grimes: How does the Inverse Duplex work?

First of all: the Inverse Duplex System is just another way of reflexing.

The particular difference lays in the signal processing sequence. While the RF goes througe 1st tube, 2nd tube to Det., the Af is first routed through the 2nd tube (which consequently is 2nd RF and 1st AF), then through the 1st tube (1st RF and 2nd AF).
The tube sequence for the complete signal chain is 1 - 2 - 3 - 2 - 1 - 4. The AF chain is inverted to the RF chain.
The straightforward arrangement for comparable performance would be 1 - 2 - 3 - 4 - 5 - 6, hence 2 tubes more.

Honestly: I cannot see any particular advantage by inversing. One idea might have been to balance the combined RF+AF peak amplitude a little. The weak RF at the input does not much harm to the higher audio level, while the 2nd tube, exposed to higher RF level gets less on top by the lower audio. May be, who knows....

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