The AM Forum

I read many complaints of resistor R35 that provides the 1614 tube screen regulation very poor in design in the Johnson Ranger transmitter.. Why has not anyone used zeners in series to hard regulate the modulator tube screens? Simply adjust the R35 voltage a little higher than normal and use zeners in series to regulate voltage to the tube screens. The 1614 screen in the Ranger is about 280 to 300 volts and draws 20ma for two tubes.

Twenty (20) transorb 1N5359B 24 volt 5 watt zener each are $2.95. Use 12 in series for 288 volts screen voltage. That is 60 watts of zener dissipation, but I would derate to 30 watts and I believe that is enough dissipation. Install a small capacitor of 10uF to clean up any zener noise and perhaps install new metal film 2% 100 ohm resistors to each screen. Due to some extra heat, mount on top of chassis. I am not a great tech, but do understand electronic circuits.

Note- I also read the LV 30uF cap produces 17 volts of AC ripple that is too high. Install a 100 ohm resistor after a new 33uF cap and add another 33uF cap. AC ripple is now about 4 volts and the 120 volts into the 115 volt transformer produces the same original design voltage with the 100 ohm voltage drop (about 10 volts)..

I’ve used Zeners in series, but not directly, for various circuit. Many others, too, in particular W9JSW even made up pc boards for this…
Using the diodes as a reference and a mosfet for a pass element. I got my mosfets from a busted 85” LCD tv (shipper said scrap it after they bent it into a curve, so I did…)
See schematic below.

Ed

Update: Here are three circuits which can be used to regulate screen voltages in Rangers I and II. Except for the external power resistors, all components can be fit on a 2X2" vector board.