AB1 Modulator Drivers

[Opcom]

I did not show an AB2 schematic, it was AB1. Others have shown possible transformerless drivers for AB2.

[Steve - WB3HUZ]

Exactly.

The main point for using RC coupling and AB1 is simplicity (check  out the skizmatic in Opcom's post - dead simple).

[AB2EZ]

I agree with Brian, Steve, et. al. about simplicity... and the difference between making modifications to an existing rig (to overcome the limitations of an inadequate coupling transformer) vs. building a one-of-a-kind home brew rig from scratch.

The little FET-based phase inverter + low impedance driver I substituted for the coupling transformer in my Ranger made a nice, reversible mod... which greatly improved the linearity of the modulation at the low frequency end. The phase inverter + low impedance driver fits in the same space that was occupied by the transformer. The 6550's in the modulator are essentially running in AB1. But, on occasional voice peaks, they may be driven into AB2. That is why I used large coupling capacitors.

If I were going to build a home brew vacuum tube driver for a push-pull modulator, I would use a coupling transformer of adequate size to handle the necessary voltage levels and frequencies down to at least 50 Hz without saturation effects... and I would run the push-pull modulator in AB1 (with, perhaps, occasional excursions into AB2 on positive peaks). I would make sure that the output impedance of the driver were low enough to deliver the higher current (to the grids of the modulator tubes), required on occasional voice peaks (which drive the grids positive with respect to the cathodes), without distortion of its output voltage waveform.

Stu