[Ron Ramirez]

Regarding use of the 25 or 70 volt line transformers as replacements for single ended audio output transformers -

Is it an ideal solution? No.

Do they work as replacements? You betcha.

And for the cost-conscious folks like me, why pay $70 or more for a transformer which will not 100% match the 7K plate impedance to a 1 ohm voice coil anyway, for use in a radio which will likely only be used occasionally (not daily, for several hours each day) when a $10 line transformer will do the job?

The only vintage radio I use on a daily basis, for a few hours each morning, is the RCA C15-3 console in my home office. I've only been using it daily since I put a Part 15 AM transmitter back on the air here; otherwise I would have no reason to listen to AM radio where I live as there is nothing good to listen to around here. There is only one local AM station, 10 miles away, and it runs an all-Spanish format. Other AM stations which may be received where I live are mostly lost in the noise which permeates the AM band here. I don't listen to the radio at night anymore - that is our TV time.

The RCA is still running on all original transformers (knocking wood). If its audio output transformer failed, then yes, I would likely look for a better replacement than a line transformer since I use the set daily.

A word or three on Philco voice coil impedance, Yes, Philco factory specs give a voice coil impedance of 0.7 ohms for the very early electrodynamic speakers such as used in models 70, 90, and 112. Starting in 1932, they bumped their voice coil impedance a bit, to 1.25 ohms for most K series speakers, and 1 ohm for most S series speakers. (FWIW, the speaker used in the OP's 37-60 is an S-7, the same speaker as used in all Philco 60 sets, and it has a 1 ohm voice coil.) Here it remained until 1939, for the most part. Higher end Philco consoles such as the 14, 16, and 17 consoles and chairsides had 2.28 ohm voice coils. Philco came out with a speaker with a 3.9 ohm voice coil for models 200 & 201, later 116X and 680X, still later 37-675, 37-116, 37-690, 38-116, 38-690, 38-1.

I think all Philco speakers made from 1939 on had 4 ohm voice coils.

8 ohm impedance voice coils are actually a more recent development. I think they began to come out in the 1950s. For many years, 4 ohm voice coils were the standard on many speakers, and have become so again more recently for subwoofers and some car radio speakers.