RadioBanter - FM from the stone age?

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"David Forsyth" wrote in message
...
Would it be possible, though not necessarily practical, to make a reciever
for the modern FM broadcast band, using only pre-WWII tube technology? I
know they had FM on a lower frequency band at that time. What sorts of
tubes could one use from the 1930's to make a reciever that could tune in
the modern FM band? I'm sure I wont actually attempt such a thing,
especially any time soon, but just wondered how difficult it might be.
Are
there any schematics or construction articles from the late 30's about
making FM radios that might be adapted over to the new FM band?

just curious,

Dave

The RCA 10T tombstone used the new metal tubes, and had an
UltraShortwave band that reached to over 60 mc. Unfortunately,
most of the RCAs had serious problems with the LO dropping off
at about 50 mc orso, very few of the radios ever seemed to actually
tune that high! I think I've seen one example that worked across the
full tuning range without twiddling the circuitry to make it work.
Bu, in 1936, probably no one even noticed, or cared.

Pete

Hi,
Dave asked:

Would it be possible, though not necessarily practical, to make a reciever
for the modern FM broadcast band, using only pre-WWII tube technology?

The Hallicrafters S-37 tunes 130 to 210 MHz and although it dates
from 1945, it uses 954 acorn tubes and could have been made with 1930s
technology. There's a front-panel selector for AM or FM (narrowband
most likely, but the manual doesn't say and I've never tried running
mine).

73, Alan

"Alan Douglas" adouglasatgis.net wrote in message
...
Hi,
Dave asked:

Would it be possible, though not necessarily practical, to make a
reciever
for the modern FM broadcast band, using only pre-WWII tube technology?

The Hallicrafters S-37 tunes 130 to 210 MHz and although it dates
from 1945, it uses 954 acorn tubes and could have been made with 1930s
technology. There's a front-panel selector for AM or FM (narrowband
most likely, but the manual doesn't say and I've never tried running
mine).

Wideband. I suspect that the S-37 and S-36 differed only in tuning range.
The S-36 had an IF bandwidth suitable for FM broadcast. The wouldn't have
been any narrow FM in that frequency range in 1945. Certainly no land
mobile, and wasn't land mobile about 20-30 kHz wide until post-WW2? Crystal
frequency stability didn't get good enough for NBFM at high VHF until the
hermetic holders like the HC-6, which are post-WW2, aren't they.

Even HC-6 and snap-action thermostat ovens could be pretty awful. Collins
built very special 400 MHz radios for an Air Force project I was on in the
early 1950s. Listening to their output frequency with BFO on you heard a kHz
or more of drift, with the fractional-minute cycling of the thermostat/oven.
Our digital modulation was 180-degree-shift PSK of the 400 MHz carrier, and
couldn't stand all that short-term drift. The solution was to disconnect
the oven's heater. We could stand slow drift.

"R J Carpenter" wrote in message
...

Wideband. I suspect that the S-37 and S-36 differed only in tuning
range.
The S-36 had an IF bandwidth suitable for FM broadcast. The wouldn't
have
been any narrow FM in that frequency range in 1945. Certainly no land
mobile, and wasn't land mobile about 20-30 kHz wide until post-WW2?
Crystal
frequency stability didn't get good enough for NBFM at high VHF until
the
hermetic holders like the HC-6, which are post-WW2, aren't they.

My S-36 has two selectivity settings, sharp and broad. The broad
position is fine for FM broadcast. I don't know how sharp the sharp
position is.

[snip]

Frank Dresser

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

Hi,
Thanks for the correction. The S-37 doesn't have any choice of
selectivity, unlike the 36 which had tertiary windings in the IFs.

73, Alan