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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.
Secondary Voltage Impedance
Ratio
1000 2.65:1
900 2.56:1
800 2.39:1
700 2.27:1
600 2.09:1
500 1.88:1
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.
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