This is the front view of the transmitter. The power supply and control circuitry is on the bottom. The next deck above that is the RF deck. It consists of a fairly typical class C RF deck, except that the negative lead is floated at audio – with no modulation, it's at about 3KV DC. Above that is the modulator deck. You can see the 833A class D modulator tube through the round peek-a-boo window. At the very top is a grounded-grid class AB2 linear amplifier, which has nothing to do with the PDM transmitter other than sharing the high-voltage supply. The supply can be switched between the PDM transmitter and the grounded-grid amp, but it's one or the other – you'd never want to use them both at the same time anyway.

Pictured below is a view inside the modulator deck. Most of the solid state circuitry is well-hidden below the chassis, where it belongs :>)

And here is the subchassis of the RF deck on which the tubes are mounted. This subchassis is connected to the “floating” negative lead, but is well-bypassed to ground for RF. The 6146 in front is used as a screen clamp tube to protect the finals from loss of excitation/grid bias.

A side view, showing the large inductor (vertical), which is the first element in the 4-pole PDM filter network. The PDM filter integrates the PDM waveform, converting it back to an analog audio voltage which modulates the class-C final amplifier in the negative lead: You can also catch a glimpse of the 2nd inductive element mounted horizontally across the rack below the vertical inductor.

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