I noticed a special quality about this Gilfillan (6x8 family)
http://www.nostalgiaair.org/PagesByModel/612/M0008612.pdf
It is very enjoyable to listen to and use because as you tune across the dial.. not only is it relatively sensitive to weak stations (or appears that it is).. but mostly I'm taken with the beautiful total quiet between stations. No static, nothing!.. As you approach each station it just starts to come in softly and as you tune to it there it is clean a pure.
This is just a five tube set... and the only time I otherwise get this sensation is when I have a six tube set. That addition rf stage seems to contribute to this quality.
But since this Gilfilan is only a "5 tuber" what do you suppose is responsible for this? I can only guess that it is the broadcast antenna bandpass coil that is blocking out all the other noise and crap like that?
If so.. this would surely be a great addition to other 5 tube radios. The difference is really astounding.
Maybe the reason it "seems" sensitve too is not that it really IS more sensitive than other radios, but only that it appears so as the noise that usually swamps out weaker stations has been eliminated?
I restores my respect for AM. After all the disturbing noise that seems to proliferate on this band I thought AM was really dead. But now I can see that a little engineering can go a long way.
Any thoughts?

Good circuit design, tube choice, transformer design, etc. The transformer may be such that it isn't letting in interference from the line cord. Good circuit design and shielding have a lot to do with it, though.

Thomas

You're right about the sheilding Thomas... every tube except the 80 and the 42 have full shields.
But the line cord does exhibit some serious static when plugged in directly to AC but quiet in the isolation transformer.
The line noise static was completely eliminated though by adding two .05uf caps connected together and across the line and the common going to the chassis.