What does a "Q Multiplier" do?

I recently picked up a Heathkit HD-11 "Q Multiplier" for free, and I can't figure out what it does. I did some research online and figured out that it was used somewhere in older transmitters that had low IF frequencies. Unfortunately, I don't understand how I can use it. Also, I was confused because it only contains one tube, a 12AX7. The strange thing about this tube is that it is not suitable for radio frequencies because of its high miller capacitance. Everything appears to be intact and when I plug it in, the tube lights up. My questions are "What does it do exactly?", "What can I use it for?", and "Should I keep it, or take the parts out and reuse them?". My camera is broken at this point, so I can't post any pictures. Please help.

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gmoon7 years ago
A "Q Multiplier" is an active bandpass filter. The "Q factor" refers to the width of the band. Higher "Q," narrower band, more rejection.

Looking a the schematic, the signal is passed directly into an LCR filter. From there, NFB is used to create the active filter.

I'm not sure there's even an output, it appears to hook directly to the signal path; the unwanted frequencies are probably fed back into the coax cable, 180 degrees out of phase. I'm not certain the miller capacitance matters, especially if it's connected to an IF transformer (as explained below.)

Couldn't find a manual, but this blurb is helpful:
First, it doesn't hook up to the antenna leads!The outer cover of the coax goes to ground (naturally). Thecenter of the coax goes to either the plate of the mixer tube orthe plate side of the IF transformer. Looking at the HE-10 schematicat the 'Bama site, the mixer tube is a 6BE6 and the plate is pin 5.Either connect there or find the pin on the IF transformer that iteventually goes to. Heath provided a template and RCA socket withthe kit for installation. You may want to add such a socket.
Xellers (author)  gmoon7 years ago
Thank you for your advice, so now the question remains - what should I do with it?
gmoon Xellers7 years ago
Well, if you're a Ham radio operator, I guess you could use it for what it was intended--a filter to fine tune radio signals.

Otherwise, it has a 12ax7 (THE preamp tube for almost all guitar amps) and a working power supply. So you could make a tube preamp...

The Real McTube is an example of one such circuit. Only it uses the "trick" of a pair of back-to-back wallwarts to get the filament voltage, and isolation on the HV. You wouldn't need the trick, the transformer on the schematic is perfect for a preamp...
Xellers (author)  gmoon7 years ago
I think that the Real McTube would be a good idea, but does the device intact have any value as it is? Would I regret taking it apart later because it has an antique value?
gmoon Xellers7 years ago
Would you regret "remaking" it? Maybe. I can certain understand the desire to keep a vintage device intact. If it makes you uncomfortable "deconstructing" it, then maybe try to sell it to someone who can use it.

Regarding the value, I don't know.

Is it rare? Probably not. There are five for sale right now on ebay, and none seem to be really expensive. But to the right person, just about anything is valuable. So you probably could find a buyer.
Xellers (author)  gmoon6 years ago
Thank you for your answers, I finally took your advice and converted it into a two-stage cathode follower similar to this: http://diyaudioprojects.com/Schematics/12AU7-Cathode-Follower-Tube-Preamplifier.htm. Paired with my TV tube amplifier, it sounds great, putting my $200 solid state amplifier to shame! =)
gmoon Xellers6 years ago
Xellers (author)  gmoon7 years ago
I don't plan on selling anything, so my best bet would probably be some sort of preamplifier.
Xellers (author)  Xellers6 years ago
Well, seeing as I've had this thing laying around for quite a quite a while now, I've decided to convert it into a guitar preamplifier. I've seen plenty of schematics out there, so its time to get to work!
Elkpelvis5 months ago

I don't know where you got the idea the 12AX7 is unsuitable for R.F. use; it has been used in amateur radio oscillator circuits for over half a century.

That being said, the Q-multiplier is used to increase the selectivity of receivers. The input and output ports are the same port, which may seem counter-intuitive, until you think of it as a closed loop oscillator, with just enough feedback NOT to oscillate.


turqb16 years ago
The thread is on the correct track. Specifically the Heathkit Q Multiplier was sold as an "add-on" upgrade super-duper accessory for Amateur HAM communications Receivers. Item was popular in the 1960's when most receivers used vacuum tubes.  Many communications were done with CW or Morse code. Morse code is very bandwidth efficient and many signals could be heard at the same time.  The tube receivers did have filters but often the many signals were heard overlapping. The Q Multiplier could be added to any tube type receiver as an upgrade. It connected to the 455 kHz second IF with just the one coax. Q refers to the Quality of a circuit resonance. Resonance occurs when a signal matches the natural frequency of a circuit. The IF transformer is tuned to its operation frequency of 455 kHz plus or minus. Everything in that bandpass is heard in the headphones. By taking a small sample of the IF down the coax, amplifying it in the 12AX7 and returning it to the IF, the losses in the IF transformer would be overcome and the circuit Q "multiplied". One signal desired could be boosted out of the background. The Q Multiplier has a mode knob that allows this Peak operation. It also has a Notch mode where instead of returning an in-phase signal an out of phase signal was made. This reverse phase made a highly selective notch and could remove a strong local signal allowing a faint distant one to be heard. The Q Multiplier has a tuning knob that lets the action be moved to the best place in the IF span. It has a gain knob to set the amount of boost or cut. Too much gain causes oscillation.  To use the machine first turn it off and tune the signal desired on the regular receiver. Then leaving the main receiver tuning set, turn on the Q Multiplier and adjust to remove interference or boost desired signal. The Q Multiplier is for communications - it can cause distortion and hollow sounds like any narrow filter. With careful use can be helpful say with overseas 2-30 MHz HF broadcasts. Q multipliers were also used by Single Side Band voice operators  to cancel interference. There is no reason it could not be used with a transistor receiver but it would need a small tube type power supply since it normally stole power from the tube receiver supply.  Also - in concept - you could change the 12AX7 to MOSFETs. Would need two as the tube had two triode sections to replace. Drop the B+ to say 15 VDC and upgrade the circuit for transistor receiver use. Would make a great add-on to inexpensive International HF Broadcast receivers sold today (Zenith etc.).  Q Multiplier theory is used now when discussing Nuclear Magnetic Resonance. See some of the Physics work out of College Station, Texas..
Also - in concept - you could change the 12AX7 to MOSFETs. Would need two as the tube had two triode sections to replace.

I have been thinking of doing JUST THAT.
Do you have any idea which MOSFETs would do the trick?
lemonie7 years ago
I believe that they improve the Quality of a signal by amplifying in the particular frequency range you're interested in. Some kind of tuned-amp. I'd use it or sell it as it is.