Cathode and Screen Modulating a Beam Pentode

Straight screen modulation of a pentode is a crap idea, so cathode modulation is usually the old stand-by for those who can’t find or afford a big mod tranny and power amp for plate modulation. Cathode mod has limitations though, even if it is done properly. Adding some screen modulation as well corrects a lot of the difficulties with cathode mod, but even this has some built in limits which disallow getting the best out of the idea. This can be overcome by a circuit to maintain a sufficient screen voltage during modulation. The end result is not only almost distortion free audio, but a significant increase in the allowable modulation level of the carrier.

The circuit below works very well in my old AM rig and can be adapted for virtually any output tetrode or pentode.

The mod transformer is a multi tapped  25watt 100volt line speaker Tx . (available anywhere) Grounding the 25w winding tap and using the common for the cathode allows the use of the lowest wattage winding for an opposite phase screen mod voltage. The screen is fed via a cap, and the voltage is also fed via a rectifier network to the grids of the 12AU7 whose combined plate current  reduces with modulation, allowing more voltage on the screen divider with each mod peak.  The 8 ohm side of the mod transformer can be driven by any old amp or, as in this case, a 6GW8 modulator using a standard speaker transformer connected back to back with the mod tranny.

Note, although not shown on the circuit, it will be necessary to select a lower impedance tap for the diode connection, rather than using the screen mod tap as shown. Mine is set to the second tap below the lowest wattage connection on the transformer. It all depends on the linearity required for the screen voltage variation. Set the tap to produce an increasing voltage on the plate of the 12AU7 which doesn’t reach it’s peak before 100% modulation. If the screen mod tap is used, then lower the value of the 100K grid resistor to set the desired dynamic range.

Note. Using the 6DQ5, the range of anode current settings for good mod seem to be within 50 to 80 MA without modulation. Under full modulation the plate current should increase approx 20%. The normal screen voltage would usually measure in the range of 40 to 60 volts with 600-700v on the anode. Best mod envelope is obtained with max drive to the grid and with good solid loading at dip centre. Under driving and/or under loading the final will usually result in distortion and lower max mod levels.

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