Old Transistor Circuit - Positive Feedback Question

Looks to be the same operating mechanism as the tube one you posted. I.e., probably nothing at all...

If CK722 fT is enough greater than the tank resonant frequency, the first transistor will probably oscillate (via C-B capacitance, tuned base configuration). If not, it's possible the "output" will feedback into the "input" by proximity.

The impedances are all quite high, implying poor bandwidth (even for CK722) and very little action at all.

If it does anything at all, it's likely a matter of the environment between the coils, not so much the circuit itself. Primitive RFID? Proximity detector? Metal detector? Regenerative radio without the speaker?

If they're between 1 and 100 turns of wire in the size shown, they'll do SFA to the impedance of much larger (kohm) resistors in series.

You'll see the same kind of thing in new designs, throwing ferrite beads anywhere possible -- including places where they actively make the circuit worse, not better. Somewhere between paranoia and superstition.

A coil is a bit higher quality than a ferrite bead, but the impedance is comparable: some kohms at the self-resonant frequency. Which is probably a much higher frequency than the transistors can operate at. At lower frequencies, they'll be a few ohms reactance, or hundreds of ohms, which does nothing compared to the 10k or 220k resistors.

Tim

So then, it may be a primitive transmitter. Given the 1940's date of the circuit, would there be any technical or production advantage using a regenerative approach rather than amplitude modulation?

Is it possible the chokes may contribute to the regenerative effect through their radiated field?

One more point.

There appears to be a couple of tunable plate capacitors across the input coil. Any indication what effect these may have on the output?

Robert Miller

CK722 was 1953 at the earliest.

"Transmitter" would be pushing it. It might make a sine wave. It won't travel any distance.

It surely would not have been in production. A hobby project, probably.

Look, what are you trying to read into these things? It's a pile of parts, not optimized for anything obvious. At best, it's an example of accidental design, not top-down intent. It's no more enigmatic than an audio amp someone tacked extra wires onto. It's very uninteresting, which makes your interest in them perplexing.

Perhaps you could explain what your interest is? Then we could try to provide answers that are more meaningful to you.

No, they don't have enough impedance, and they're too small to radiate per se.

Also, without a mechanical diagram, it's impossible to make any guess at coupling between any of the coils, including the "large" ones (which don't have any winding info at all, remember).

Well, tuning, of course! ;-)

Tim

where do you see feedback?

I don't think it was necessarily drawn by a madman or crackpot electro-healer; looks like it was drawn by a teenager/newbie who wanted to make their own s*it but didn't quite have the theory down to do so yet, so it's a mishmash of things they'd seen but weren't really appropriate.

I'd have a bunch of drawings/Spice files like that from back in the day if I'd bothered to save them.

Besides, the thing is very unstably biased. Just jettison it with the pentode monster.

To the OP: Where do you dig these uglies from?

Like the tube version a few days back my guess is some kind of pseudo science medico nonsense. You use it in conjunction with a nearby AM broadcast radio receiver and listen to weird whistles and whoops from the device. The two spaced antennae/loops are so you can wave bits of your body in the space between and imagine from the change in tone that you have diagnosed or cured without malady afflicts?

piglet

The CK727 PNP germanium probably didn't have a nominal beta of more than around 20, and the curve of beta vs. collector current probably headed straight for the dirt at low collector currents. 220k ohms from 3 volts to the base won't provide anywhere near enough base current to bias the transistor into any kind of operating point sufficient for HF work, and if it turns on at all its operating point will be totally unstable wrt temperature, anyway.

That's not far from weird music instruments of the yesteryear: Ondes Marthenot and Theremin.

That reminds me of something I read in TIME magazine in the days when transistors were where robots are today - widely used in some fields but still mysterious cutting-edge technology to most people (Iron Man comics often mentioned transistors overheating). The article spoke about an exhibition in which some piece of techno-art wowed visitors by emitting whoops, whistles and lights when they went near it.

Ya most major US cities have those these days, they call it the "Apple Store"

The emitters feed back to the bases through a choke and R1 + R2.

I might simulate it in LTSpice and see what happens.

Robert Miller

On another scrap of paper the input coil is specified as 7T 2 1/2" dia. and the output 7T 6" dia. Presumably air core close wound on a former.

I was hoping to find something unusual, mostly just as a learning experience. The schematics were in fact drawn by an electrical engineer (now deceased).

Robert Miller

No feedback there... See Rd and Cd? They decouple the battery supply to the first two transistors. The whole circuit is probably a TRF radio reciever of unknown frequency, since no values are given for the input tuning tank.

Dave M

Bingo, that's the same feeling I get from it. :)

Ha, living in the information age, I still have all those on hard drive... (in my earlier years, that was QBasic; I picked up SPICE in college).

Tim

Interesting. So was the small coil placed inside the large coil?

Rd,Cd seems to be blocking the positive feedback. maybe also functioning as an AGC?

chokes on the collector increase gain, chokes on the base reduce feedback thorugh the bias network (probably no effect in this circuit)

Hmm.. someone else was collecting circuits, this one was also in the doc I posted a link to. If you don't know what these circuits were for, you might be in for a surprise...

Ran into stuff like this about 25 years ago, wild claims, circuits that looked like amplifiers but supposedly did magical stuff. Actually built the 2-stage pentode circuit, added a gain control and output control, input and output jacks, (filtered!) HV supply - awesome guitar overdrive. I used 12SJ7 tubes. As far as the claims.. well it made me feel good when I played through it...