Disclaimer:
The device(s) and circuit(s) described herein may generate and/or be connected to dangerous sources of voltage and current which may be capable of causing shocks, burns or death. Other hazards may be present in the building, use, repair or troubleshooting of these device(s)/circuit(s), including but not limited to thermal burns from hot tubes and components, broken glass hazards, fire, exploding capacitors or other parts, loss of insurance underwriting or coverage for use of nonapproved equipment, fines, confiscation or other legal consequences for violation of FCC or other rules, causing harmful interference with licensed transmissions, loss of critical power systems due to nuisance tripping of GFCI breakers, etc. Projects on this or any linked or referenced site should only be attempted and/or used by those intimately familiar with these and other hazards and with the proper construction and use of electrical and electronic equipment. The individual constructor and/or operator of this equipment is responsible for the determination of safety and suitability for a particular purpose. No warranties are expressed or implied with respect to suitability for any application. The author/creator of this site cannot be held responsible or liable, directly or indirectly by any party or entity, for any legal, financial, personal, property or other losses or consequences to any party or entity arising directly or indirectly from the use, misuse or abuse of, or for inaccuracies in any of, the information contained herein. The act of viewing, by any means electronic or otherwise, any portion of this page or any page or portion of a page linked or referenced therefrom, constitutes your agreement that you accept the terms as set forth herein and agree to hold the author/creator, site host(s) and any third parties harmless and free of all liability in accordance with the above.
Now that the lawyers are happy, let's get to the meat with our first project, ideal for getting one's feet wet in the CC arena...
A Simple 2 Watt Tube Carrier Current AM Broadcast Transmitter
The small, basic tube carrier current transmitter shown below will give excellent audio quality thanks to the use of series modulation; no modulation transformer or choke is used. This also removes one of the difficulties often encountered in building tube AM transmitters as transformers or chokes suitable for modulation purposes are hard to obtain and are often expensive when found.
This transmitter makes use of the "neutral feed" system which may extend range over the typical "hot feed" method in certain situations and in any case will reduce or even eliminate the hum which typically plagues carrier current systems using the latter means of coupling to the powerlines. No coupler is needed with this unit; since the injection of RF into the neutral wire is internal, the unit simply need be plugged into an ordinary outlet.
RF carrier output is between 1 and 2 watts; although designs similar to this are often called "5 watt" transmitters, that is going by the power input, which many authors seem to like to do to "pump up" the specifications! Nonetheless, it is capable of providing a solid signal to any AM radio throughout a typical residential neighborhood, if the neutrals are not broken up at each distribution transformer.
The entire transmitter fits neatly on a 6" x 4" x 1-1/2" "Bud" or similar aluminum chassis and is wired in typical tube equipment fashion using point-to-point connections to phenolic 5-position tie points or terminal strips (available at Radio Shack, among other sources.) See below for chassis layout used by the author; this layout results in short, direct wiring and leads when tube socket pins are lined up correctly (plates of 12AT7 facing grids of 6AQ5s, the plate of the modulator facing the cathode of the final, etc.)
Parts List
| R1: 39k 1 w | C10, C13: 180 pF 500v ceramic disc |
| R2: 27k 1w | C11: 25uF 25 v electrolytic |
| R3, R6: 22k 2w | C15, C16: 100uF 350v electrolytic |
| R4, R10: 470k 1w | C17: .01uF 2kV ceramic disc |
| R5: 100 ohm 5w wirewound | D1, D2: Radio Shack PTC-205; 1kV PIV, 2.5 A |
| R7: 47k 2w | T1: Hammond # 266-J12 power transformer; dual primary 117-117 volts; dual secondary 6.3-6.3 volts at 2 amps |
| R8: 1.5k 2w | RFC1, RFC2: 1 mH 100 mA |
| R9: 100k 1w | RFC3, RFC4: 1 mH 300 mA |
| R11: 10 ohm 2w wirewound | MOV1: 130LA20A (130 volts AC) |
| C1: 22 pF 500v silver mica | P1: 3-prong 120-volt line cord plug |
| C2: .001uF 500v ceramic disc | F1: 250 mA, slow-blow |
| C3, C4: .005uF 500v ceramic disc | J1: RCA phono jack |
| C5: 260 pF 1kV silver mica | Z1: 6 turns # 22 enam., space-wound over body of 100 ohm 2w carbon or metal-film resistor, solder ends of coil to resistor leads |
| C6, C12: .01uF 500v ceramic disc | XTAL: fundamental parallel-resonant crystal in HC-6 holder for AM broadcast band; ~20 pF load capacity |
| C7: .01 uF 2kV ceramic disc | L1: For middle of AM band (~1000 kHz), 100 turns #30 enam., closewound on 1/4" diam. ferrite slug tuned shielded coil form. Slug must be at least 1-1/4" long |
| C8: .005 uF 500v ceramic disc | L2: 21 turns # 26 enam., closewound over cold end of L1, in same winding "sense" or direction of wind; insulate from L1 with heat-shrink tubing and shellac entire assy. after winding. |
| C9, C14: .1 uF 250v metallized film poly |
The tube lineup is a 12AT7, one triode of which is used as the crystal oscillator and the other triode as first audio stage; followed by a 6AQ5 series modulator and another 6AQ5 used as the final RF power amplifier. These tubes are readily available and give good performance at relatively low plate voltages. Despite the lack of a buffer or "IPA" stage between the oscillator and final, isolation is good as no appreciable FM-ing could be detected under full modulation.
The power transformer is a special type available from Hammond (www.hammondelec.com) for approx. $12. One of the 117 volt primaries is used for its intended purpose and the other as the "plate" winding, feeding a full-wave voltage doubler consisting of D1-D2, C15-C16 and R11 to provide approx. 225 volts under load. The filament or heater windings are separate, one for the osc. and audio stages, and the other for the final RF stage, and this is necessary since the filament of the final RF stage only must be tied to the cathode as shown, to minimize heater-cathode potential difference.
This transmitter's final tank values as shown above were designed for the middle of the AM broadcast band (~800-1200 kHz) but can be changed to any desired portion of the band, by simply increasing the number of turns on L1 for a lower range and decreasing them for a higher frequency. It is also possible to make the present L1 tune lower by increasing the value of C5, to 330 pF for example, or tune higher by decreasing it to say 180 pF, instead of changing the number of turns. L1 was made from an old IF can and as such the builder may have to make do with whatever they can find, build or buy; whatever type coil is used, it should be shielded. If operation on a much lower frequency is desired the RF chokes RFC1-RFC4 should also be increased to 2.5 mH, with current ratings the same as the originals.
Only fundamental type, parallel-resonant crystals should be used with this circuit; for frequencies below about 800 kHz, the value of C1 may need to be increased to stimulate enough feedback to "kick" the crystal into oscillation; but be careful as too much feedback will overheat and perhaps crack and ruin the crystal.
RFC1 also may be increased in value without detriment if this helps oscillation with lower-frequency crystals.
To operate the unit, plug it in and wait for the tubes to warm up, then check for oscillation with a nearby receiver tuned to the crystal frequency. Tune the slug in L1 for maximum signal strength on a receiver having an S-meter or by using a field strength meter (at 1000 kHz L1 should tune when the slug is approximately centered in the coil given the specifications shown.) Then apply audio to J1 (line-level output from a board, or small mixer should be adequate) and increase the level until distortion is perceptible, then back down until the audio just cleans up again. The audio should be full, loud and clean with very little or no distortion evident when adjusted properly, and there should be little or no hum. A small amount of hum is normal in some cases, while in others the hum will be imperceptible; but objectionable hum is a sign of either a problem within the transmitter or nonlinear device demodulation, a phenomenon whereby incidental (or intentional) rectification occurs in appliances on the same circuit, causing modulation of the carrier with 60 (or 120) Hz energy. RF getting into rectifiers or motors with sparking brushes, or arc discharge lamps (fluorescent and HID) is the main culprit.
As with any tube transmitter do not operate the unit in a non-oscillating condition, or off-resonance, as these conditions will cause the tubes to draw excessive current and eventually cause them and possibly other components to overheat and burn out.
When moving the unit from one outlet or branch circuit to another, retuning of L1 may be necessary. Also, if range outdoors along lines is not up to par when using the unit on a particular outlet, try another outlet on a different circuit. If range still is not satisfactory, it may be that your power system neutrals are not continuous throughout the entire grid or area and with each distribution transformer but are separated or "sectionalized" at each, or every few transformers. In such a case, there is nothing which can be done to increase the outdoor range of the unit along the lines.
Outlets MUST be wired correctly, and the ground wiring MUST be intact for this unit to work properly and be as safe as possible to use; correct wiring can be verified with any of the readily available 3-prong plug-in neon lamp or LED outlet testers (NOT the two-wire test lead type), which can be found in home centers and hardware stores. Test the tester itself on a known-good outlet before using it to test the outlet into which the transmitter will be connected.
If it is desired to make this transmitter a "hot feed" unit instead of a neutral feed, simply unground the cold (grounded) end of L2 and feed via a .01 uF, 2 kV ceramic disc capacitor to the hot 117 v wire in the transmitter between the fuse and RFC3. No other changes need be made. These coupling capacitor values are not "etched in stone" and may be changed to tune out reactance in the lines if found to be necessary (in fact experimentation here is encouraged). Make sure your experiments are done SAFELY! DISCONNECT ALL POWER and carefully DISCHARGE ALL CAPACITORS before working on this or any equipment. Let hot tubes cool. Be smart, be safe!
A 10-20 Watt, "Full-Featured" Tube Carrier Current AM Transmitter
This unit incorporates a number of important safety features and is purposely "overdesigned" to help assure continuous, 24/7 reliability as would be standard practice in professional AM broadcast transmitter design. Of course, it also features great tube sound. While not a beginner's project it is felt that anyone with a reasonable level of experience in building tube RF and audio equipment should be able to duplicate the design presented here; but of course, experimentation with "pet" ideas is always encouraged.
coming soon
A 50 Watt Max., Solid-State Variable-Power CC AM Transmitter with PLL Frequency Synthesis
coming a little less soon :-)
A Universal Line Coupler For Carrier Current Use (Both "Hot" and "Neutral" Feed Methods)
Because of the great demand for a coupler design which will serve all methods of feeding AM broadcast energy into AC power circuits, a configuration is presented here which is felt to be about as "universal" as one can get. This basic topology of coupler, with a few necessary changes in part values (such as fuses and MOVs) can be used for neutral-ground feed on any system and for hot-neutral feed on single and three-phase wye or delta systems up to 600 volts. It can also be used to inject RF into low voltage circuits such as fire and burglar alarm, low voltage lighting, etc. but should NEVER be connected to power and low voltage wiring simultaneously.
DO NOT attempt to connect this (or any) coupler directly to a primary (over 600 volts) distribution system; it is for secondary use only. 600 volt-or-less secondaries are extremely dangerous to begin with; voltages higher than that are even more so. For primary feed applications (where permission has been obtained from responsible parties) much larger components must be used such as high-voltage oil switches and capacitors, which is beyond the scope of this article.
This coupler design is a 'bare-bones' minimum, and includes no SWR or power metering; these functions if desired will be left up to the user to apply externally. Nonetheless, it will get the job done, with almost any wiring system the user is likely to encounter.
also under construction