Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
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 |
#2
![]() |
|||
|
|||
![]() "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 |
#3
![]() |
|||
|
|||
![]()
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/ |
#4
![]() |
|||
|
|||
![]() "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 |
#5
![]() |
|||
|
|||
![]() "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
![]() |
|||
|
|||
![]()
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
![]() |
|||
|
|||
![]() "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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|