Why AM? FM took the country by storm, due to its vastly improved music quality and stereo. The FCC did not take the steps in time which were necessary to revive musical programming on AM: the elimination of interference and the adoption of a stereo standard. Nevertheless, the popularity of talk radio and consolidation of FM ownership is already fueling a revival of interest in AM. Talk stations are the highest rated station by a large margin in many large markets. Niche formats are finding their only refuge on AM.
When I approach the subject of how to build a really great AM tuner, I was faced with a problem. With the exception of the GE Super Radio series, and McKay / Dymek - there have been virtually no good AM tuners made in almost 40 years. With no good tuners available, the AM enthusiast is stuck with antennas and pre-amplifiers designed to squeeze every last ounce of performance out of a modern tuner. Unfortunately, most AM tuners - even in expensive home stereo systems, are squeezed in on a budget, and are barely enough to receive strong local stations. Most lack RF amplifier sections, most combine the local oscillator and the mixer into a noisy "converter" stage, most have miserably inadequate IF and detector stages. Internal loop antennas disappeared years ago, replaced by smaller and smaller Ferrite rods. You can pack all the gain you want to into these cheap tuners, and maybe pull out some distant station, but the performance will come at the price of overloading, crosstalk, and frequent "tweaking". A number of enthusiasts have resorted to using car radios in their homes. Car radios, particularly older Delco models, use a 262.5 kHz IF instead of the conventional 455 kHz, which helps selectivity a great deal. Frequently, though, there are too many noise sources in the average home for a car radio on a 12V adapter to work as a decent "DX" model.
Rather than spend time trying to construct their own tuner, I recommend that the serious AM enthusiast look for older, tube type receivers. Yes - the realm of "fire bottles", paper and wax capacitors, and even stranger components, such as power resistors with multiple taps, and multi-section electrolytic capacitors. Construction techniques that are based on wiring from point to point (surprisingly efficient), and to phenolic insulated terminal strips - with some components neatly riveted to a metal chassis. Extra care must be taken with these old receivers, as they have hazardous voltages throughout the interior. I learned first hand how unpleasant a 450V "B" voltage can feel, when the adjustment slug on an IF transformer shorted to a winding on the coil! I still have a faint outline of the screw on the tip of my finger.
The "state of the art" in AM reception is probably 40 to 50 years ago, before FM was a serious contender. Many times, an old communication receiver that combines AM with shortwave can be found at local "hamfests". I found a Hammarlund SP-600 at a local electronics hobbyist store. At 75 pounds, it is quite cumbersome! After installing it in a rack with wheels, it is movable, though hardly portable - taking up a previously wasted corner of my bedroom, making a handy table.
There were a number of AM only tuners available on the market in the 1950's and even into the 1960's. These tuners, combined with an outdoor loop antenna, provide excellent reception, and will not tilt the scales. Most are quite compact, on a scale with modern stereo equipment. Finding them, however, is quite a challenge - I got two units from different friends of my father. Sadly, one such unit finally burned out castastrophically, and other was lost (along with a lot of other equipment) in a move. I still own a Fisher AM-80, however. With modifications, I have been able to receive many trans-Atlantic stations on "split channels", using a 5 foot loop antenna.
I found the following article in a library archive. I include it here not so much as a contruction project, but for historical reasons. Designing electronics with "fire bottles" (tubes) is a dying, possibly dead art. It may be possible to obtain the parts and assemble this receiver, if anybody does so successfully I would love to hear from you: ! Notice that the article uses the archaic "kc" nomenclature instead of "kHz". For those not familiar with the old unit "uuF", it is the same thing as pF. Mica capacitors are still available, but NPO ceramics are probably OK. Paper and wax capacitors, of course, are not, but suitable polyester and polystyrene capacitors should work. Your best bet may be to find an old tube type radio, which probably has the necessary power transformer, RF/IF coils, and tuning capacitor - strip the chassis, and build this tuner instead.
Remember, tube circuits deal with hazardous voltages! If you shock yourself seriously, I don't take any responsibility. You are on your own. This is one case where I would NOT use an anti-static band!!!!
Most listeners still rely on standard AM broadcasts for the greater part of their radio entertainment. There is some pretty good fidelity in AM, if you have tuner and an amplifier to reproduce it Make the amplifier versatile by simply adding a selector switch and a bass control, and you can reproduce any hi-fi signal, whether AM, FM, or disc or tape recording.
This AM tuner doesn't depart from the conventional in too many respects. The superhet circuit, when it has variable-bandwidth i.f.'s, can pass all you'll get on the antenna, with little sideband cutting. Add a couple of other features and it will do a surprising job!
The schematic for this tuner (Fig. 1) shows some of these little additions which give it hi-fi at low cost—a feature most of us are interested in. For one thing, eliminating the conventional cathode bias on the r.f. and i.f. stages does away with numerous resistors and bypass capacitors. This makes a cleaner wiring job and reduces the over-all number of components. A bleeder voltage divider puts the correct voltage on plates and screens, confines the dissipated heat where it won't bother coils and impregnated parts, and adds to stability. To compensate for all the parts and money saved by this feature, separate decoupling networks are used in all plate and screen leads. But without them some serious common-impedance-coupling problems might arise, especially when the gain is rather high. The screens are run near their maximum voltage for just this reason—a compromise between high plate resistance and a high transconductance.
The diode detector was scrapped in favor of the infinite-impedance detector which is just as capable of handling large signals as the diode and which handles small signals with less distortion. The infinite-impedance detector is nothing but a cathode follower with a large cathode resistance to hias the tube almost to cutoff. This detector presents a very high impedance across the secondary of the last i.f. transformer so that there is no loss in Q as with a diode detector which has a relatively low impedance.
There we have the most important features of a good AM tuner. There are some others, like variable a.v.c., which we will mention.
The construction—whether it's on a new aluminum chassis, as the author’s unit was, or on a well-baked cake pan— must be done carefully. The parts layout shown in the photos has short, direct leads, adequate separation of components and shielding, and gives stability in the r.f. and i.f. stages where it’s really needed. And a hot r.f. stage can generate a lot of things beside a strictly class-A signal. The 6SK7 is used in the r.f. stage because it keeps the grid circuit where it belongs, down under the chassis with the antenna coil; and if the layout shown is fol lowed, there should be no unwanted coupling between circuits. The 2,200- ohm resistors and the two O.1uF capacitors in plate and screen leads are decoupling units.
The converter uses a 6SA7. This tube has a higher gain at broadcast frequencies than similar converters because of a fairly high conversion transconductance and a high plate resistance. Because of its construction it is reasonably stable under a.v.c. conditions, and with a separate oscillator its operation can be further improved.
A separate oscillator tube (a 6J5 in thiS case) reduces the chances of freuency drift due to the a.v.c. action on the converter. At broadcast frequencies this may be splitting hairs, but the theory says it can happen even there. So as a refinement worthy of a good AM tuner, a separate oscillator it is! As in the r.f. stage, separate decoupling networks are used in the plate and screen leads.
The i.f. stage has the older type 6K7. This puts the grid lead above the chasis and away from the plate lead, and reduces the chance of regeneration. Although the shielding of the single ended type tubes is good the 6K7 is better for the variable i.f. transformer used since the grid lead is on the top of the can. And here we have another chance at getting some fidelity. The secondary of the input i.f. has a tapped auiliary winding which broadens the selectivity curve and lessens the side-band cutting which otherwise would be rather severe when the i.f. is peaked. Other methods of doing this same thing use variable coupling between the two windings, but the result is the same-the top of the i.f. curve is broadened, and the skirts fall less sharply.
Following the i.f. stage is the 6C5 infinite-impedance detector. Since we need some a.v.c. voltage, which this detector can't supply, a separate tube, a 6H6, is used. This also provides a diode for the d.c. bias on the grids of the r.f., converter, and if, stages. One half of the 6H6 is led through a 50-uuF mica capacitor from the primary side of the i.f. transformer. This voltage will be higher than that delivered to the grid of the detector, and will give delayed a.v.c. This a.v.c. voltage is fed through the 1-megohm resistor in the a.v.c line, and then to the grids through their isolation resistors.
The R-C constant of the a.v.c. bus is made variable, along with the variability of the i.f. transformer. On position 1 of the bandwidth switch, the R-C constant is fairly short (0.24 sec.), so that the a.v.c. will follow normal moderately rapid fading. But as the bandwidth is increased (positions 2 and 3, the a.v.c time constant is progressively increased, giving 0.36 and 0.66 sec. at maximum. This is necessarily 1ong give good bass response: but once hi-fi broad position is normally used on local or other strong stations, this long R-C constant does not impair the a.v.c. action, such strong signals being usually fairly constant in level.
On strong signals the gain is reduced of course, and the long time constant takes appreciable time to return the gain to its normal high level. For this reason, tuning with the bandwidth switch in BROAD position will give an almost dead response if you tune from a strong signal across the band. until you hit another strong signal or until the a.v.c. voltage falls and increases the gain. It's a kind of unintentional quench circuit, but not bothersome since tuning is normally done in the SHARP position.
Along with the a.v.c. voltage, a fixed bias of about 3 volts is supplied from the other half of the 6H6, through the 0.1-uuf capacitor coupled to the 6.3 heater voltage. The two 100,000-ohm resistors and the 50-uF electrolytic form a filter and divider network for this bias voltage, which is fed through a l-megohm resistor to the a.v.c. hus. Thus, the proper bias is provided without the conventional cathode resistors, and there is no chance for the cathode to go more negative than the grids when a strong signal hits the grids.
When the tuner is completely wired, plug in the a.c. and connect a pair of phones to the audio output. The detector will provide a healthy signal for tuning purposes via phones if you haven't an amplifier handy.
If you’ve got a scope and sweep generator handy, aligning the i.f.’s is easy, and they’ll have about 20-kc bandwidth in the BROAD position, and 10 kc or less in the SHARP position. Tracking and other alignment procedures are conventional.
|Materials for Tuner|
|Resistors: 6—2200, —22000, —30,000, —47,000, 5—100,000 ohm, 1/2 watt; 2—1 megohm, 1/2 watt; 1— 25,000 ohm, 50 watt, wirewound adjustable; 1—1 megohm potentiometer.|
|Capacitors: 3—50 uuf, 2—.001 uf, mica; 2—.05, 4—0.1, —.25 uf, 200 volt, paper: lO—O.l uf, 600 volt paper; 2—16 uf, 450 volt l—50uf. 50 volt, electrolytic; —350 uuf trimmer; l—365 uuf, 3-gang tuning.|
|Transformers and coils: 1—antenna; 1—r.f. Interstage; 1—456 kc if, input with variable bandwidth; 1—456 kc if. output; 1—oscillator coil; 1—20-h, 70-ma choke; —350-0-350 vac., 70 ma, 5v at 2 amp, 6.3 at 3 amp power.|
|Miscellaneous: 1—2-gang 3-position switch; 1— s.p.s.t. switch; tubes, sockets, chassis, fuse, fuse holder, dial, hookup wire, and assorted hardware.|
Interesting article! Notice that the author talks about recycling an old baking pan as a chassis?! Perhaps, though, that was as common to hobbyists 50 years ago as recycling IC and transistors out of discarded projects is today.
For those brave enough to attempt a tube project, I also include the schematic for a dear, departed friend, my old Bogen tuner (I wish I could find where it went).
Click to enlarge
This circuit may offer some advantages, because it uses more commonly available tubes. Finding the tubes for the design project above could be time consuming and expensive!
A similar, but better tuner is the Fisher AM-80:
Click to enlarge
Beware of exactly copying this design - I tried to be careful reverse engineering, but I can't guarantee accuracy. I do have an Orcad Capture version of this design available: . This circuit is superior due to its use of 3 IF stages instead of 2. That this schematic already shows some of my modifications:
NOTE: Drive voltage for the relay can be derived off of the filament winding of the power transformer, which is a very convenient 6 volts AC! All it takes to convert it to DC, useful for all sorts of added circuitry, is a diode and a filter cap. If you place voltage sensitive circuitry such as logic inside the receiver, you can also use a small linear regulator to control the voltage.
After restoring the Fisher AM-80, I was surprised to find that a GE Super Radio III had better performance in just about every way except for selectivity - WITH ONLY A FERRITE LOOP ANTENNA! This gives rise to a new project - reverse engineering the GE Super Radio and using it as the basis for a new, souped up AM solid state tuner design. The GE Super Radio III uses varactor diodes and an analog scale, I may decide to retain this design, or I may make a digital tuning version. One strong argument in favor of an analog tuning design would be that a lot of DX'ers will be after split channels from Europe and Asia. Just as analog tuning helps separate out closely spaced FM stations, it will help with AM as well. I may decide to use a tuning capacitor instead of varactors, or may make it optional.
One thing I promise - when I do post a Super Radio design on here, I will post if for free - unlike a lot of people who will try to sell it to you. I have even seen a lot of people who even try to charge for simple the Super Radio 3 fix - to correct a sensitivity problem some early serial numbers had. BTW - the fix is to reverse the polarity of the speaker connections. Take THAT - you greedy scoundrels! True DX'ers want to help their fellow hobbyists be successful, without asking for money in return. Lets make the hobby easy for Junior High kids (like I was) to get into, lest the information die out and everybody be stuck with what the Discount stores want to sell us.
The expansion of the AM band to 1700 kHz has opened the door to some exciting DX opportunities. The older AM tuners described here, of course, top out at 1600 kHz. It should be possible to adjust them to have a higher top frequency without too much trouble, the only limits might be the limit of travel on the trim cap. If there is a small capacitor in parallel with the trim caps, they can be removed without much danger of raising the low end RF frequency response of the tuner. Remember, though, that the antenna, RF, and oscillator / converter stages must all track, or receiver performance will suffer badly.
Another headache will be the dial scale, usually a piece of glass. These old dials are often works of art, and it might be a little disappointing to just put a piece of plexiglas with the revised scale in there. If you have a scanner, you can always scan the old dial in, deleting the scale itself, and then adding back an expanded band scale. You can get color transparency paper and print the new scale in color, then place it in front of a piece of window glass the right size. Don't cut yourself on the glass - old or new! And be careful not to break your original, you can never replace it.
Do the best you can - but remember you will get more co-operation by being nice than by being belligerant (except perhaps for Redneck Power, Inc., in Lubbock, TX).