FM Transmitter and Stereo MPX


FM Transmitter Sections


This is the basic oscillator and reactance tube modulator section. The grid of V1BA is fed a 90 degree phase shifted sample of the oscillator output, which then appears on the plate, and, through C5, is injected into the oscillator tank. But total tank current cannot remain out of phase so it is forced to shift frequency in proportion to the injected signal. Audio impressed onto V1Bs grid varies the amount of injection and, so, oscillator frequency is modulated by the audio.


First Shot Breadboard of 100MHz Osc/FM Modulator

Subbed an on hand 6BL8 for the 6CU8, 10 uH for L1 and 22 pF for C6. Audio input is through a 10 K pot from the bench notebook sound card, which can manage about 1 V out.

125V B+ was from a Granco power transformer and simple 47 uF-5.6 k-47 uF RC filter (Gort AM Transmitter breadboard supply).

Coil is 10 turns of 20 gauge over 1" on a .5" dowel. 100Mhz was achieved at 6.5 turn tap.

The air coil radiates enough for 10ft-20 ft transmission, even over a local station, and audio was received around 101 on the FM dial.

The rather flexible air coil is quite microphonic and classic acoustic feedback can be achieved by placing the radio nearby and turning up the volume.



22 pF replaced with correct value 10 pF

Coil cut down to 7 turns.

Hum issue solved with 4.7 nF RF bypass caps across power supply bridge rectifier.

Put pre-emphasis in Winamp's equalizer.

Re-tweaked coil length to 106MHz for a less crowded spot on the dial and with just the air coil we're now able to reliably receive from the second floor workroom to the first floor den.

I've decided to break the project into two parts, with the first being a two tube standalone mono FM transmitter suitable for a UHF Converter chassis and entirely usable on it's own. An "add on" Stereo Multiplexor would then be an optional second project.

A standalone mono FM transmitter needs its own pre-emphasis network and modulator linearity is poor. Both are corrected with the addition of a 6AU6 preamp and negative feedback (NFB) from the modulator.


Input stereo is combined by R11+R12 and fed to the NFB node through R15, which provides 3-27 dB of gain adjustment. Negative feedback comes from R2 sensing modulator current. The pre-emphasis network is, then, in the feedback loop so all gain not used for pre-emphasis, or input gain, goes into NFB reducing modulator distortion.

R13 shelves the pre-emphasis filter, so it does not keep boosting ad infinitum. C9 cuts extreme HF content and improves loop stability. C14 provides an RF ground for the audio side of R4. C15 on the input cuts above 15 KHz.

The transmitter coil has been revised to a"ballpoint pen" size wind, 5/16" diameter, with the antenna coupler wound in the same direction and spaced as if it were a continuation of the primary. Currently a wet noodle wire antenna is employed but the antenna winding allows a typical 300 ohm "FM Antenna."

Audio quality is excellent.

It should be noted that gain comes at the expense of NFB so it is best, when possible, to keep R15 at minimum and use the audio source's volume control to set modulation. Increase R15 gain only when the source is insufficient, such as with cell phones and MP3 players.


I'm still using the simple Gort style FW rectifier-CRC B+ filter on the transmitter. The preamp is run off a second RC to split the current draw and increase it's B+


6BL8 FM Transmitter with 6AU6 L+R Pre-emphasis Preamp

breadboarded on my home made tube socket expanders. It may not be pretty, but it works.

The antenna coil is coaxially located to the right of the primary. The blue wire flying off to the top is the antenna.

"Dollar Days" FM Transmitter

A "Dollar Days" version using 6AG5 for the preamp and 6KE8 for the reactance tube/oscillator, both of which are 1 buck each from online tube suppliers.

The 6KE8 not only has almost twice the 6BL8s gm but its voltage control range is only 1/3 as wide (note the lower, 220 ohm, R2), meaning the gm slope is dramatically steeper resulting in increased gain. So much so, estimated around 12-15 dB, the input resistors are increased to 100k for lower preamp gain yet small MP3 players and cell phones can easily drive the transmitter.

Checking the pre-emphasis/de-emphasis loop with a cell phone's FM radio showed frequency response to be essentially flat with an expected 2 dB drop at 15 KHz due to the pre-emphasis shelf which, in addition to C15, prevents more than a 17 dB boost above 15 KHz.



Stereo Multiplexer Speculations



Plus some filters (15KHz LP and 19KHz notch pilot protection), composite audio mixer, and miscellaneous glue circuitry.



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