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transmit ...music stations to other stereo systems around my house.
Then get one of the many dongles sold for iPod and similar home transmission, and be happy.
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learn something about what I am doing in the process.
Unless you plan a career in FM transmission (which has become DSP and spec-sheet work), an FM transmitter will teach you many thing you will never need to know.
If you must fling solder, get a
Ramsey Kit.
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plans that use vacuum tube circuts!
Ah. I don't have plans handy, but I do remember what FM VT transmitters looked like. BIG!
Simple: get a VT FM radio. It will probably be hot-chassis:
get an isolation transformer!!!
Follow the original signal path. RF amp, mixer, IF strip, ratio-detector, volume pot, audio amp. To the side of the mixer is an ocsillator (LO). Next to that (if the radio was any good) is a reactance tube (plain pentode plus reactor) for AFC. Its grid runs back to the ratio detector, through an R-C network and possibly an AFC switch.
Snip the AFC at the reactance tube grid. Feed audio voltage. The oscillator will make FM.
The oscillator runs 10.7MC below the signal, so it tunes ~80MC-~100MC. You can pick it up on an FM radio, BUT the LO output was carefully designed to NOT radiate outside the set. It would muck-up other people's radios. Oh, you can catch it if you are very close, but for whole-house use you'll need to boost it. Seems to me you could shuffle the RF stage parts and build a power output stage. Even drive 75/300-ohm antenna (if the input coil is air-core; ferrite would saturate at mighty-low transmit level). Output power is unknown, spectral purity may be very bad: A $10 transmitter is legal only if you have a $100,000 test facility to verify FCC Part 15 compliance.
And an L-C tuned VT VHF transmitter will wander all over the dial as tubes heat and supply voltage drifts. Even with stabilized voltage, temp-oven, etc, stability will challenge most analog radios, and some digital-tuned radios refuse to grab a signal that isn't right on a legal frequency.
The Way They Did It in broadcasting: start with a crystal, solid frequency. Too solid: you can't pull a crystal off-pitch enough for wide-band FM. But a Phase Shift network can cause short-term frequency change without any possibiility of long-term variance from crystal freq. A Phase Modulator is a guitarist's Wah-Wah pedal except voltage controlled and many-many more stages. Looks like a small VT computer. In fact it takes too many stages to be practical: you phase-modulate to the equivalent of say +/-5KC deviation, and then to a string of frequency multipliers. 3MC at +/-5KC, multiplied-up 30 times, gives 90MC and +/-150KC deviation. A Phase Modulator ran many-dozen tubes.
Stereo is an add-on. Fisher made DEmodulators to convert the last FM Mono receivers to FM Stereo. Just three tubes, but a lot of coils. Making stereo is a lot harder than decoding it: gmarsh skimmed the surface. It "is" ordinary vacuum tube design, and perhaps easier than getting a solid FM signal to transmit. It isn't a trivial hobby.
If you haunt eBay used test gear areas, you can find signal generators that can test FM radios, sometimes FM Stereo, sometimes with external audio inputs. Restoring one of these would be more-sane than total DIY.
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