Jesse French Junior Model 8 TRF

You're looking both at a nomenclature change ca. 1932 and, in the case
of the 24, a design change for the tube.

I'll deal with nomenclature first. In the 1920's, tube types were
designated by base type, and three digits, beginning with a single
manufacturer identification digit. Thus, an 80 rectifier was
originally (1927) a UX-280. "UX" is a 4-pin plug base, and the
initial "2" is "RCA design." Note that until 1928, RCA was a patent
pool holding patents for Westinghouse and GE and (originally)
AT&T/Western Electric. They entered manufacturing with the purchase
of the Victor Talking Machine Co. in 1928. As I recall, Western
Electric used 1, the RCA pool partners used 2, and Cunningham used 3.
The UX base designator was for the standard 4-pin base, and UY for the
standard 5-pin, both with the two filament/heater "fat pins" adjacent
for locating and current-carrying. The "V99" was originally "UV199,"
an AT&T/Western Electric design from about 1919 originally used as a
telephone signal amplifier. It actually predates common receiving
tubes from the early/mid 1920's. The "UV" base is the nub-pin bayonet
type used on that tube.

That nomenclature was dropped in 1931-32, leaving only the two-digit
design number as the identifier. Thus "UY-224A" became "24A", "UX-245"
became "45," and "UX-280" became "80." A follow-on to this was
changing the nomenclature to a three part format, such as "6D6", where
the first "6" meant heater/filament voltage; the "D" meant high-gain
amplifier, and the last "6" meant the total number of elements in the
tube. Except for the heater voltage number, there wasn't much in the
way of standardization in the assignments that followed.

24 tetrode redesign to 24A:
The original 24 tetrode (1927, as I recall), had a problem with
secondary emission from the plate. Note that the second grid is
called "screen grid" because it creates a constant high-voltage field
beyond the control grid, so it "screens" the effect of plate voltage
changes from the control grid and cathode, increasing dynamic plate
resistance. However, it also accelerates the electron stream to a
velocity that bombards the plate, causing the plate to emit electrons
(secondary emission). This put a negative resistance dip in the
dynamic plate resistance. That makes it difficult to use the tube in
a circuit driving a following tuned circuit, as the circuit will break
into oscillation fairly easily.

The 24A uses a different material for the plate that reduces the
secondary emission from the electronic bombardment. This reduces the
oscillation instability problem. The ultimate solution was to add a
third "supressor" grid beyond the screen, to "suppress" collection of
secondary emission electrons by the screen. The type 57 design was
the result. The 35 tetrode and 58 pentode are a similar evolution
with a varying-pitch in the control grid winding to give remote
cutoff. Otherwise, the original 35 was enough later that it used the
24A technology in its construction.

There are a ton of discussions about tetrode and pentode "screen" and
"suppressor" grids in various texts, but I'd look at Carl Spangenberg,
"Vacuum Tubes," (McGraw-Hill, 1948). Spangenberg was Terman's
student, but, coming a generation later, used a much more "modern"
physics in his presentations. Some of the older texts give
explanations that sound more along the lines of "caloric" and
"phlogiston" in combustion and heat engine theory.

Hank