[Ron Ramirez]

Why do you call it "power detector"?

To fully answer that question, let's turn to The Radio Handbook, Editors of "Radio," copyright 1941:

The plate detector or bias detector (sometimes called a power detector) amplifies the radio-frequency wave and then rectifies it, and passes the resultant audio signal component to the succeeding audio amplifier. The detector operates on the lower bend in the plate current characteristic, because it is biased close to the cutoff point and therefore could be called a single-ended class B amplifier. The plate current is quite low in the absence of a signal, and the audio component is evidenced by an increase in the average unmodulated plate current.

Steve has already told you what to look for in model 90 sets as to which one has this "power detector" (that is, only the early models with two 45 outputs and the normal-max. toggle switch in back). And as Steve said, only the early 70 sets have the "power" detector. Later 70 models do not. These can be most easily identified by having a 35/51 RF tube, 35/51 IF tube, and 27 detector.

In nearly 40 years of restoring radios, I've only had one late version 70 cross my bench. All of the others have been early versions.