Most AM rigs from this era did not address the potential failure of the modulation transformer due to over modulation in the negative direction. Many "fudged" the solution by deliberately crippling the modulator design in such a way as to limit the maximum modulation power available. This also provided an ersatz high level clipping function that probably caused less splatter than permitting over modulation. Many hams of that era did not have scopes, since they were expensive.

NO DESIGN IMPROVEMENT INCREASING THE MODULATOR POWER TO A VINTAGE RIG SHOULD BE DONE WITHOUT ADDING NEGATIVE CYCLE LOADING. With modern solid state diodes, it is very easy to do, such that ignoring this essential circuit is penny wise and pound foolish.

I added negative cycle loading to my Valiant. The modulator originally did not develop enough power to endanger the modulation transformer by forcing the level to exceed 0% negative peaks. Now the transformer may drive the final to less than zero volts on the plate, causing it to shut down (its a diode for this purpose). Without the approximate 2K load of the RF final, voltages soar and the modulator transformer or components may arc. The rig I have originally DID arc to the point that the original owner put in an extra set of standoffs to attach the transformer leads with sufficient insulation. This condition can also result from tuning up at full power in AM mode rather than first carefully loading in CW mode. The negative cycle loading consists of sufficient 1N4007 diodes (for Peak Inverse Voltage Rating) and a 2K resistor which is switched in by them to continue loading the transformer.

W8JI states that a diode in series with the modulated plate voltage feed to the final does not stop splatter. This diode is featured in some handbooks from the 50s. My rig contained one of these when I received it, as well as a large choke in series with the screen (which was not switched out in CW). I bet the clicks and waveform on CW were really ratty. I disposed of all this crap in short order, putting the rig back to fully stock to get it running right before modifications.

W8JI comments on the multiple diode super modulation scheme and dismisses it. Some people like non symmetrical modulation, but vintage receivers only tolerate a little bit. I chose not to pursue it, because I retained the clipper circuit and its associated low pass filter. This limits splatter, a good thing. You do not constantly have to ride the gain and stare at the scope to keep your audio from hitting the baseline. The mistake people make employing the clipper is to run it at 25 dB clipping to obtain more punch. What they get is fuzzy distorted hard to understand audio. Using 3 to 6 dB will not be noticed. Properly adjusted, the clipper prevents splatter. The LC filter in a pi network configuration blocks harmonics generated during clipping and undesired high frequencies from passing to the power stage. This prevents excessive bandwidth. Even the external audio chain enters before the LC filter. A small capacitor across the level control and a small capacitor across the phase inverter aid in the suppression of excessive bandwidth. Additional shunt capacitance is across the modulation transformer to limit excessive bandwidth, as discussed later. A high level splatter filter may be added later using the J8 connector for an external box, if it is warranted. The 1950s handbooks show these. By about 1955, the handbooks no longer showed a diode in the high level clipper circuits.

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