Build an AM Radio Transmitter

Have you ever wanted to listen to modern music or old-time radio broadcasts on your vintage radio? Now you can!

Phil's Old Radios presents the Li'l 7, a high-quality AM broadcast transmitter that you can build at home for less than $50. It can broadcast anywhere on the normal AM radio band and it accepts ordinary audio input. Just plug in your CD player or iPod, fire up the Li'l 7, and hear your favorite music playing on an antique radio anywhere in your house.

Here's a picture of Li'l 7 as built by Phil.

Douglas Herigstad made a more decorative enclosure for his Li'l 7, recycling an empty cabinet from a 1936 Canadian Viking radio.

Still another approach is seen in this Li'l 7 built by Volker in Italy. He used old-school techniques, building the transmitter on a wooden breadboard with open wiring.

You can read more about Volker's version at the end of this article.

The Li'l 7 was designed by fellow radio collector Walter Heskes. It provides good audio quality and is constructed from inexpensive, readily available parts. Powered by a single 117L7 tube (hence, the name), this transmitter has enough oomph to reach any spot in your home, but it does not require any license to operate.

In these pages, you'll find complete building instructions, schematic diagram, parts list, and an explanation of how the transmitter works.

The Li'l 7 can be built in an evening or two by anyone with average soldering skills. If you have never built an electronic kit before, it's best to get help from someone more experienced (a local radio collector's club is an excellent place to find help). Since the Li'l 7 involves 120-volt house current, please observe normal safety precautions in building and using it.

Both Walter and I have built working transmitters using this design. We hope that you enjoy the Li'l 7 as much as we do. If you make one, send some email to let us know how you like it!

If you're interested in a portable AM transmitter, check out the A-1 Minicaster project elsewhere in this section.

Parts List

The parts with AES part numbers are available from Antique Electronic Supply, 6221 South Maple Avenue, Tempe, AZ USA 85283 (480) 820-5411. The other parts are available from Radio Shack and Mouser Electronics at the time of this writing. Note that every supplier changes inventory from time to time, so some parts may have different part numbers and prices in the future. Other good parts suppliers include Allied Electronics and Digi-Key.

Part
Description
Part No.
Quantity
Price (ea.)
C1
220pf ceramic capacitor
AES C-D220-6000 1
0.44
C2 .01mf 600v capacitor AES C-RD01-600 1 0.59
C3, C4 22mf 160v electrolytic capacitor AES C-ET22-160 2 0.98
C5 68pf mica capacitor AES C-SM68 1 0.45
C6 150pf mica capacitor AES C-SM150 1 0.51
J1 RCA quad phono jack Radio Shack 274-322 1 1.69
J2 8-pin tube socket AES P-ST8-209M 1 2.70
L1 RF coil AES P-C70-RF 1 7.95
P1 12v lamp assembly Radio Shack 272-336 1 2.59
R1 47K ohm 1/2-watt resistor AES R-A47K 1 0.08
R2, S1 100K potentiometer & switch Mouser 31VM501-F 1 1.72
R3 10K ohm 1/2-watt resistor AES R-A10K 1 0.08
T1 120v/12v Power transformer AES P-T442 1 16.95
V1 117L7 or 117M7 tube AES 117L7 1 6.60
Misc. 120v power cord AES S-W104 1 1.50
Misc. Antenna wire (24 ga. enamel) AES S-WL3-612 1 3.00
Misc. Alignment tool kit AES S-T9304 1 1.25
Misc. Perforated project board   1  

The alignment tool kit will include a tool that you can use to adjust your coil.

Depending on what you plan to use as an audio source, you may also need to get a suitable adapter, to connect your source to Li'l 7's audio input (J1 in the schematic). Radio Shack sells a wide assortment of adapters to convert between phono plugs, stereo mini plugs, etc.

A rectangle of perforated board will make a convenient base for mounting components inside your cabinet. If you use a non-conducting plastic cabinet and mount things carefully, you may be able to do without this part, wiring smaller components directly from point to point between the major components.

If you plan to experiment with different antennas, you can also add a binding post or similar terminal to simplify connecting and disconnecting antenna wires.

Consider adding a 2 amp fast-blow fuse in series between the AC line input and the power switch. This fuse will protect the components in the circuit from possible damage if a short circuit occurs.

Use a two-pronged power cord for this project. A three-pronged cord would serve no purpose. If you happen to use a three-pronged cord because you have one lying around, leave the third prong connected to nothing.

Schematic Diagram

This schematic will print on a single 8.5 x 11 sheet of paper if you print it without any margins or header/footer information. We suggest that you save the picture file and print it from a paint program rather than printing this web page from your browser. To save the picture in Internet Explorer, right-click anywhere on the schematic, then choose Save Picture As.

If you are unfamiliar with schematics, the ARRL has a good beginner's article on how to read them (Part 1, Part 2).

Building Instructions

The first step in building the Li'l 7 AM transmitter is to acquire all the needed parts. If you're an experienced builder, you may find many of these parts in your junkbox. For best results, we recommend that you stick with the values given in the parts list.

If you haven't already done so, print these instructions and the schematic. Read all of the instructions before you start to build anything.

Enclosure

You can build the Li'l 7 on a breadboard or in any plastic or metal box roughly 6 x 6 x 3 inches in size. Both AES and Radio Shack supply suitable enclosures. Walter built his unit in a custom-fabricated metal box, while I salvaged an old plastic box that just happened to be the right size. If you don't have metal-working tools, plastic is easier to work, of course.

If you choose a metal enclosure, you may want to add a perforated board to your shopping list, to provide a safe mounting surface for your components. If your enclosure is plastic, you may be able to mount your smaller components point-to-point between the major components and skip the perf board.

Here is a picture of Walter's workbench, showing the original Li'l 7 in action. It's visible at the lower right, housed in a custom-built metal enclosure. Visible on the top of Li'l 7 are the 117L7/M7 tube and transformer T1. Walter's Zenith TransOceanic Model 1000 serves as the test receiver. Also shown in the picture, although not in use at this time, are a homemade test speaker used for radio repairs and a Dim-bulb Tester.

Connector options

The parts list suggests a 4-jack gang phono connector to provide handy connection points for your antenna and any input connector(s) that you plan to use. Phil uses a portable CD/cassette player's earphone jack for input, using a stereo mini-to-mini cord from the player to the Li'l 7, and then a Radio Shack stereo/mini-to-mono/phono adapter to complete the connection. Only two of the four available jacks are used (one for audio input and the other for output). Depending on what you want to use as input, you may want to replace the gang radio/phono jack assembly with screw binding posts or other types of connectors. Or, if you want to use your transmitter with more than one input device, you can wire input jacks in parallel with each other using the suggested gang jack.

If you plan to experiment with different antennas, you may also want to use a binding post or similar connector at the antenna end, rather than soldering the antenna wire permanently.

Layout

Parts layout makes a big difference in ease of construction. Layout is not too critical in this circuit. Just try to keep related parts close together without making the assembly so tight that it's difficult to work in your enclosure. Notice that a number of components connect to the chassis ground; this may help you come up with a reasonable layout.

Walter and I both built our units with the tube sticking out the top, which makes it easy to mount the tube socket, but also exposes the tube to possible damage. You could also mount the tube inside your box, mounting the tube socket sideways using an angle bracket. If you enclose the tube, be sure to provide some ventilation holes.

Keep in mind that you'll need to adjust the RF coil L1 once the transmitter is assembled. One way to mount the coil is to drill a small hole in the back or top of your enclosure and glue the coil's small tube through the hole, allowing access for adjustment. You can also glue the large part of the coil directly onto your enclosure. Glue may not be necessary if the coil is supplied with a mounting lug or clip that fits into the chassis through-hole and locks the coil assembly to the chassis. Use glue sparingly and do not cover the coil with glue; the glue can affect the inductance of the coil.

Enclosure

Once you have a sensible layout, make a sketch and set all the parts aside while you create the needed openings or attachment points in your box or breadboard.

After creating the attachment holes and points, do a test assembly of all the attached components to make sure everything will fit as planned. This is the time to make any last-minute revisions before you fire up the soldering gun. You should also plan the order in which you'll attach the parts.

Assembly

Now it's time to start hooking everything up. Work slowly and check off every part against the schematic and parts list as you go. Remember the old joke about, "I haven't got time to do it right, but I do have time to do it over!"

Begin by mounting the major components—the tube socket, transformer, potentiometer and switch, pilot light, and the jacks. Once these big parts are in place, connect the smaller ones.

It's important to hook up the coil correctly—if you don't, the transmitter may not work at all. The numbers 1-2-3-4 shown in the schematic match the numbers of the pins on the RF coil as shown on the data sheet included with the AES part. The red dot on the coil helps you identify the pins; it is located between pins 1 and 2. A diagram of the pins and the red dot appears in the lower left corner of the schematic. Important: the pin numbers on the schematic are given as if you are holding the coil assembly with the larger of the two coils closer to you.

In your initial assembly, leave the coil connections unsoldered until you have tested the unit and confirmed that the coil is working correctly. That will make it easier to change things around if you got something backwards.

In the schematic, note that three points are labeled with the ground symbol. These should all be connected together (this schematic-drawing convention is explained in the schematic tutorial referenced earlier). Do not connect them to anything else.

When everything is hooked up, take five minutes to recheck every connection against the schematic, paying special attention to the values on similar-looking components.

Antenna configuration

The best antenna for this transmitter is a vertical antenna four feet in height. Just about any kind of wire will do, although a cheap whip (telescoping) antenna attached vertically to the enclosure will give a neater appearance. There are many local factors that will affect a small transmitter, such as the wiring and plumbing inside your home, nearby radiation sources, and so on. Feel free to experiment with your antenna configuration. A longer antenna may not necessarily give better performance, however.

Tuning the transmitter

The moment of truth has almost arrived. Now it's time to tune the transmitter to broadcast on a quiet spot on your local AM dial. Before you plug in the transmitter, turn on a nearby AM radio and try to find the quietest possible spot on the dial between about 800 and 1000. Make sure that everything else is in place and connected—your input source, the antenna, and the adjustment tool to adjust the coil L1.

Here's how to tune the transmitter:

  1. Remove antenna wire from transmitter.
  2. Turn on Lil'7.
  3. Connect modulation source (casette player, phono, etc.) to the Li'l 7 transmitter's input jack.
  4. Turn on your modulation source.
  5. Turn on a nearby AM receiver and turn the volume up about halfway.
  6. Wait two minutes while all three units warm up.
  7. Tune the receiver to a clear spot between 800 and 1000 on the AM broadcast band.
  8. Using your coil adjustment tool, slowly turn the core of of the coil until you hear the modulation source playing through your receiver.
  9. Adjust the tuning dial of your AM receiver so that you have the smallest possible amount of heterodyning (whistling or interference) with adjacent stations on the dial.
  10. Readjust the coil core as needed to obtain the maximum sound volume at your AM receiver.
  11. Reconnect your antenna wire to the Li'l 7 AM transmitter.

Did it work?

At this point, you should have a working AM transmitter. If you don't hear any sound at all, power everything down, take your Li'l 7 back to the workbench, and check all the wiring against the schematic in case you miswired something. Remember, a miswired coil may prevent the transmitter from working. If the signal is audible but weak, check the input levels on your modulation source and repeat the tuning procedure from the very start. If you're using an earphone jack as your source, you may need to turn up the volume pretty high.

If your transmitter works properly, finish soldering the coil connections. You're done!

Again, antenna characteristics can greatly affect the performance of low-power transmitters. You may get very different results using different lengths and configurations. Your antenna's characteristics also affect how directional it is (i.e., how strongly it broadcasts in a particular direction). There's no single antenna that's be perfect for every location, so be prepared to experiment a bit before you find the best solution for your particular setup. Antenna design can be quite complex. If you get interested in the topic, check out your local library—there are a number of books on the subject.

Refinements

Since we first published this design, fellow collector Ron Olexa wrote in to suggest adding a fuse as an extra safety factor. A fuse will minimize damage in the event of an accidental short circuit while you're testing or using your Li'l 7. We recommend connecting a 2-amp slow-blow fuse between the on/off switch and the 120v AC line cord.

If you come up with other improvements or suggestions, please send them along so that we can share them with other experimenters.

How the Li'l 7 Works

The heart of the Li'l 7 transmitter is the 117L7/M7 GT tube. (Note that 117L7 and 117M7 tubes are interchangable. Some references may list one or the other, or use the number 117L7/M7.) In a single envelope, this tube combines both a diode and a pentode. Its filament runs on 117 volts, which simplifies the power supply. Here are the tube internals.

The diode portion of the tube rectifies the incoming AC line voltage, changing it to a series of DC pulses. The electrolytic capacitors C3 and C4 soften these pulses and provide smooth B+ voltage.

Isolation transformer T1 is strongly recommended. Although the transmitter will work without it, it prevents a possible shock hazard when the chassis or any other normally grounded point is touched. Any small transformer delivering up to 150 volts at a few milliamperes will work. The transformer also supplies 12 volts AC, which we used to power a pilot light.

The pentode portion of the tube combines both the oscillator and modulator. The novelty of this circuit is that the pentode screen is returned to ground rather than to B+ as might be expected. The modulation input varies the pentode screen voltage about 1 volt above and below ground potential.

The circuit across the cathode (pin 8) and the suppressor grid (pin 4) oscillates at the frequency determined by the LC circuit composed of capacitor C6 connected across the primary of the tunable coil. This becomes the carrier wave. The carrier wave is modulated (or shaped) by the electrons injected onto the pentode screen by the modulation source. Electrons are both oscillated at the carrier frequency and modulated at the signal frequency before they hit the plate and step off the antenna. The receiver then detects these electrons and strips away the carrier wave from the modulation signal, which is amplified and transformed back into audible sound.

You can shift the Li'l 7's broadcast frequency either by moving the iron core of the coil or by changing the value of capacitor C6. With C6 at 150pf, the oscillator tunes to a vacant spot around the midpoint (800 - 1000 KHz) of the AM broadcast band. A smaller value for capacitor C6 increases the oscillator's frequency; a larger value decreases the frequency. You can experiment by placing a 68 pF capacitor in series with the 150 pf capacitor to drop the total capacitance to about 50 pF. At that value, you may be able to tune Li'l 7 to the upper end of the broadcast band, possibly around 1700 KHz or higher. To tune the lower end of the band, try placing a 68 pf capacitor in parallel with the 150 pf one to raise C6 to about 220 pf. The idea is to customize your transmitter for the frequency that works best in your setting.

If you're really adventurous, install a rotary switch in series with different stepped values of C6 (for instance, 50 pf, 150 pf, 200 pf, 300pf), to allow tuning across a wide range of broadcast frequencies. That selectability feature would make the transmitter universal for regions where it is nearly impossible to find a vacant channel in the middle of the broadcast band. It's also fun to refine a working design.

Possible modulation sources include the speaker voice coil of a radio, a crystal microphone or piezoelectric phono cartridge, the tape output terminals of an AM/FM receiver, or the earphone jack of a radio, tape machine, or CD player.

Li'l 7, European Style

In 2014, fellow collector Volker from Italy sent me photos of his Li'l 7, along with a schematic translated into German and Italian.

He used techniques that were employed in the earliest days of radio, basing the transmitter on a wood platform and dispensing with a cabinet. He also used Fahnestock clips and right-angled buss wiring remeniscent of days gone by.

              

Like old-fashioned breadboard projects, Volker's version has exposed wiring that presents a shock danger. Obviously, this style is not recommended for children! Do not employ this technique if your household includes anyone who does not have the common sense to avoid touching bare wires.

If you have built a Li'l 7 transmitter and wish to share your photos, kindly send me an email.


This radio construction project, including all descriptions, diagrams, photos, and the underlying electronic design, is published here for the noncommercial use of radio hobbyists. You may print and reproduce these project instructions for your personal use. Commercial use of this material is strictly forbidden.

©1995-2014 Philip I. Nelson, all rights reserved