"David Forsyth" wrote in message
...
So basically if I could somehow use 30's tubes to tune to 100 MHz more
or
less, all I would need to do then is to add a suitable FM detector and
bingo? Hmm I think I will need to do a lot more research on this
one....


Dave



You should be able to get good performance with acorn tubes in the front
end. Acorn tubes pre-date octals, if I recall.

The prewar FM setup not only used a different band, but also a different
audio pre-emphesis. 100us as compared to the current 75us.

Frank Dresser

In article ,
"Frank Dresser" wrote:

"David Forsyth" wrote in message
...
So basically if I could somehow use 30's tubes to tune to 100 MHz more
or
less, all I would need to do then is to add a suitable FM detector and
bingo? Hmm I think I will need to do a lot more research on this
one....


Dave



You should be able to get good performance with acorn tubes in the front
end. Acorn tubes pre-date octals, if I recall.

The prewar FM setup not only used a different band, but also a different
audio pre-emphesis. 100us as compared to the current 75us.

Different pre-emphasis is hardly a big deal, the important thing is that
the only significant difference was the frequency band used. The FM
deviation was the same for the pre war and the post war systems, so a pre
war IF strip and detector would work on the post war band, the only
potential problem being image responses due to the lower IF frequency used
pre war.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

In article ,
"Frank Dresser" wrote:

"David Forsyth" wrote in message
...
So basically if I could somehow use 30's tubes to tune to 100 MHz more
or
less, all I would need to do then is to add a suitable FM detector and
bingo? Hmm I think I will need to do a lot more research on this
one....


Dave



You should be able to get good performance with acorn tubes in the front
end. Acorn tubes pre-date octals, if I recall.

The prewar FM setup not only used a different band, but also a different
audio pre-emphesis. 100us as compared to the current 75us.

Different pre-emphasis is hardly a big deal, the important thing is that
the only significant difference was the frequency band used. The FM
deviation was the same for the pre war and the post war systems, so a pre
war IF strip and detector would work on the post war band, the only
potential problem being image responses due to the lower IF frequency used
pre war.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

R J Carpenter wrote:

"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.

About half the present frequency.

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

It was something of an effort for them to reach even 50 MHz. Tubes like the
6AC7 would have some gain. No one building entertainment radios had heard
of noise figure, so their sensitivity was terrible by post-war standards.
One of the most popular and best prewar tuners was the GE JFM-90. Its front
end was two pentode mixers, one after the other. Its noise figure must have
been astronomical. At least 20 or 30 dB worse than today's radios.

I'm sure I wont actually attempt such a thing,
especially any time soon, but just wondered how difficult it might be.

The main problem would be the large cpapacitance in the big octal tubes.
IIRC, the first all-glass 7-pin miniature tubes didn't appear until 1940-41,
the 9001, 9002, and 9003. You could build a fair set with them given
today's knowledge.

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?

The new band wasn't chosen until about the end of WW2, so prewar sets
weren't designed with it in mind. There were only a dozen or so operating
FM stations before WW2, so there wasn't a lot of discussion of the
technology. I have/had a just-prewar book of build-it-yourself radios put
out by Popular Science (I think) which had an FM set. It was pretty
complicated. They hadn't learned how to cut corners. The IF in old-band sets
was 4.3 MHz, not today's 10.7 MHz. The lower frequency allowed better gain
with the tubes at hand, and the narrower FM band didn't result in inband
images. Modulation, etc, was the same as today.

Some of old VHF Hallicrafters sets used the acorn 954, 955, etc, tubes.
They would do ok at 100 MHz, but were exotic and never used in consumer
equipment.

Articles I have seen about early attempts at VHF talked about
removing the tube bases to reduce lead inductance but gain was still
very low, even around 50 MHz. The early, hand made tubes used widely
spaced elements that didn't perform very well at higher frequencies.
Remember the old "Let the hams have 160 meters an up because its
useless? It was, because there was nothing available at the time to make
use of higher frequencies. Remember too, that early RADAR was at lower
than optimum frequencies, as well. The 15E was an early RADAR
transmitter tube that pushed the limits of the day. If 100 MHz or high
band VHF was reasonable technology at the start of WWII you would have
seen a lot of equipment surplus for those bands, rather than the HF
band.
--


Michael A. Terrell
Central Florida

R J Carpenter wrote:

"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.

About half the present frequency.

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

It was something of an effort for them to reach even 50 MHz. Tubes like the
6AC7 would have some gain. No one building entertainment radios had heard
of noise figure, so their sensitivity was terrible by post-war standards.
One of the most popular and best prewar tuners was the GE JFM-90. Its front
end was two pentode mixers, one after the other. Its noise figure must have
been astronomical. At least 20 or 30 dB worse than today's radios.

I'm sure I wont actually attempt such a thing,
especially any time soon, but just wondered how difficult it might be.

The main problem would be the large cpapacitance in the big octal tubes.
IIRC, the first all-glass 7-pin miniature tubes didn't appear until 1940-41,
the 9001, 9002, and 9003. You could build a fair set with them given
today's knowledge.

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?

The new band wasn't chosen until about the end of WW2, so prewar sets
weren't designed with it in mind. There were only a dozen or so operating
FM stations before WW2, so there wasn't a lot of discussion of the
technology. I have/had a just-prewar book of build-it-yourself radios put
out by Popular Science (I think) which had an FM set. It was pretty
complicated. They hadn't learned how to cut corners. The IF in old-band sets
was 4.3 MHz, not today's 10.7 MHz. The lower frequency allowed better gain
with the tubes at hand, and the narrower FM band didn't result in inband
images. Modulation, etc, was the same as today.

Some of old VHF Hallicrafters sets used the acorn 954, 955, etc, tubes.
They would do ok at 100 MHz, but were exotic and never used in consumer
equipment.

Articles I have seen about early attempts at VHF talked about
removing the tube bases to reduce lead inductance but gain was still
very low, even around 50 MHz. The early, hand made tubes used widely
spaced elements that didn't perform very well at higher frequencies.
Remember the old "Let the hams have 160 meters an up because its
useless? It was, because there was nothing available at the time to make
use of higher frequencies. Remember too, that early RADAR was at lower
than optimum frequencies, as well. The 15E was an early RADAR
transmitter tube that pushed the limits of the day. If 100 MHz or high
band VHF was reasonable technology at the start of WWII you would have
seen a lot of equipment surplus for those bands, rather than the HF
band.
--


Michael A. Terrell
Central Florida

"John Byrns" wrote in message
...

Different pre-emphasis is hardly a big deal, the important thing is
that
the only significant difference was the frequency band used. The FM
deviation was the same for the pre war and the post war systems, so a
pre
war IF strip and detector would work on the post war band, the only
potential problem being image responses due to the lower IF frequency
used
pre war.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/


Yep. But the original poster might as well take the difference into
account. Also, I think a 10.7 Mc IF strip would still be allowed under
a pre-war tube technology stipulation.

Frank Dresser

"John Byrns" wrote in message
...

Different pre-emphasis is hardly a big deal, the important thing is
that
the only significant difference was the frequency band used. The FM
deviation was the same for the pre war and the post war systems, so a
pre
war IF strip and detector would work on the post war band, the only
potential problem being image responses due to the lower IF frequency
used
pre war.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/


Yep. But the original poster might as well take the difference into
account. Also, I think a 10.7 Mc IF strip would still be allowed under
a pre-war tube technology stipulation.

Frank Dresser

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

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 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.