Mostly likely it's a 12AU7/12AT7/12AX7. Any of them could be made to work as a BFO and an audio amp. Because of the low B+, I'd lean towards a 12AU7 or 12AT7.

I don't think there is an AM detector. The remaining triode is wired as an audio amp, with a 4.7K cathode bias resistor, the signal going to the control grid, the headphones acting as a plate load. The BFO heterodynes with the incoming signal to make an audio frequency, no need for a detector.

Rob

Thanks Rob. Those tubes were mentioned here earlier by Mr. Detrola. I have both tubes and in the next day or so will attempt to power this up and see how it works. Thanks for the help. A lot going on in what appears to be a simple circuit!

Jeff

FWIW, here is my attempt at redrawing the schematic of this unknown homebrew receiver. See attached. I suspect that the open end of the pot in the cathode circuit of V2 should be grounded. Also, a 12AU7 is probably a better choice than a 12AT7 for V3. Looks like a straight forward superhet with minimum decoupling resistors in the B+ line to allow for operation at low B+ voltage.
The 12AT7, 6AK5 and 12AU7 are good choices for operation at low B+. Max

Thanks Max for "cleaning up" my messy schematic. Much easier to look over, that's for sure!

Well, good news. Today around mid-day, I put a 12AT7 in the missing socket (I have several 12AT7 & 12AX7 tubes but unfortunately, no 12AU7), connected to the power supply, connected the receiver to my 100' long wire antenna, earth ground and high impedance magnetic headphones. Nothing received then but I was getting some very weak reception of possible beacons. But about an hour after sunset today and tonight this radio came alive! I can receive WWV at 10 & 15 MHz, weather broadcast,
amateur radio communications at 7-7.5 MHz, morse code, beacons (?), WWCR, Nashville, TN and other unidentified shortwave stations with foreign talk and music.
The BFO pitch control, using the 30pf trim capacitor, doesn't seem to be working when attempting to clear up voice communications on SSB. Minor adjustments with the band spread capacitor (70pf) and the gain control help to clarify speech. Maybe the BFO coil needs adjustment? With headphones, the audio is legible. The low B+ supply does seem to be sufficient. I need to get a 12AU7 to compare with the 12AT7 I am using presently. I also plan to replace all those vintage capacitors and any out of tolerance resistors with new ones.
Much thanks to everyone who helped me understand this HB receiver. I will update radio performance when the 12AU7 is in and the vintage components are replaced.

Jeff

I'm glad this saga has continued. It's been fun.

Comments on the design:

-I don't see any direct coupling between the HF LO (high frequency local oscillator) (V1B) and the mixer (V1A). So I assume there is internal coupling within the 12AT7. The 12AU7/AT7/AX7 series does not have an internal shield between triode sections, so this coupling is possible. Possible, but not the best way to do this. Injection is usually too weak. Might have to modify that part of the circuit. Not difficult.

-Likewise, the BFO is not directly coupled to anywhere in the circuit. Not normal. I suppose there's some stray coupling between the BFO and the 6AK5 in play. You could try adding an insulated wire from V3B grid to the 6AK5 control grid. Don't connect the wire directly to the 6AK5 control grid. Instead wrap a turn or two around the tube socket lug to make a gimmick cap. This wire should technically be a shielded cable, but if you run it close to the metal chassis, it should work all right to see if it improves BFO injection.

-Any idea what the IF is? 455KHz? 1700KHz? etc.

-Can you actually hear the HF LO on another receiver? For example, when receiving WWV on 10MHz, can you hear the LO at 10.455MHz, or some other frequency?

-Can you hear the BFO on another receiver? (Btw, this is where those cheap Chinese digital readout SW receivers come in handy).

-There's no AM detector, so I assume you're receiving AM by zero beating the carrier with the BFO.

-That 70pF VC is not a bandspread cap, although it may act like one because of interaction with the HF LO ('pulling'). That 15pF antenna trimmer VC might make a suitable bandspread control. It would be connected across the LO VC in parallel. Of course, being located on the side of the chassis, it's not in a very convenient position.

Several basic changes suggest themselves: first, better LO injection. Second, add a simple diode for AM detection (1N34, BAT46, etc). Third, better BFO injection. Fourth, move the 15pF VC to the front panel, use it as a bandspread. Fifth, use a voltage doubler in the PS to get the B+ up to about 50~60VDC, and add some decoupling RC networks.

The above are just my ideas. Think of them as suggestions, maybe try one at a time.

Rob

Jeff, Glad to hear that you got the set working. I'm tempted to make a copy of one for myself. Of course I'd make a few changes. Since it was apparently never intended for portable use, there's no need to cram everything together on such a small chassis. Placing the coils so close to the front panel is a Q-killer. Also, the whole point of raising the coil sockets above the chassis is to avoid the Q-killing shorted-turn effect of the socket holes in the chassis. The use of metal-rimmed coil sockets in this set defeats the whole purpose of the raised-above-the-chassis sockets. All-ceramic or all-phenolic sockets should have been used for the coils.

I would not use a diode detector as this would load the IF output resulting in some loss of selectivity. V3A appears to be functioning as a plate detector, power detector or anode-bend detector (all different names for the same thing). This presents a higher impedance to the IF output which is a good thing. Also, its non-linear characteristic allows it to function as a mixer for the BFO which is capacitively coupled to it in the same tube.

If you have a 6AJ5, you might try it in place of the 6AK5. It is essentially the same as a 6AK5 but was designed for operation on 28 Volts B+.

Perhaps when you have some time, you could provide some data on the coils: Form diameter, number of turns, winding height, estimated wire diameter.

Is the gain pot grounded on one end???

Are there any markings on the IF transformers such as Miller or Meissner part numbers??? I sure would like to know what the IF is.

Good luck with this interesting set and thanks for your posts on it. Max

Yes, that's it. I at first had thought the 4.7K cathode resistor was too small for plate detection. However, the B+ is only 30V, so it won't take much to bias the triode nearly to cutoff. And at the same time, it provides a non-linear response for mixing the BFO with the signal. Plus much reduced loading of T3. Clever. This is also how the HF mixer works. This may also indicate that V3 should be a 12AT7, the same as V1, not a 12AU7. Jeff, can you measure the voltage at the V3A cathode?

This also means my suggestion of doubling the B+ to 60V would throw all this off. It was designed to work at 30V (not that it could not be redesigned at a higher B+ and I still think that would be an improvement).

There's more to this radio than I first thought.

Another idea: make the first triode of V3 an infinite impedance detector. This retains the high input Z of the plate detector. The cathode bypass cap value can be chosen to make the stage oscillate for CW. The second triode can then be used for audio amplification. The advantage would be quite a bit more audio gain. The disadvantage, the CW beat note cannot be independently tuned.

Rob

Last night I was looking at the 12AU7 tube equivalents and saw the 5963 was essentially a 12AU7 designed for cut-off use in computers (long ago) and was made to maintain emissions quickly between "on/off" intervals. I had several 5963 tubes in my inventory and found the volume seemed to increase with this tube. So at the moment, I am using the 12AU7 equivalent 5963 with improved volume in V3.

Max mentioned the coil sockets with the metal stand-offs and screws might be affecting Q. I have ceramic 4-pin sockets BUT they are a bit too large to fit on this chassis. I just did the simplest alternative fix, I replaced all coil socket stand-offs with threaded nylon stand-offs and used very short machine screws to attached to chassis and coil socket. Now the metal around the coil sockets are insulated from the chassis. Also, I removed the (2) aluminum panel supports. They really were not needed and they were just millimeters from the coil windings. The coils remain close to the front panel but to resolve that would involve moving the panel forward and lots of modifications. I'm not sure the gain, if any, is worth all that effort. New nylon stand-offs and removing the panel supports was simple. Maybe it will help? I know that the metal panel is helping curb hand capacitance when tuning so it has that benefit.

Rob mentioned moving the antenna trim capacitor to the front panel. The original was a 1.2 -35pf "postage stamp" type trim capacitor requiring a screwdriver or plastic trim adjustment tool which is a PITA to use. So I installed a 4-35pf "air trimmer" with knob to the edge of the right front panel so the adjustment will be easy.

A rainy day in Northern Oregon today so I went through the chassis and replaced all the vintage capacitors. The (2) electrolytic capacitors were close to their specified values but I wanted to replace them due to age. The black beauty capacitors were all reading higher values so they were replaced. Two of the seven carbon resistors were out of tolerance (high) while the remaining five were testing accurate resistance. Found several bad solder connections and excess leads that needed to be cut. Now the chassis is updated with all components functioning as originally designed. Yes, the gain pot wire was grounded before I first powered up radio.

Here are some of the plug-in coil specifications:

Coil forms are 1.25" diameter and 2.25" high (not including pins)
4-pin UX-type base
Antenna coil - 14 turns of 31ga magnet wire (3/16" width) *****the antenna coil in photos has a larger second winding but it is NOT used in circuit.
Local OSC coil - 8 turns of 31ga magnet wire (1/8" width) and 23 turns of 31ga magnet wire (1/4" width)

I have lots of different size magnet wire and the 31ga alongside the coil windings looks to be a match (under a magnifying glass). Yes, it is fine wire!

Here is what I found on the IF cans:

T1 - Input IF - "B00-23126"
T2 - BFO - M-76J468-1
T3 - Output IF - no numbers or markings

Tomorrow I will check on some of the things Rob asked about, such as IF frequency and voltage at V3A cathode. I may try the BFO coupling with insulated wire too. And, I have a "6AJ5" incoming in a few days to compare with the 6AK5.

Jeff

BINGO!

I found this article "A Compact Superhet Tuner" in the November 1948 issue of "Radio & TV News". Thank you World Radio History! What a valuable resource.

I just found this article and have not read through entirely yet. But there is no doubt this is the original builder's project, from almost 75 years ago. Of course, my receiver is not that old.....but the project is. I see that the V3 tube is a second 12AT7 tube, although a 12AU7 seems to work better for me. I need to try some other 12AT7's I have, maybe it is the individual tube?

Anyway, this article will explain the original design and answer many questions.

Jeff

Congratulations on finding the original source; that hardly ever happens.



The standoffs and their screws are a minor problem. The big problem is the metal ring around the socket itself. That forms a shorted turn around the coil and affects the inductance as well as the Q. It doesn't matter if it is grounded, it's still a shorted turn. One possible solution is to move the windings on the coils away from the base and toward the top of the coil form. That moves their magnetic field farther away from the socket. Besides increasing the Q, it will probably also change the frequency they tune to.

One noticeable change by the builder: the RF tuning VC and the LO VC are ganged. In the original, they are separately tuned. Separately tuned is not a problem for a ham band receiver (and this was designed as a ham band rx), but for 'general coverage it could be a nuisance. Depends on how much knob twisting you can tolerate. It seems your version was intended for general coverage.

I noticed the writer mentioned adding regeneration to the 6AK5. I was also thinking about that. Since the IF amp has two IFTs, the easiest way would be to switch to a tube with a separate connection to the suppressor grid. 12/6BA6 is an obvious choice. Then connect a 2K pot wired as a variable resistor between the suppressor and ground. Turns the amp into a TGTP oscillator. (Historical note: a Hallicrafters engineer holds the US patent for this trick ((1958??)), but Japanese builders were already using it by 1950 or so). This is not an ideal Q-multiplier, it falls into the category of 'quick and dirty'. But quick and dirty is sort of the theme of this kind of receiver. A more typical version of this kind of receiver would be mixer+LO, a regenerative IF detector, a stage of audio. No BFO. Just one IF coil.

Interesting too that he says it works on such a wide B+ range. But I wonder about that. Higher B+ does change the cutoff point for the mixer and the plate detector, and that is somewhat critical.

Also curious about the choice of the plate detector. OK, far less loading than a diode and some audio gain. OTOH, fairly insensitive to weak signals. Zepler (The Technique of Radio Design) says it should have about 2V to work properly. High level of distortion too. Yet despite all that, it seems to work good enough for headphones.

Btw, Jeff, my suggestion about coupling the BFO to the IF amp was before I realized what I thought was an audio amp was actually a plate detector. It may work better to couple the BFO to the 6AK5 control grid, but it will almost certainly detune the IFTs.

I really wonder when your version was built. It looks too good to be about 75 years old. Yet it is so close to the original.

Thanks for posting all this. Very interesting design.

Rob

Just an UPDATE on the "Compact Superhet Tuner" from the Nov. 1948 article in the "Radio & TV News" magazine after some experimentation mentioned in the article.

Biggest improvement, in fact major improvement, was increasing the B+ voltage to just over the recommended 67.5 volts. The volume on weak stations now becomes much louder, more stations received overall and the "mixer tuning" has become very responsive. Mixer tuning is key to maximizing station reception and clarity. I would say the higher plate voltage is a game changer. (That was one of the first recommendations early in this thread. I'm not sure why the original builder used approx. 30 volts, the article recommends B+ voltage between 67.5 to 90, with little advantage to go much beyond the 67.5 volts. I do believe the home-brew power supply that came with it was built for this receiver by the original builder.) I am using the specified 12AT7's and the 6AK5 tubes.

I am now using (8) 9v alkaline batteries in series for B+ 72 VDC and (4) D-cell alkaline batteries for the heater 6 VDC.

I tried adding a small amount of grid to plate capacity using twisted wire (gimmick) on the 6AK5 IF Amp but got either uncontrollable "squealing" or with just one wire turn or no wire turns, no change at all. The radio worked better without adding G-P capacitance.

Next, as the article mentioned, I tried attaching a short wire from the mixer grid to the near vicinity of the BFO pitch control. I got some oscillation in the tuning range but it doesn't function like a "BFO pitch control" on my National and Hallicrafters communication receivers. I am not hearing any changes to voice pitch when listening to the amateur band. So I am using *super* fine tuning on the main oscillator dial to clarify voice communications.

So the biggest improvements were increasing B+ voltage AND some minor adjustments to the IF coils. I am satisfied with the current performance and don't plan to make any further modifications. The original design performed as advertised when following the original 1948 magazine article.

Jeff

Again, thanks for posting all this. It turned out to be a very interesting design. As I mentioned before, the standard approach from this era was a mixer/LO or converter front end, followed by a regenerative detector, lastly a stage of audio amplification. There are lots of examples in the magazines from that time. So you have to admire the guy's independent streak.

Concerning adding regeneration to the 6AK5. Much easier to use a pentode with a separate suppressor grid connection, like the 6BA6 or 6AU6. (Why the 6AK5? War surplus. The military used a lot of 6AK5, they were cheap and plentiful). The gimmick cap trick is very difficult to get right: you either get so little feedback that there's virtually no effect or you get too much feedback and the stage goes into oscillation. Which is just what you noticed. Even if you get the feedback just right, moving the IF gain control will alter it. There are also other problems when adding regeneration to an IF amp with double tuned IFTs front and back.

Do you have a license? It'd be great to pair this with a simple one tube tx, a couple of xtals, and a 40 meter dipole.

Rob

Rob,
Thanks for your comments. No, I am not a ham but just a hobbyist with an interest in vintage home brew radio.
BTW, the author and designer of this circuit is John F. Clemens (formerly W9ERN) who was 25 years old at the time and authored many
articles in the magazines of the era. He started Clemens Manufacturing and produced the Mosley CM-1 communications
receiver and the SG-83 signal generator. I saw some earlier threads on the CM-1 and the SG-83 here on ARF and you
participated in the conversation, so I'm sure you will recall John Clemens.
Unfortunately, he passed away in 2018 at age 94 after a long career in electronics. He was interviewed a couple years earlier - see attached link to brief video.

Jeff

https://www.youtube.com/watch?v=VtMMCHYbKBE

The tuning dial looks very familiar, like this one from and old WW2 clandestine radio I picked up at a yard sale.

Jeff, many thanks for that info. I didn't notice that it was John Clemens. Damn. Now the design makes sense: John wasn't going to simply copy the then current homebrew trend, he went his own way.

I much wanted a CM-1 back then, but it was beyond my means. I still think that if it had looked a little more 'stylish' and less homebrew, it would have sold more.

Rob

That dial is generic; I have a couple of them in my junk box.

The dial is a National Type B dial. It's the only radio dial with a continuously variable reduction drive that has a gear shift (actually a variable friction drive). By moving the lever located behind the tuning knob, you can go from fast motion to slow motion. Arguably the best-ever dial for regenerative radios. Used in the National SW-3 series of regenerative and other National receivers as well as some non-National brands. It was sold as a generic part by National. BTW, Jackson Bros. made one with a two-step reduction drive (6:1 and 36:1). Most people modify it to remove the 6:1 feature due to the high friction in that ratio that makes it difficult to tune. By contrast, the National Type B has equal ease of tuning at all ratios. Max

SW-3 or some variant? Not "really" a spy radio, some short comings if used for that.

An all purpose compact radio yes indeed. The model and variations lasted many years. Still a very good performer when restored.

Chas

WA1JFD

One probable (safety) reason for the low B+ is the use of B+ on the headphones.
An isolating audio transformer is one possible cure and would allow feeding an audio amp.
-=-=-=-
On another subject, the metal ring around the coil sockets can be cut in one place with a fine hack saw blade or Dremel blade.
It might be interesting to measure how much the modification would change things.
Pat