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