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Making The Modulation Transformer
Fig. 7

plate modulation- Consider for a moment the effect of having the grid connected above the modulation transformer. In this case we have ordinary plate modulation except the transformer is connected on the cathode side of the amplifier. Since it is a series circuit it would make no difference on which side the transformer is connected. Suppose for an instant that we have sufficient audio to plate modulate 20If we now advance the grid down the adjustable resistor tied across the modulation transformer we get additional grid modulation. If adjustments are carried out as outlined in grid modulation we can get the additional 80%, making a total of 100%, In this case, due to the presence of some plate modulation we will not have to double the efficiency. So we can run at a higher unmodulated efficiency, Depending on the amount of audio power available, the efficiency may run up to

Fig. 8

50% or so. This is an expedient that may be used when one has a public address system, or similar piece of gear, having insufficient power to plate modulate the final amplifier.

Screen Grid Modulation

There are assorted forms of screen modulation but they all hinge on lowering screen voltage, in some cases into the negative region, and adjusting for the proper coupling to get linear performance. Many screen grid type tubes are not capable of 100% screen modulation. Some specially built tubes do a very good jobt particularly on speech. In general it is difficult to get a distortion free scope pattern on a pure tone; however speech may be quite acceptable. In many ca^es, judging from what this observer hears on the air, the positive and negative peaks are badly unbalanced and considerable flattering occurs on the positive peaks. Iiis is probably the result of adjustment without the use of a scope- This particular form of modulation has become popular in controlled carrier systems of which there are too many for the scope of this article. They all trade on running with very little output and plate dissipation with no modulation. When modulation is applied input, output, and plate dissipation go up. With a sustained sine wave of modulation many of these circuits would have plate dissipation far exceeding manufacturers ratings. But with speech, ratings are only exceed momentarily. Fig. 9 is a conventional circuit. Quite an article could be written on variations of screen modulation.

Suppressor Grid Modulation

This form of efficiency modulation was very popular back in the 30Js, but is almost unknown today. Yet it is perhaps the easiest of all the systems of this kind to adjust for linear operation. Its chief claim to fame is the

tSCREEN I

relative unimportance of the loading. Almost all of the variable efficiency is derived from adjustment of the suppressor grid voltage. If you are not concerned with efficiency it is possible to get linear modulation at any value of load whatever just by reducing (making more negative) the dc potential on the suppressor grid to the necessary level. In practice the optimum loading for an unmodulated signal will not have to be changed. Just bias the suppressor grid sufficiently negative to get a linear pattern by observation. This usually requires a modulating voltage that never drives the suppressor into the positive region; hence, little or no audio power is required. As is the

^another First from Finco

Fig. 10

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