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Old November 13th 03, 11:48 PM
David Forsyth
 
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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



"John H. Smith" wrote in message
...
Subject: FM from the stone age?
From: "David Forsyth"
Date: 11/13/2003 3:56 PM Central Standard Time
Message-id:

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


Look up some ham / swl construction articles from the 30's. The "5

Meter"
ham band was quite popular among home constructors and magazine authors.

Only
very slight modifications are needed to cover the modern FM band with

these
circuits, some of which used the newest tubes out at the time, but many of
which used what would have been cheap surplus tubes from the late 20's,

early
30's.
Keep in mind that the Super- regenerative circuits you will find are

very
effective radiators of RF when they are receiving, so be careful about

slipping
into the aircraft band or bothering the neighbors. A short antenna is a

1/4
wave at these frequencies, so you can really "get out" with one of these.

Look
at the tranciever circuits, there isn't much difference between the

receive and
the transmit circuit, since a SPDT switch often was all the switching
rerquired! These will detect FM about as well as AM.
If you go with a TRF or superhet design, you'll need to add an FM

detector
of some sort or modify the AM detector a bit for slope detection. You can

see
examples of slope detectors and simple FM detectors in early broadcast FM
radios and in ham gear for 2 meters from the 30's.
There were many tricks for making early tubes work at higher

frequencies,
and the ham's articles detail them nicely. An advantage of articles from

the
30's is that they tended to lean heavily on household items, hand made

items
and surplus components due to the Depression. This makes recreating them
somewhat easier.
The reprints of Gernsback's "Official Short Wave Manual" of 1934 and

1935
are loaded with 5 meter circuits for home construction articles and

commercial
products. These are available from Lindsay and others. Old ARRL manuals

are
plentiful on Ebay or if you request one here, someone will ususally have

one to
sell reasonably.
Another source of early FM info is in the schematics of early TV

sets. The
kit sets especially, shaved tube and part counts as much as possible and

had
simple FM detectors for the FM audio.
Hams were quite active on the 5 meter band while they had it, going

by the
results, they made the available tubes work fairly well.
Neutrodyne



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Old November 13th 03, 11:55 PM
Frank Dresser
 
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"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


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Old November 14th 03, 12:08 AM
John Byrns
 
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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/
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Old November 14th 03, 12:34 AM
Frank Dresser
 
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"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


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Old November 14th 03, 12:34 AM
Frank Dresser
 
Posts: n/a
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"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




  #6   Report Post  
Old November 14th 03, 12:08 AM
John Byrns
 
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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/
  #7   Report Post  
Old November 13th 03, 11:55 PM
Frank Dresser
 
Posts: n/a
Default


"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


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