This was a most unusual early radio design

Den tirsdag den 15. december 2015 kl. 01.23.27 UTC+1 skrev boomer#...@none.com:

The concept was used into relatively modern times... I certainly can recall, in the early '60's, automobile radios being "slug-tuned". ...Jim Thompson

Variable inductors aren't that unique.

The large single-layer coils weren't just because they didn't have machine-winding. It was also because people hadn't figured out how to make good small coils (Google "pi wound coil").

Those spherical variometers look very cool.

It was once common for car radios to be slug-tuned, with variable inductors instead of capacitors. That made preset push-button tuning easier.

look at these contraptions

-Lasse

Coilcraft has slug tunable variable indictors up to around a millihenry, cheap. I may use one of them soon.

Large diameter coils require less wire for the same inductance, because inductance is proportional to area, but wire length to diameter.

I did that, and saw some pictures (and a bunch of apparently unrelated metal-detector coils called Pulse induction"), but nothing really definitive.

Do you have a better source?

Clifford Heath.

How about this one:

Surface mount. On the floor.

.... Phil

disagrees with you. See the r^2 term in there?

Clifford Heath.

This on-line calculator shows what happens:

So same wire length with double diameter gives less L.

BTW:

Air coils used in early AM radios needed to have a self resonance frequency above the highest frequency being tuned and this dictated the use of large diameters and single layers.

... Phil

Wow, you just brought back a memory of mine from about 50 years ago. I used to play around with darn near anything electrical or electronic when I was in my teens. I went to a nearby auto junk yard, and got a car radio from a junked car, including the speaker. The guy that ran that junk yard liked having me around because I helped him with stuff and I always wanted to learn about cars, so I spent a lot of time there. He gave me that radio because I had never played with a car radio and wanted to see what was in one of them.

Anyhow, that radio was probably 1940s vintage. I remember opening it, testing all the tubes and getting it to work using a 12V power supply. I recall the speaker cone (from the same car) was bad, so I got another speaker from the junk yard and it worked well. It had push buttons, and I can almost visualize those variable inductors now. That radio was large and heavy, and built like an army tank. There was a lot of metal around those push buttons causing them to take considerable space inside the radio chassis.

Those were a brilliant invention. You tune the station using the knob, which pulls the cores in and out of the LO and RF tuning coils. Then you pull one button way out, releasing the lock on its cam, and push it back in, locking the cam in contact with the tuning bar at exactly the right place.

After that you could change stations without taking your eyes off the road.

Cheers

Phil Hobbs

I found they always needed a little tweak after pressing the button, but yes, they were clever.

Cheers

I've always wondered how they shrunk coils down so far to fit in these tiny modern transcievers, having been of the view that nothing beats large, open, silver-plated turns for Q and consequently selectivity. That's why most of my ham gear is 40 years old when coils looked more like car springs. I still don't see how you can beat the old-fashioned way of doing it from the reception quality pov, but suspect I'm about to be corrected. ;-)

Answer 1: it explains why today's cheap receivers don't as well as the not-so-cheap receivers of 40 years ago

Answer 2: the modern receivers use software instead of coils. while that cannot solve all far-off selectivity problems due to dynamic range issues, it surely beats coils at channel selectivity.

Yep, I remember them well. ...Jim Thompson

My Audi has the electronic equivalent. It too about a year to figure out how it works.

I had an old tube/pushbutton car radio when I was a kid. It really sounded great in my bedroom, with a "sweet 16" speaker array in a homemade wooden box. I got rid of the vibrator and jammed the filament winding of a TV power transformer into the radio primary. It ran really hot (must have been saturating the radio transformer) but it worked fine.

Junk electronics was useful and free back then. A local sign company would give us an old 15KV neon sign transformer for the asking. The chemical supply company would sell us anything. Imagine something like that now.