AM Amazing
Transformation

by
Tim Smith, WA1HLR

There comes a time when the modulation transformer in your DX-100, Apache, B&W 5100, 32V, Viking I or II, Valiant or other 50-200 watt power class transmitter zorches out. Maybe you are interested in building a low bucks transmitter or modulator for your CW rig. The biggest stumbling block is getting a modulation transformer. Well, fear not! The TV power transformer has come to the rescue.

  There are a number of methods in using these beasts as shown in Figure 1 and Figure 2. These are two variations of a similar configuration. The original 120 volt primary is connected to one leg of the high voltage secondary (600-800 volts). The windings have to be in phase or it won't modulate worth a damn.

Figure 1

Figure 2

  A test voltage can be applied to the center-tap and one end of the secondary, as shown in Figure 3. The test voltage can be from 6.3-120 volts AC. The total voltage obtained from the combination of the primary winding and one-half of the secondary should be greater than the test voltage applied. If not, the windings are out of phase. In this case, just reverse the lead on the primary or connect the test voltage to the other end of the secondary.

Figure 3

  In tests conducted on an 800 volt, center-tapped unit, with a 115 volt primary, the impedance ratio was approximately 2:1. This is a perfect ratio for the usual setup with a common high voltage (HV) power supply, and typical 6164's modulated by a pair of 807's, or other lash-ups where the HV does not exceed 800 volts DC. The limiting factor is the high voltage break down rating of the transformer insulation. Most are designed for 1500-2500 volts breakdown. In some designs the primary is the first winding on the core. In others, the primary may be wound on the inside and is more susceptible to crap out. The transformer may also be mounted or insulated above ground in order to accommodate higher working voltages.
  You may ask yourself, "How does a power transformer behave at audio frequencies?" Amazingly enough, most will have very good response characteristics. There are so many variables that it would be difficult to accurately predict what it will do. The power handling capability of the usual TV power transformer is in the order of 150-300 watts depending on the size of the core. It can generally be assumed that one pound of core material will handle 10 watts of audio for good bass response and 20 watts per pound with higher impedances (i.e. PP 811's modulating an 813 final amp).
  In Figure 1, the final amplifier DC current flows through the transformer. This uni-directional current flow can cause the core to saturate. Remember, the secondary is center-tapped. In its original use, current flowed in both directions through the secondary, thus the magnetic flux canceled, and no core saturation. Therefore, the transformer must be modified to handle the uni-directional current flow, or for short, unbalanced DC.
  If you are observant, you will notice that typical modulation transformers, filter reactors and audio output transformers for single-ended operation have a core constructed with "E" shaped laminations stacked together next to "I" shaped laminations. The E and I shaped stacks are separated by a heavy paper spacer. This arrangement enables the transformer to handle (without core saturation) a certain amount of unbalanced DC. There are drawback, however. The inductance of the core and windings falls off rapidly, as the spacing or gap between the laminations is increased. This causes a marked drop in the low frequency response of the transformer. In the case of the TV power transformer, a gapped unit will work okay over the frequency range of 70-5000 Hz.
  In order to modify the transformer, the whole unit must be disassembled. With careful use of a hammer and several sizes of screwdrivers, it is possible to do the job within an hour. Remove the bolts holding the the cover bells in place. It may be necessary to give the core stack a couple of well-placed whacks with the hammer to loosen up the laminations. Some transformers come apart easily and other take more patience. Exercise extreme caution so as to not damage the windings.
  Once the laminations are apart and are clean of foreign matter, group the E and I laminations in separate stacks. An educated guess will have to be made on the desired thickness of the paper for the gap. For plate currents of 200-300 milliamps, try a piece of paperback book cover (or paper of similar thickness). For currents of 75-150 milliamps, a sheet of of standard weight copier or printer paper should do. Assemble the E laminations into the core of the windings. If you can't get the last few pieces in, don't worry, it will have little effect on performance.
  Cut the gap spacer paper to the proper dimension and epoxy it between the E and I lamination stacks. Insert the bolts and compress the laminations with a large vice or heavy weight. This will force the epoxy to flow into all the crevices between the E and I laminations. Thisstep must not be skipped or loud talk back will occur when the transformer is put in use, resulting in feedback. If the work has been done properly, you now have a modulation transformer!
  In Figure 2, the final amplifier DC current does not flow through the transformer but instead is handled by the inductance. This configuration is known as modified-Heising modulation. The value of inductance can be determined by a this rule of thumb: 10 Henrys for every 1 kOhm of finals amplifier modulating impedance (final voltage divided by the final current). A single choke or reactor is not required. The required value of inductance can be obtained by connecting several chokes in series. Power supply filter chokes are plentiful and can be used. Smoothing chokes (NOT swinging chokes) must be used. You can get away with as little as 5 Henrys per 1000 Ohms of final modulating impedance with some loss in bass response. Since there is no DC current flowing through the secondary of the modulation transformer, no modification of the transformer (as described above) is required. Low frequency response is also better. Overall, this approach is superior to that shown in Figure 1. The draw back is that room must be found or made for the choke(s).

  The table below shows the computed impedance ratios for voltages commonly found in TV power transformers.

 Here's how you would use the table if you wanted to plate modulate a DX-60 with a pair of 807's. Assume the final amplifier is to be run at 700 DC plate voltage and 120 milliamps current. This yields as modulating impedance of 5600 Ohms. The 807's want to see a plate-to-plate load impedance of about 10 kOhms when run in class-AB1. With 600 volts on the 807 plates about 45 watts of audio can be obtained. An impedance ratio of 2:1 is required. Thus, a TV power transformer with a secondary voltage of 500 or 600 volts is required. If the modified-Heising approach is used, a 10uF cap, several filter chokes can be series together. Three typical values like 10 Henrys at 150 mA, 16 Henrys at 150 to 200 mA, and 8 Henrys at 200 mA will add up to 144 Henries, which is sufficient.
  Once assembled, it is found that your modulator is performing much better than you thought. The component that usually cost the most or is the hardest to find, cost you nothing, and you had to look no further than that junk TV in your garage!
  In the case of replacing the modulation transformer in you DX-100 or Apache, where space is limited, you can solid state the power supplies and move around components to make room for the additional Hiesing choke(s). The extra space for the chokes is not required if you feel like performing the gap modification. A TV power transformer was used as a modulation transformer in the modified-Heising configuration in an Apache, recently.
  In the case of a Viking Valiant, converting the HV supply to solid state and moving the screen dropping resistor provides plenty of room for two typical 10 Henry/300 mA chokes. An old power transformer can be squeezed in place of the original modulation transformer.
  You may be wondering what to do with any filaments on the TV power transformer. They can be used for a variety of things. A 600 VCT unit, with a 6.3 VAC filament winding, when used with a 5 kOhm plate-to-plate load impedance, will present a load of 0.5 Ohms at the filament winding. This makes an ideal pick-off point for monitor headphoniums. The voltage step down ratio is such that enough level will be available at the filament winding without blowing your ears off. You can also use the filament winding for a loop a negative feedback into an earlier audio stage, in order to improve the modulator frequency response and lower distortion. The winding can also be used as an oscilloscope take off point to produce a trapezoidal pattern for testing and monitoring modulation quality and level.

From The AM Press Exchange, August 1990.