[Ron Ramirez]

The dual 47 90 chassis does not have a separate oscillator tube. It uses the same circuit as the 89 and 19, with a tetrode (a 24 in this case) acting as mixer and oscillator.

Philco would not have installed the newer autodyne circuit in the older single 47 90 chassis, else they would have a leftover 27 tube socket with no need for a tube in that position. It was a new (late 1931) development, and it went into Philco's newest version of the 90.

I think the confusion comes from the documentation which was inconsistent over the years, with some sources claiming the single 47 model 90 used a 260 kc IF when in fact it was 175 kc. 260 is not double of 175 - 175 x 2 = 350. It is quite possible that some people have attempted to align their single 47 90s at 260 kc, with poor results, due to the misleading documentation.

Off subject a bit, I always wondered why Philco continued to use the less than ideal autodyne circuit in the 89 with the difficulties they had with it even back then, after the development of the 6A7 pentagrid converter tube. I've converted a 71 and an 89 to use 6A7 tubes, and they work very well.