I recently helped an email acquaintance upgrade his DX-100 modulator system, which I affectionately call the, “Pre-Apache.” While the two rigs share very similar circuitry, the DX-100 uses the 12 volt equivalent of the 807, the 1625, while the Apache’s modulator tubes are EL-34/6CA7. The 6CA7 takes less control grid drive than does the 807 in Class AB2, so we dispensed with the 1k grid parasitic resistors and replaced them with 100 ohm resistors which are just as effective but increase grid drive. We regulated the modulator screen voltage with the zeners he had on hand since without regulation the screen voltages can surge to 350 volts or more shortening the life of the tubes. The capacitive coupling at the driver-to-transformer primary removes any problems with DC core saturation and its ensuing distortion. With the component values shown in “” the Peak-to-Peak voltages for 100% modulation were as follows: 12BY7 control grid Pin 2: 6 volts p-p, 12BY7 Plate Pin 7,8: 133 volts p-p, Full Primary of 124D: 130 volts p-p, Full Secondary (Grid-to-Grid) of 124D: 195 volts p-p. The Modulator transformer was wired as per the original schematic, except for the soft-knee limiter.