input can tell what is connected to the
output. If you examine, for example, the broadband feedback amplifier that Wes uses
between the mixer and crystal filter in the Progressive Receiver, he includes a 6 dB pad on
the output. That keeps the input impedance of the feedback amplifier from varying all
over the map when the output is connected to something like a filter with wild fluctuations
in impedance across the frequency range.
In null mode, the input to the second triode stage sees the output through a feedback
resistor, typically 220k. That seems like a high resistance to solid state engineers, but is
typical for vacuum tube circuits. If you are comfortable thinking about op amps, imagine
that the second triode stage is an inverting op amp, with a 220k feedback resistor and a
high Q LC parallel tuned circuit from the inverting input to ground. Now imagine signals
at various frequencies above, below, and exactly at the resonant frequency of the tuned
circuit--connected to the output. It is a great thought experiment, and will lead to
understanding how the null functi
2356 2008-11-03 17:05:28 rotfunkblau Re: Q MULTIPLIER QUESTION Hello,
Q-multipliers are also used in spectrum analyzers to make variable
bandwidth filters with xtals. Does anybody know how this works? Or
have at least a schematic. Maybe this would also be a great topic for
experiments and simulation (please not with tubes). I am very
interested in Q-multipliers, because there are almost no publications
2357 2008-11-03 17:05:28 Paul Re: Q MULTIPLIER QUESTION Hello Bob,
Please take a look at
It's amazingly similar to the Heath circuit.
I suppose you mean DC-return for the right-hand triode.
Switch as in the diagram is in the "off" position. In the other
positions pin 8 kathode goes via 8.2k resistor and switch to the
peak and null adjust pot's.
2358 2008-11-03 17:36:21 Russell Shaw Re: Q MULTIPLIER QUESTION rotfunkblau wrote:
> Hello,It's used in the HP8552B 141T spec-an IF section. There's a description
> Q-multipliers are also used in spectrum analyzers to make variable
> bandwidth filters with xtals. Does anybody know how this works? Or
> have at least a schematic. Maybe this would also be a great topic for
> experiments and simulation (please not with tubes). I am very
> interested in Q-multipliers, because there are almost no publications
> on it.
in the service manual IIRC, but i can't remember how detailed.
2360 2008-11-03 19:48:03 cwfingertalker Re: Q MULTIPLIER QUESTION Hello Rick,
Very good description of the Q-Multiplier. I wonder if a modern
solidstate version would be practical in a homebrew receiver using a
low IF system such as the 455 khz.
I still use my Drake 2-B with matching Q-mult/speaker combo. With a
little patience you can get single signal reception with it on CW.
2361 2008-11-03 21:17:38 Rick Re: Q MULTIPLIER QUESTION Hi Bill,
Solid State Q-multipliers were common in the Radio Handbooks of the 1960s. The circuit
in EMRFD figure 1.16 might be a good place to start. George Dobbs has published a
number of related circuits in Sprat over the years, and Wes has recently played with a
simple Q multiplier on a basic crystal set.
From a basic radio performance standpoint, a single Hi-Q tuned circuit has several
limitations, and a simple ladder filter at HF using a few crystals has several advantages.
But often the most practical circuit may not be the most interesting. I used a receiver for
a while with two independent Q multipliers, one at the beginning of the IF amplifier and
one at the tail end. That was fun to play with, but perhaps not very practical. After I
removed the Q multipliers and replaced them with a Collins Mechanical Filter I quickly lost
interest in that particular receiver. With only a tuning knob and gain control it wasn't as
Another interesting topology might be a single conversion receiver with low overall gain to
a very low frequency IF--50 kHz or so, with a Q-multiplier
2364 2008-11-04 06:16:43 Michael Clarke Re: Q MULTIPLIER QUESTION Quote >>the Q-Multiplier. I wonder if a modern>> solidstate version
would be practical in a homebrew receiver using a>> low IF system such
as the 455 khz.>>
While not modern, but 1970s, the Heathkit SW 717, a simple kit for a
general coverage SW receiver, AM, SSB, CW, used 'reaction' in its 455
IF, the feed back providing sharper tuning and sensitivity and also
BFO, as it were, when into oscillation for SSB and CW. I have been
refurbishing one, a rally bargain, and this 'BFO' works OK.
2365 2008-11-04 17:54:33 Allison Parent Re: Q MULTIPLIER QUESTION Hello,
It's been done multiple times both commercial and HB.
One very good example of the Allied/RadioShack AX(or SX)190 reciever
all solid state (transistors both BJT and FET) has mechanical filters
and Q-mult that can do both peak and reject. I have one and it's
still a viable RX for ham bands today. It's not an R7000 but
effective, stable and runs off 12V very nicely.
I also have a homebrew design that uses a tube RX and the Q-mult was
added later using a pair of transistors.
2366 2008-11-04 20:05:40 rcbuckiii Re: Q MULTIPLIER QUESTION The Hallicrafters SX-140 did the same thing with its 1650 IF. They
simply referred to it as a Selectivity-BFO control. I used it in the
early 60s and worked the world with it. I bought one off of Ebay a
couple of years ago for nostaglic reasons. It is amazing to compare
the poor performance of those 60s receivers to today's and to realize
that we were actually able to work DX with that type of equipment. :-)
2369 2008-11-05 09:40:12 bkopski Re: Q MULTIPLIER QUESTION Thank You, Everyone for your inputs and assist.
I now have some better - but not yet complete - insight into
the "peak" but mostly re: the "null" operation of Q Multipliers. In
particular, I now have two representative schematics and associated
text (Heath and Drake) and have been pouring over both. From these
two, which are the "same" but "different", it seems clear that
successful operation depends on careful tune up and careful
application - not much of a surprise given the "regenerative" nature
of this old circuit application / approach. I've even taken a shot
at a solid state attempt at the "null" part - without success - yet.
To those responding in particular to my quandry re: the "no plate
current" in the Heath QF-1, I was "mislead" by the rotational arrow
direction shown on that mother of all rotary switches! It's now
clear that viewing it "the other way" establishes current flow.
Anyway, this pursuit is not for a contemporary applicati
2378 2008-11-10 19:26:56 jr_dakota Re: Q MULTIPLIER QUESTION Even the lowly S-120 used that circuit at 455Khz, crank it full and it
made the IF oscillate at 455Khz making it effectively a BFO ... Crank
it up until just before oscillation, kind of like you do with a regen
and it raised the sensitivity AND increased the selectivity of the IF
... Nothing dramatic but it did make a difference for DXing AM BCB and
shortwave stations but it was crude as a BFO