In article , Jack Smith
writes:

On Thu, 13 Nov 2003 17:33:19 -0800, Roy Lewallen
wrote:

As I recall, Armstrong invented and patented the FM radio before WWII.
If I'm correct, his patent should show a schematic of the circuit he
used. It's probably still available from the patent office.

Roy Lewallen, W7EL

Roy-

Armstrong's classic paper "A Method of Reducing Disturbances in Radio
Signaling by a System of Frequency Modulation" was published in 1936
in the November Proc. IRE. (The paper was presented in a demonstration
at the Nov 1935 IRE New York meeting.)

His field work started 1934, at 44 MHz, with a 2KW 44 MHz transmitter
on the Empire State Building shortly thereafter (the article is a bit
vague on the timing of this part of his operation). (Also, the
frequency was changed to 41 MHz at some point during the trials.)

Armstrong also notes the problem with receiver RF amplifiers at this
frequency and thanks RCA for its provision of experimental VHF
receiving tubes.

His detector circuit looks like a conventional discriminator to me;
two detectors each coupled to a tuned circuit with the outputs summed.
One detector gives + and the other a - output. One tuned circuit
resonates on the high side of the IF passband the the other on the low
side. Each detector fed by an independent buffer amplifier. The
receiver was a double conversion, with the first IF at 6 MHz and the
second IF (and detection) at 400 KHz and a passband of 150 KHz.

It's not necessary to go back to just a few years after the first
audion. :-)

The Galvin/Motorola SCR-300/BC-1000 used 7-pin glass base
"miniature" tubes, battery type filaments, and was 40 to 48 MHz.
Tube types were 3A4, 1T4, 1R5 in the VHF range. It started into
full production in 1942 and about 50,000 were built. FM. Dan
Noble migrated to Chicago to work for Galvin in 1940...after finishing
a design for the first mobile police radios...pictures show those
tubes to be glass, octal-base. Noble is the chief architect of the
SCR-300 walkie-talkie. Interesting little gem of a then-tiny
transceiver.

The argument against that might be that 1942 was "after" the entry
into WW2. But...those battery-filament (directly heated) tubes
were in existance in late 1939. Galvin/Motorola used them in the
SCR-536/BC-611, the HF-range "handie-talkie" which was started
in design in 1940.

Perusing the MIT "Rad Lab" volumes ("Amplifiers" is the one I have)
will show that 6AG5s were already around for radar set IF strips
of 30 and 60 MHz. Another 7-pin all-glass "miniature" envelope but
with an indirectly-heated cathode. See also the 6AK5. Both are
the "short" version (physically short compared to the "All-American
Five" types with 7-pin bases and all-glass).

One of the reasons one didn't see those (then) advanced tubes was
that the consumer market products still went in for the cheaper
octal base kind...sockets were also cheaper, many being made out
of melamine-laminate stamp-outs riveted together to hold individual
tube pin grabbers (first cousin to solderless joining gadgets). Most
of the consumer radio equipment of the pre-1950 era were indeed
low-quality "Model T" kind of things.

In pre-WW2 days, a 7-pin all-glass envelope for a tube cost more to
produce than the older octal base with pins soldered to tube
element leads. All metal envelope tubes cost more than glass
envelope types. Once tube makers ramped up for the all-glass
7- and 9-pin little ones, they got cheaper and were used in post-
war consumer radios.

EVERYTHING in radio was already in continuous change from the
1930s onward...even if it didn't show up in the consumer marketplace
right away.

I suppose it is very interesting for antique boatanchor folks to play
with those "stone-age" tubes. Ho hum, I say to self, broke into
vacuum tubes using a 201A from an ancient radio. Phooey. :-)
My wife often uses a little AM-FM set with ear-bud listening things,
the set about the size of a cosmetic compact case. Was a Give-
Away sales promotion thing in the mail. Cost nothing. Obviously
no tubes in it. Fine FM, including stereo sound.

Len Anderson
retired (from regular hours) electronic engineer person