A short time ago, I completed restoration of a Meissner 9-1047 pre-war FM tuner with the kind help of many members who participated in my restoration topic. One member, however, asked why anyone would want to restore something that would be useless upon completion. I said something about doing it for it's historical significance blah blah blah, etc., but it made me think how neat it would be to have this set actually receive a high quality signal off the air. So I set out to design an old-band FM transmitter that would be easy to build yet produce the kind of signal this radio was made to receive. It would use tubes, of course.

Well...it's done!



I don't have a schematic drawn yet but will post one soon. While it looks involved, it really isn't and can actually be stripped down to using only one tube and still work fine. A full explanation as to how it works and building tips will be forthcoming

Dave

I look forward to your sharing this with us

Funny thing too, I have been wondering about this as well as the "Japanese" bands. Been a few times the last few weeks the FM that is not broadcast here or any longer has come up in conversation.

That's cool Dave but that filter cap looks "kinda" old, maybe you should consider recapping it???

Tom

I thought about that but it seemed to work OK. 450 volt cap operating at 150 volts. But I did notice some stuff oozing from one of the sections. I figured it was good enough for experimental purposes. Here's the chassis as found in my junk pile:




After I removed the stuff I didn't need, it looked like this:




I've almost finished drawing the schematic and will post tomorrow.

Dave

That's great. There's been a lot of talk about building an FM transmitter in this forum, but I think you're the first to do it. Looking forward to seeing your schematic.

That Mallory electrolytic looks kinda cool. I'd just restuff it. Or leave it in place, and put a new one under the chassis.

A couple 6C4's, a couple 6AQ5's and... 6BE6 maybe? (i'm completely guessing )

A couple of 6C4's and a 6AU6. Finally...something useful for the gazillion 6AU6's in my stash. Hard to believe they were being back-ordered in the '50's.

The one-tube version uses a 12AT7 but there were no 9-pin sockets already on the chassis. Besides, you can get a little more power out of a 6C4.

Dave

The tall ones on right are 6X4 rect(in rear) and a 0B2 gas reg(type is on top ), a 6C4 is no doubt in the left foreground, one closest to can is maybe a 6AU6, won't guess the other...

EDIT oh well I'm late... Dun good though... LOL

Tom

Yes - 6X4 rectifier and 0B2 regulator. Two 6C4's on the left. One is RF and the other is modulator. The 6AU6 is just an audio preamp. I'd better get busy finishing the schematic but I'll be some of you already have it figured out.

Dave

I think it is neat because you would have the channel to yourself. No garbage to compete with. John
Unless I misunderstood!

I'm looking forward to seeing the schematic.

I've read a bit about using a "reactance tube" as a modulator for FM, but I can't wrap my head around FM without a varactor

OK - Here's the schematic:



A reactance tube modulator works the same way as a varactor. Internal capacitance of the tube varies with grid voltage.

Dave

What kind of coil and variable cap did you use?

I knew you would ask that. I'll be giving a detailed description of the coil and cap tomorrow.

Dave

Seems too easy

Guess I'll have to play with a similar circuit sometime soon!

I confess this is not the first time I have built the oscillator-modulator circuit. As a kid, I first saw plans for it in a monthly issue of Science and Mechanics magazine back in the late-'50's which always had at least one article featuring an electronics construction project. This was for an FM "phono oscillator" consisting of a 12AT7 whose modulator section was driven directly by a crystal cartridge. I found it worked so well that I could transmit an FM signal around the neighborhood using our roof-mounted folded dipole FM antenna. By that time, of course, FM had moved to the new band. Later, in college, I re-built the circuit in a weatherproof box and mounted it with an antenna on the roof of one of the dormatories. It covered the whole campus with a decent FM simulcast of the college's AM "carrier current" station. (Carrier current is in quotes for reasons I won't go into here.)

I have never forgotten this circuit and how simple it was to build. I modified it for the old band and added some embellishments here like the audio preamp with variable pre-emphasis. Using 6C4's has the potential for more power. I fiddled with component values to achieve the lowest distortion while maintaining full 75 khz modulation deviation as observed on a spectrum analyzer.

For the coil, I used a 1/2" dia. ceramic, slug-tuned form found in my junk box. I wound 10-turns of 18ga enameled copper wire with the B+ tap 4-turns from the grid connection. Spacing between the turns is about 3/32". The wire is stiff enough where you probably don't need to use a coil form, although it's frequency might change somewhat with vibration.

I didn't use a variable cap across the coil as shown in the schematic. Instead I used a 7 pf mica cap which made it oscillate at 50 mhz with the slug all the way out. Turning the slug in, I could get it down to 42 mhz. I believe a 5-30 pf trimmer cap would work equally as well for tuning without a slug.

The one-turn of 16ga bus-wire surrounding the coil form is the antenna pick-up coil which leads to an RCA jack on the back panel.



Dave

Dave- are you sure about this? The 0B2 is only regulating the voltage supplied to the neon bulb, and it does not need regulation and it will do a fairly decent job of its own regulating. I think that wire going to the rest of the circuit should come right off the 0B2 instead of ahead of the 4700 ohm current limiting resistor.
Curt

Curt - sharp eyes - yes the OB2 is connected only to the neon lamp as kind of a joke. The tube was on the original chassis and it looks cool but it only provides 105 volts. I should use an 0A2 there (150 volts) but the wimpy power transformer will only provide about 140 volts with the load - not enough voltage to fire an 0A2 and regulate. The regulator tube really isn't necessary at all since there are no changing loads in the circuit. I just wanted to fill the tube socket with something. But if someone wants to build it with a better power supply, it wouldn't hurt.

Dave

Actually I had passed over the schematic earlier and it was Norm Leal who mentioned it to me. I figured I better say something, or else there would be others trying to duplicate it in the future and would have some questions about it.
Curt

Someone might also wonder why the .01 uf caps between the plates and cathode of the 6X4 are needed. After I built the circuit, there was a serious buzz in the audio. RF was being detected and chopped by the 6X4 then re-radiated into the low level parts of the circuit. Those .01's and the 100 pf to ground on the grid of the 6AU6 eliminated this condition.

Dave