RadioBanter - FM from the stone age?

RadioBanter (https://www.radiobanter.com/)

- Homebrew (https://www.radiobanter.com/homebrew/)

- - FM from the stone age? (https://www.radiobanter.com/homebrew/21630-fm-stone-age.html)

"Roy Lewallen" wrote in message
...
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.

It was actually posited in a presentation in 1935. See Wikipedia entry on
FM.

David Forsyth wrote:

Would it be possible, though not necessarily practical, to make a receiver
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.

I have a few pre war FM sets, see
http://www.geocities.com/wa2ise/radios/fm45.html

What sorts of
tubes could one use from the 1930's to make a receiver that could tune in
the modern FM band?

Focus on the front end. Once you get to the IF, then my Emerson 460 pre
war set
would have the same technology as your "what if" set. I think acorn
tubes were pre war,
and were designed for VHF service. 2 or 3 such tubes (one RF amp,
another local
osc, and the 3rd the mixer) should make a good 100MHz front end. The
above Emerson
had an IF around 4 or was it 8 MHz, used octal tubes like 6SG7 and 6SH7
and a 6H6
for an FM detector.

Brenda Ann wrote:

"Roy Lewallen" wrote in message
...

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.

It was actually posited in a presentation in 1935. See Wikipedia entry on
FM.

FM back then was on frequencies around 25MHz for experiments. Ordinary
higher frequency HF SW front ends would have sufficed, with a higher
frequency IF strip. Today hams have a
small 10 meter sub-band for FM somewhere around 29MHz. Later a
broadcast band
from about 41 to 50MHz was set up. See
http://members.aol.com/jeff560/jeff.html for more FM history.

In article , "David Forsyth"
writes:

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?

I doubt you will find much in amateur radio periodicals. They do
exist in various corporate libraries, though, since the VHF and up
radios were being pioneered in the late 1930s, some of the first being
police mobile radios. Those would evolve into the SCR-300 backpack
"walkie-talkie" of WW2 Army use and those shiny chrome button
channel select "tank radios" used mobile back then. The AN/TRC-1
through TRC-4 of WW2 times was low VHF (70 to 90 MHz) using
octal base tubes, PM with lots of multiplier stages following a MF
crystal oscillator.

Of course there are lead length problems with big bases as well as
the electrode connecting leads within bigger tube envelopes. What
most folks don't consider is the electron transit time within the tube
structure. That is slower than the speed of light (don't have an exact
value handy) and will cause a significant phase change between
grid input and plate output at VHF and above. The "lighthouse" triode
structure (ultimate may be the 2C39) has terribly short tube electrode
spacings allowing operation on up to 2.5 GHz...very quick electron
transit time internally.

Note: Klystrons and magnetrons go much higher in frequency. In the
case of the klystron, an integral tuned cavity structure is an absolute
requirement for operation. The magnetron depends upon both the
diode spacing (it is only a diode) and the magnetic field and the voltage
and the output tuned cavity structure to oscillate at X-band.

Anyone can play games with old, big tubes used at low-VHF and
arrange all kinds of neat tuned circuits to work with long leads and the
long electron transit time. Problem is, the amount of extra components
isn't really worth it. Whenever a tube has to be replaced (happened
often due to filament technology still lagging) then you would need to
do a humongous amount of retuning. About the only thing workable
for the oldie tubes is the distributed amplifier wherein LOTS of tubes
were arranged along tapped delay lines for grid inputs and plate
outputs. [the Tektronix 54x series of oscilloscopes uses such an
arrangment as the final CRT voltage driver for vertical deflection
plates] Very good as a space heater for a residence in winter...

The electron transit time thing is akin to a low f_sub_t in transistors.
Such low f_sub_t bipolars might be okay on low HF but their
characteristics don't allow good amplification or easy oscillation at
VHF and higher. Some of the newer SiGe bipolars have f_sub_ts in
the tens of GHz range.

Electron transit time depends on filament-cathode temperature, tube
geometry (and element spacings), and accelerating potential (screen
and plate quiescent voltage). There's no precise value that fits all
tubes. To view what did work at 70-90 MHz, find an old AN/TRC-1
Technical Manual and see which octal base tubes were used there.
The transmitter box final amplifier was an 829, good for about 50 W,
but not an octal base tube.

Len Anderson
retired (from regular hours) electronic engineer person
[and caretaker of TRC-1s and TRC-8s a half century ago, among
other tube-based radios]

You're going to have a bit of trouble with the older tubes. They had long
skinny leads leading down to those high-capacitance octal bases.

Hams were able to get things going up around 140Mc/s, but it was mainly
super-regen receivers.

If you want to try it, I'd find the smallest dual-triodes of that era, say
6SN7's, and make up at least one maybe two cascode Rf amplifier stages, then
another triode mixer, then maybe another cascode first IF amp. By then you
should have enough signal to start using old pentodes as IF amplifiers. If
localts are allowed, the 7F7 IIRC is probably a much lower capacitance
dual-triode.

I do recall one post-war Sparton FM set that used a 6AC7 as the RF
amplifier, a 7Q7 (loctal 6SA7) as the osc/mixer, then three 6SJ7's for IF
amplifiers and limiters. Worked surprisingly well.

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.

Jack K8ZOA