The PDM AM XMTR at NE1S
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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