Have you considered a test bed for your hi-fi AM radio experiments? Bill's
comments about IF transformers got me thinking that one approach would be to
get an old 70s era stereo receiver with IF transformers and no
crystal/ceramic filters in the IF strip. These things are pretty
inexpensive at the Salvation Army type stores now. You could try stagger
tuning the IF transformers for a wide bandwidth. Ideally, you'd use a sweep
generator, but you could probably do a passable job with a standard signal
generator and some patience. It's not a tube setup, of course, but you'd
get wide bandwidth AM with a reasonably low noise wideband amplifier.

Frank Dresser