Mod Tranny

8/10/2019 Mod Tranny

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Using a Plate Transformer for High-Level ModulationBy Mike Cowart WA5CMI

Have you ever wanted to build a high-powered homebrew transmitter but stopped when

you started looking for the mod iron? Lets face it; finding a modulation transformercapable of modulating a one-kilowatt amplifier is becoming very difficult. What if youcould use that old 4 KV plate transformer you have out in the garage that has been takingup space for the past 10 years?

A Little TheoryAs a rule, to plate-modulate a class C amplifier, the modulator must furnish a peak-to-

peak voltage that is twice the plate voltage (B+). When impressed onto the platevoltage, the positive swing adds to the plate voltage to generate a peak voltage up to twotimes B+, and the negative swing subtracts from the plate voltage to reduce B+ to zero.

The final B+ on my Globe King 500 is approximately 1,500 volts. I would need atransformer capable of 3,000 volts peak-to-peak at a little better than 250 watts. I found atransformer rated at 5,000 volts center tapped at 500 mA: a big honker! The primary can

be set for either 110 or 220 Vac.

Because I needed only 3,000 volts p-p to fully modulate the Globe King, I knew I wouldnot have to supply the full 110 volts to the primary. Remember power transformers arerated in RMS volts; 5,000 volts RMS is equal to a little over 14,000 volts p-p, and 110volts RMS is equal to a little over 300 volts p-p. The transformer has a turns ratio of110:5,000 or approximately 1:45. To obtain 3,000 volts p-p, I needed 66 volts p-p on the

primary. An audio amplifier capable of delivering at least 66 volts p-p (23 volts RMS) at250 watts (the equivalent of a 2-ohm load) should do the trick. An amplifier of that

capability, however, can be expensive, and this was , after all, an experiment.Fortunately, I have a Peavey solid-state amp that I bought last March at the MidlandHamfest that has two channels, both capable of 130 watts RMS into a four-ohm load. Byreconfiguring the amplifier to use both channels to drive the transformer primary, I wasable to obtain the voltage and power levels needed.

Mono-BridgedUsing both channels of a stereo amp to drive a single transformer winding (or speaker,for that matter) requires that they be driven 180 o out of phase. This means that onechannel is driven directly by the audio signal and the other by the inverse, making amodification to the amplifier necessary, unless it has the capability to be configured thisway. Many stereo amps today have this feature.

By using the both amplifiers to drive the load with 180 o phase shift, the effective outputvoltage doubles, yielding four times the power that a single channel can provide. This istheoretical, of course, and assumes that the power supply and output transistors canhandle the increase. Be careful; assumption is a fools game! The reason for the increaseof four is because of the square law:Power = E 2 / R.


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Figure 1 Mono-Bridged Amplifier

The ExperimentI built a simple operational-amplifier circuit with its own voltage regulation, powering itfrom the internal power supply rails in the Peavey. I mounted the phase inverter board ona printed circuit board in the amplifier on which the volume controls are mounted. Thisallowed me to wire the phase inverter output directly to the potentiometers, keeping thewires short.

Testing the ModulatorAfter installing the phase inverter, I connected an audio generator to the input of themodified Peavey. I then connected the two power amplifier outputs to either side of the

plate transformer primary as shown in Figure 1 . I then connected the secondary of the plate transformer to a 2700-ohm, 225-watt resistor. This value is very close to thecalculated load impedance of the Globe King final amplifier. Finally, I connected myoscilloscope across the load resistor.

Upon powering up the amplifier, I set the audio generator output to 1,000 kHz andadjusted the gain until the waveform on the load resistor began to flatten out. The peak-to-peak voltage across the load was in excess of 3,500 volts, more than the 3,000 voltsrequired to fully modulate the Globe king. I let the amplifier work for a long period oftime, and after approximately 30 minutes, the thermal protection circuit shut the amplifieroff. This was certainly understandable, as the amplifier had delivered over 500 watts ofaudio for that period of time! Needless to say, the resistor was rather warm.

When the amplifier and the load resistor cooled down. I ran a frequency response test byfinding the frequency that yielded the maximum output (approximately 300 Hz), thenadjusting the frequency of the audio generator while measuring the output voltage. Theresults are given in the following table. The data are plotted in the graph after the table. Iused 400 volts p-p as a starting point (0 dB).


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Figure 2 Peavey Amplifier with Phase Inverter Installed

Figure 3. Frequency Response Data

From the above curve, you can see the frequency response of the plate transformer rollsoff rapidly above 2,000 Hz, but this can be easily compensated for using a graphicequalizer or some other form of equalization built into the amplifier.


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The Smoke TestAfter I was convinced the amplifier would hold up and produce enough audio power forthe Globe King, I simply connected the secondary between the high-voltage deck and theRF deck. I left the audio generator connected so I could look at the modulated outputusing a sine wave modulating voltage. Running the Globe King (using a 4-400 as a final)

into a dummy load at full power, there was plenty of audio power to over modulate therig.

The Proof Is In The PuddingI connected the audio rack to the Peavey amplifier and tried it out on the troops in theAM window. One of the things in the audio chain is a parametric equalizer, socompensating for the roll off is easy. Having had this modulation system on the air foralmost two weeks has given me time to solicit opinions. Many have said that the audio

produced with this system sounds as good as the stock modulator in the Globe King 500.After one of my old-buzzard transmissions, the amplifier heat sinks are warm, but nothot, to the touch. I could not be happier with his system. It surpassed all my expectations,and I may use this system, with a more powerful audio amplifier, in my upcominghomebrew 4-1000 transmitter project.

Figure 4. Globe King 500 With Outboard Modulation Setup


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