For all practical purposes I am a radio amateur. I know I give ya'll advice on things I pretend to know about, but when it comes to the mathematical side of radio, I'm really bad. I have all of the theories down to some degree, which is how I get by, but I'm not an expert on this stuff.

So, that said, I have to ask the experts out there something. Why can't I make an FM radio with an intermediate frequency of 455 KC? I want to try this with one of my radios. I'm sick of AM. There's nothing on it anymore. I also don't want to butcher my radios. I was thinking that maybe I could add some stuff to the oscillator and antenna circuit (coils--slug tuned probably since I don't want to modify my variable condenser). Then I was thinking about having a 455 KC ratio detector underneath the chassis. I want to make use of all the other IF transformers on the set, though. Using a ratio detector would eliminate the need for a limiter. I don't know if an AM radio with 2 IF amplification stages can compete with an FM radio which has several IF stages plus an RF stage, but I thought this might be worth a try. FM usually has more stages of amplification, though. I also was thinking about placing something on the original IF transformers to broaden their range (say about 19 KC). All of these modifications would only be for FM reception. The radio would otherwise be able to perform on AM broadcast and short wave as it originally did.

Anyway, with all of this done, will the radio work properly? Will I get FM stations with at least moderate fidelity (I'm not looking for 20 to 20 KC fidelity), or will I have a harmonic mess using the 455KC IF?

I hate it when people butcher antique cars and turn them into street rods, and I really don't like it when people modify antique radios or houses, either, but I'm really getting sick of AM, and I'm sick of not being able to listen to my old radios.

Thomas

Thomas,
FM radio was propagated to rid early days AM radio of its many shortcomings, and hence the popularity of that medium today. Using the existing tuning cap (typ. 65-365mmf)would require a 9 nanohenry or .009uh inductor (thats 9 with 8 zeros in front) to parallel resonate at 88 mc with 365mmf. You want to wind one of those. A straight piece of wire would suffice. The same inductor would parallel resonate at 400mc with the low end cap of 65 mmf, and thats just for openers. Add the circuit and extraneous mmf, and the inductor would become even smaller. We're just talking about the front end RF amp circuitry, required for FM reception. AM freqs are easier to propagate/receive, and typical ERP is higher. Tracking and tuning would be a severe problem. Even a 10:1 mechanical ratio would make tuning almost impossible. FM radio has a bandwidth limit of +/- 75kc, thats 150kc total channel bandwidth for max deviation. To realize full fidelity, a 150kc IF bandwidth at 455kc would be especially difficult, if not impossible. Thats a 3:1 ratio of bandwidth to center freq. Existing IF transformers rely on the physical spacing of the primaries and secondaries to establish IF bandwidth. Thats over, under or critical coupling in laymans talk. The existing IF transformers in todays AM radios don't possess that capability. FM frequencies require more attention to the circuit details than AM. Physical layout of the AM components and components Q factors will all lead to disappointing results if pursued. The best option for a 455kc IF of that bandwidth ratio might be active filters, but then again that requires quite a modification, an intimate grasp of active filter design, and requires extensive re-work. I agree that AM and even FM radio is at a crossroad, but I vote at the on/off switch. I find few TV and radio offerings entertaining, since every commentator, anchor, spokesperson has an axe to grind, a sponsor to satisfy and personnally the sound of silence is sometimes golden. I rest my case.
marv

:For all practical purposes I am a radio amateur. I know I give ya'll advice on things I pretend to know about, but when it comes to the mathematical side of radio, I'm really bad. I have all of the theories down to some degree, which is how I get by, but I'm not an expert on this stuff.
:
:So, that said, I have to ask the experts out there something. Why can't I make an FM radio with an intermediate frequency of 455 KC? I want to try this with one of my radios. I'm sick of AM. There's nothing on it anymore. I also don't want to butcher my radios. I was thinking that maybe I could add some stuff to the oscillator and antenna circuit (coils--slug tuned probably since I don't want to modify my variable condenser). Then I was thinking about having a 455 KC ratio detector underneath the chassis. I want to make use of all the other IF transformers on the set, though. Using a ratio detector would eliminate the need for a limiter. I don't know if an AM radio with 2 IF amplification stages can compete with an FM radio which has several IF stages plus an RF stage, but I thought this might be worth a try. FM usually has more stages of amplification, though. I also was thinking about placing something on the original IF transformers to broaden their range (say about 19 KC). All of these modifications would only be for FM reception. The radio would otherwise be able to perform on AM broadcast and short wave as it originally did.
:
:Anyway, with all of this done, will the radio work properly? Will I get FM stations with at least moderate fidelity (I'm not looking for 20 to 20 KC fidelity), or will I have a harmonic mess using the 455KC IF?
:
:I hate it when people butcher antique cars and turn them into street rods, and I really don't like it when people modify antique radios or houses, either, but I'm really getting sick of AM, and I'm sick of not being able to listen to my old radios.
:
:Thomas

You have a good point. I wasn't thinking of using the original tuning condenser for FM, though. I was thinking of going the Zenith route and mounting slug tuned coils next to the condenser which are operated by a cam. This would allow me to use appropriate coil/condenser set-ups. The condensers would be fixed. I thought, though, that the FM band width was about 19 KC, not 150 KC. I guess I misread something in my book. I know that the audio coverage for FM is only about 19 KC, but I thought that the actualy band width of the frequencies going through the IF transformers was 19 KC as well.

What I do right now is just pipe FM in through the amplifier from a portable FM radio (high quality FM headset). I suppose, too, I could mount an FM tuner inside the radio to be operated by the regular tuning mechanism. This defeats the purpose of having tubes, though. At this point it'd be better to buy one of those ugly antique radio replicas with the cassette player on the side. I guess we just have to push for better programming on AM or something, if that is at all possible.

Thomas

:Thomas,
:FM radio was propagated to rid early days AM radio of its many shortcomings, and hence the popularity of that medium today. Using the existing tuning cap (typ. 65-365mmf)would require a 9 nanohenry or .009uh inductor (thats 9 with 8 zeros in front) to parallel resonate at 88 mc with 365mmf. You want to wind one of those. A straight piece of wire would suffice. The same inductor would parallel resonate at 400mc with the low end cap of 65 mmf, and thats just for openers. Add the circuit and extraneous mmf, and the inductor would become even smaller. We're just talking about the front end RF amp circuitry, required for FM reception. AM freqs are easier to propagate/receive, and typical ERP is higher. Tracking and tuning would be a severe problem. Even a 10:1 mechanical ratio would make tuning almost impossible. FM radio has a bandwidth limit of +/- 75kc, thats 150kc total channel bandwidth for max deviation. To realize full fidelity, a 150kc IF bandwidth at 455kc would be especially difficult, if not impossible. Thats a 3:1 ratio of bandwidth to center freq. Existing IF transformers rely on the physical spacing of the primaries and secondaries to establish IF bandwidth. Thats over, under or critical coupling in laymans talk. The existing IF transformers in todays AM radios don't possess that capability. FM frequencies require more attention to the circuit details than AM. Physical layout of the AM components and components Q factors will all lead to disappointing results if pursued. The best option for a 455kc IF of that bandwidth ratio might be active filters, but then again that requires quite a modification, an intimate grasp of active filter design, and requires extensive re-work. I agree that AM and even FM radio is at a crossroad, but I vote at the on/off switch. I find few TV and radio offerings entertaining, since every commentator, anchor, spokesperson has an axe to grind, a sponsor to satisfy and personnally the sound of silence is sometimes golden. I rest my case.
:marv
:
::For all practical purposes I am a radio amateur. I know I give ya'll advice on things I pretend to know about, but when it comes to the mathematical side of radio, I'm really bad. I have all of the theories down to some degree, which is how I get by, but I'm not an expert on this stuff.
::
::So, that said, I have to ask the experts out there something. Why can't I make an FM radio with an intermediate frequency of 455 KC? I want to try this with one of my radios. I'm sick of AM. There's nothing on it anymore. I also don't want to butcher my radios. I was thinking that maybe I could add some stuff to the oscillator and antenna circuit (coils--slug tuned probably since I don't want to modify my variable condenser). Then I was thinking about having a 455 KC ratio detector underneath the chassis. I want to make use of all the other IF transformers on the set, though. Using a ratio detector would eliminate the need for a limiter. I don't know if an AM radio with 2 IF amplification stages can compete with an FM radio which has several IF stages plus an RF stage, but I thought this might be worth a try. FM usually has more stages of amplification, though. I also was thinking about placing something on the original IF transformers to broaden their range (say about 19 KC). All of these modifications would only be for FM reception. The radio would otherwise be able to perform on AM broadcast and short wave as it originally did.
::
::Anyway, with all of this done, will the radio work properly? Will I get FM stations with at least moderate fidelity (I'm not looking for 20 to 20 KC fidelity), or will I have a harmonic mess using the 455KC IF?
::
::I hate it when people butcher antique cars and turn them into street rods, and I really don't like it when people modify antique radios or houses, either, but I'm really getting sick of AM, and I'm sick of not being able to listen to my old radios.
::
::Thomas

I really don't get much use out of my DeWald television, either (the one from 1947 with the RCA 630TS chassis). There just isn't anything exciting to watch on TV, especially with only 13 channels. I recently purchased a working UHF converter, which helps. I guess I should get cable!

Thomas

Thomas,
On those rare occassions that I require audible entertainment, I pull out my collection of 50's, 60's and 70's LP's and 45's, and older 78's for real enjoyable sounds. Its amazing that those records have actual words that I can decipher. No mumbo-jumbo, nothing PC, just plain English and beautiful orchestral background, and that's progress in reverse. I've given up on commercial broadcast. Its no longer a public service as was the original intent, but a method of commercial salesmanship.
marv

:I really don't get much use out of my DeWald television, either (the one from 1947 with the RCA 630TS chassis). There just isn't anything exciting to watch on TV, especially with only 13 channels. I recently purchased a working UHF converter, which helps. I guess I should get cable!
:
:Thomas

:Thomas,
:On those rare occassions that I require audible entertainment, I pull out my collection of 50's, 60's and 70's LP's and 45's, and older 78's for real enjoyable sounds. Its amazing that those records have actual words that I can decipher. No mumbo-jumbo, nothing PC, just plain English and beautiful orchestral background, and that's progress in reverse. I've given up on commercial broadcast. Its no longer a public service as was the original intent, but a method of commercial salesmanship.
:marv
:
::I really don't get much use out of my DeWald television, either (the one from 1947 with the RCA 630TS chassis). There just isn't anything exciting to watch on TV, especially with only 13 channels. I recently purchased a working UHF converter, which helps. I guess I should get cable!
::
::Thomas

Gentlemen,

Could one build a low power AM transmitter for the broadcast band? If so, one could control the programming content their personal radio station on their classic AM radio.

Radiodoc

::Thomas,
::On those rare occassions that I require audible entertainment, I pull out my collection of 50's, 60's and 70's LP's and 45's, and older 78's for real enjoyable sounds. Its amazing that those records have actual words that I can decipher. No mumbo-jumbo, nothing PC, just plain English and beautiful orchestral background, and that's progress in reverse. I've given up on commercial broadcast. Its no longer a public service as was the original intent, but a method of commercial salesmanship.
::marv
::
:::I really don't get much use out of my DeWald television, either (the one from 1947 with the RCA 630TS chassis). There just isn't anything exciting to watch on TV, especially with only 13 channels. I recently purchased a working UHF converter, which helps. I guess I should get cable!
:::
:::Thomas
:
:Gentlemen,
:
:Could one build a low power AM transmitter for the broadcast band? If so, one could control the programming content their personal radio station on their classic AM radio.
:
:Radiodoc
:Thats the best idea, build a low power transmitter and rebroadcast the FM programming

Tom

Thomas, don't get me started about the quality of broadcasting.

My objections are not limited to "commercial" or AM stations. Can you receive WFMT, the Chicago public "fine-arts" station, in Milwaukee? Like other FM stations, their signal is way over-compressed. ppp sounds like p and fff sounds like f, losing all sense of the dynamics that were originally written by the composers.

About six months ago, as a supporter of WFMT, I complained. Their broadcast engineer got back to me and basically agreed with me. It seems that broadcast stations over-compress so that quiet passages can be heard on car radios, above the background noise? Also, he said that individual show hosts can fiddle with the compression, adding ever more problems to the mix.

About the only thing I can stand to listen to, music-wise, are old LPs.

More: I visualize the glorified DJs at WFMT, with their stilted pronunciation of composers' names and foreign words or with their phoney British accents, watching analog needles or graphical equalizers, and adjusting the compression and who knows what else to suit their fancy.

:Thomas do you have a Pilot FM tuner? Although a phono jack on your AM sets are required they bring in a great big sound from a nice little wood box. Coming from a pure amatuer..another bonus to vinyl is that way too much harmony is deleted out when digital reissues are made on CDs. I love organ music. Too often the harmonics between pedal changes are edited out in order to eliminate the air hiss. The same can be said with 78rpms. I think it is an audio art to edit out the pops and hisses without losing tonal quality.

More: I visualize the glorified DJs at WFMT, with their stilted pronunciation of composers' names and foreign words or with their phoney British accents, watching analog needles or graphical equalizers, and adjusting the compression and who knows what else to suit their fancy.
:

Well, I can tell ya'll that I do have a collection of over 1,000 78s, so I play them a lot. I just really miss being able to listen to the radio. On AM I have someone telling me how to run my life politically, and on FM, well, FM has a lot of variety. I guess that's why I want my radios to be able to pick up FM. There are a lot of stations on FM which I do not like, but there are a few. We have a pretty nice classical station here in Milwaukee, and there's a classical station from Chicago which I can pick up around 88.7 or something. Maybe it was 89.something. It's decent, though. Also, on Sunday mornings WMSE (Milwaukee School of Engineering) plays 1930s and 1940s music. WOKY on AM used to have a really great program on Saturday nights, though, with Tom Collins, that played 1930s and 1940s music. It was more of the popular stuff that I like and not so abstract. They cut him after our economy went sour. Now it's just the same clear channel crap over and over again.

Even if I don't have phono jacks on my radios, most of them I have a wire going out the back which goes to the volume control, so I guess I could use a Pilot tuner. I just want to be able to turn the knob on the radio to tune the station.

Thomas

Thomas, I have a suggestion that may suit your needs, but doubt if you would go to the expense and trouble to find out. This would require a few cheap FM tuners, each connected to an AM transmitter. This would allow you to tune your favorite FM stations on each one, then transmit them simultaneously at different AM frequencies. This method would allow you to turn the knobs on any AM radio in your collection to receive them.

The reason I suggest cheap FM tuners is because your AM radios will probably not be capable of reproducing the fidelity of any FM signal, regardless of its quality. I don’t know if it would be best to tap into the tuner or the audio output, but I imagine that would depend on whether the transmitter has a suitable preamp.

I’m making all this up as I type, so don’t know how practical this idea will be or how well it will work. However, it’s sounding better all the time, and I may even try something like this one of these days. If anyone can come up with a good reason why this is a real stupid idea to pursue, speak up quick so that I will quit thinking about it.

:Thomas, I have a suggestion that may suit your needs, but doubt if you would go to the expense and trouble to find out. This would require a few cheap FM tuners, each connected to an AM transmitter. This would allow you to tune your favorite FM stations on each one, then transmit them simultaneously at different AM frequencies. This method would allow you to turn the knobs on any AM radio in your collection to receive them.
:
:The reason I suggest cheap FM tuners is because your AM radios will probably not be capable of reproducing the fidelity of any FM signal, regardless of its quality. I don’t know if it would be best to tap into the tuner or the audio output, but I imagine that would depend on whether the transmitter has a suitable preamp.
:
:I’m making all this up as I type, so don’t know how practical this idea will be or how well it will work. However, it’s sounding better all the time, and I may even try something like this one of these days. If anyone can come up with a good reason why this is a real stupid idea to pursue, speak up quick so that I will quit thinking about it.
:

Thomas,
I've seen it (AM transmitters, modulated by a tuner or CD player, or whatever) done several times using a home built transmitter, which is the most practical way to handle the issue. If the radio engineers had a simple way to use only the AM band tube/mixer/filter/detector functions to run an AM/FM radio, believe me, they would have done it, to reduce cost. FM requires 300 KHz of IF bandwith,which is one of the principal reasons the quality is so good. They were able to share a few functions, but in general, a typical relatively modern low cost AM/FM radio has about 3 more tubes than in AM alone.

AES sells a transmitter/modulator kit as do several other sources. I hear that they are not very good at audio quality, but I have heard some pretty good quality music coming out of old AM radios using this method, so I know it can work well. Toys-R-Us used to sell a "radio DJ" toy transmitter that I heard had good audio quality. I was thinking of simply trying to use the external modulation input on some of the several signal generators I have collected over the years & pick the one that sounds the best, hook up a long antenna to the generator, find a clear channel and, should be in business (beware of FCC requirements on transmit power and antenna length). I imagine that it would be possible to improve the music quality by playing around with the levels & using an o-scope, setting bias, so clipping/distortion does not occur. This is something I have been meaning to try. I'm wondering if anyone else has tried it & what the results were.
Do a search on "AM Transmitter" in this site & you will find plenty of info on the subject.

The AES transmitter works well. I built one years ago. It has fairly good fidelity. It is not at all strong, but if operated near the radio, it is fair. I've also built some other stronger transmitters.

I have one in a book that uses a 6A3 as an output. I want to build this one, but haven't, yet, due to cost.

Thomas