[tom2]

I am in the process of reworking a 1946 Electronic Laboratories, Inc. (EL) 6 tube wooden table radio. Model 2701. Case shows wear but the chassis was clean and complete. The unusual feature for me, is that instead of a tuning cap, it uses a vertical rack system that moves ferrite slugs in and out of the oscillator and RF coils. I am about to give it a full alignment and have the data. How unusual is this style of tuning in consumer radios like this? I know that comm radios use this scheme to tune the preselectors sometimes, but I wonder what was the rationale for using this instead of the simpler var. caps? I know it is from 46, postwar materials shortages might explain the cheap enclosure, and the use of window screen for a speaker grille! And would explain why it has a combo. of carbon comps and dog bones underneath(latter actually cked OK). I recapped it and replaced resistors as needed. 6 octal tubes with an RF stage. It is a listener, not a showpiece! Window screen...

[BILL]

This sounds like an auto radio design, Some auto radios ,with the war time shortages, were converted by the factories to house radios and run on 110AC, I have some examples of these myself, BILL

[Doug Houston]

Many people are astonished to come upon slug tuned radios, other than automotive ones. I have an RL set that has the slug tuning, as well as a Delco table set with it. The slug tuning rig used less chassis space than conventional condensers, though that was not much of an issue in chassis design. I doubt if manufacturing cost was ever an issue with slug tuning.

My feelings are that it's just another way of tuning the set, that works, and surprises people.

[TexMac]

I have a Bakelite Stewart-Warner set made this way from 1946. I also own a Mantola (BF Goodrich) that has a conventional tuning cap but a slug tuned RF section with a linkage lever connected to the tuner from about the same time. The only thing about slug tuned front ends you need to be careful of is that the radio be allowed to equilibrate to its environment before alignment. They seem to be a little more subject to moisture in the controls. Why this did not create problems in automobile sets is still one of those little mysteries I have not figured out yet, unless the closed space and temperatures redried them quickly enough that no one noticed a little drift.

[exray]

I had a Truetone like that. D2210 I think was the number. Plain-Jane octal AA5. I'm surprised the scheme wasn't used more often. Looks simple and cheap.
That radio had a metal cabinet with a curved speaker area. It too had something similar to window screen to protect the grille cloth.

-Bill

[tom2]

Oh yea, I forgot about those old car radios that used slugs. Have not had one of those open in years. Decades. I set the slugs per the instructions for initial alignment as the set was off at the low end. Seems I had to turn them all the way down then to get it to track the bottom. Maybe I need to pad some capacitance across the coils or something but it seems to be OK now. It sure is not needed to save space, I don't think. Not a compact radio or chassis at all. Probably just some design engineers that wanted to show they were different. Presumably did not get them market share as I am unaware of much product ever coming from that company. Or references to them. I did install a fuse and a polarised plug in this thing for some measure of safety.

[MinnesotaHam]

I have a model 540A Emerson midget radio, circa 1947.
It's tiny and cramped, and chassis space is definately the reason for using slug tuning.
http://www.tuberadioland.com/emerson540A_main.html

[exray]

That Truetone I referred to earlier had an adjustable mounting for the coils. You could slide it in increments as part of the alignment - it had little notches on the assembly where it was mounted to the chassis.

-Bill

[gary rabbitt]

The common Detrola 571, in all sorts of brand names had slug tuning. At least in the Imperial branded one I have.
The 571 came out IIRC 1946-47 era.

[Ron Ramirez]

I have a Firestone Air Chief 4-A-22 here, that belongs to Barry Jones. Made by Stewart Warner. It has slug tuning, and mice have destroyed at least one of the four slug coils along with the antenna coil.

Waiting on delivery of a Stewart Warner 62TC16 (identical) chassis I bought from an ARF member, so I can restore it instead of the unrestorable Air Chief.

[tom2]

I was just reading/flipping thru some old collectors mags and they had something in there about mfr. costs of radios per 1000 or the like. Some mfr. was using slug tuning because it cost 90 some cents for a tuning cap in volume at the time and half that for inductor tuning of some sort. This is for home not auto and I think it was prewar 30s too. I would think that the labor costs to set up and fiddle with those tuning slug arrays, as well as making them would be more expensive than mass produced universal type tuning caps, but assumptions can be wrong I guess. Dang. I keep reflexivly clicking "save" instead of "submit" when I finish a posting, then I have to go thru and figure out how to submit the draft again! Well websites vary in how they work. Gotta think.

[Arran]

It seems like there was a bit of an engineering fad surrounding permeable slug tuning in the late 1940s, there were a few makes and models that used them and then the fad didn't go any further, maybe because the big three didn't go along with it . But in the 1950s everyone went back to the conventional LC condenser tuning except for car radios and FM sets, even most of those used a tuning condenser of some sort.
Best Regards
Arran