After looking at a few sets of mine, I noticed a different way of placing the station numbers on the dial. On a larger number of portables the stations are numbered with an expanded space on the low end & a compacted space on the high end, 74 is in the middle of the dial. On a smaller number portables the stations are different, 90 is in the center, & the high end is stretched out a little better. I have a few old car radios where the low end is the same as the high end as far as spacing is concerned,the center of the dial about 1050. I looked at the car radios & they have a different type of tuner. It is a black "slug" (or round tube) that moves back & forth. Let's forget the car radio for now. On the portables, what causes the difference in dial spacing? The sets work OK (or seem to). After reading "fun with tubes", all the sets are "superhets". What changes inside the set? Is it the size of the set, the manufacteur or the useage where in the US they are used or sold? I'm guessing that most of the tube sets have the same type of circuits. The transistors have different circuits. I have "exploded" the pictures on the radio attic archives. Zenith has the 90 in the center on most of the portables (tubes & transistors). Only the small ones have "screwey dials". On other makes the dials are like I described "up above" & there is a lot more of them! Did ZENITH do something better? I know they had quality but does the dial calibration spacing change the quality? On the high end it would seem that the stations would be very hard to seperate ESPECIALLY in the bigger cities. One station would be on "top" of the other one!(New York City area for example.

Mechanical tuning capacitors aren't linear with respect to their settings. For example, the capacitance of a fully-meshed capacitor isn't necessarily double that of a half-meshed capacitor. Radios that use direct connections to the tuning cap tended to be inexpensive and a linearized dial scale was traded off in favor of keeping the price point low. More complex radios have schemes for linearizing the tuning cap's response, which generally requires noodling around with the cap's plate shapes. This necessitates designing and building a custom tuning cap. Not so much a "quality" issue as it is a "feature" thing.

Higher-end sets have reduction gears in the tuning mechanism which allow fine separation of close stations. Sets designed for radio hobbyists like hams and short-wave listeners incorporate bandspread for the same purpose, which is just a small adjustable set of caps in parallel with the main tuning cap.