[Ron Ramirez]

If you compare the schematics of the 37-690 and 38-690, you will see that the tweeters are driven by the output tubes in the 37-690, as is the woofer. This is the reason the tweeters are so loud in the 37 model. (Too loud, in my opinion; even taking the NRSC pre-emphasis curve into consideration.)

In the 38-690, the tweeters are driven by the DRIVER tube; there is a special winding in the audio interstage transformer just for the tweeter voice coils. For this reason, the tweeters are more subdued in the 38 model.

I think this was an attempt by Philco to give the tweeters just enough "presence" without being "in your face" on the high end, unlike their 37-690 model. Now, some of us do not hear the higher frequencies as well as others, so the 38-690 may seem as if its tweeters are not working, when in fact they may be working just fine (even with the phase correction I discuss on my site). Syl has pointed this out already.

I also agree with Syl when he says the tweeters in these models are more like midrange speakers. The set was not designed to reproduce audio above 10 kc, anyway.