In my travels I've discovered a source of technical tests right here in the
UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter, but they also do most if not all of the classic
tests.

They do require you to register for a (generic) userid and password, but its
via autoresponder and all they want is some name and an email address.

At any rate I noticed a test that makes me smile: atest of a legacy Leak
20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one has
seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response pattern
characteristic of modulation distortion around the fundamental and first few
harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons of
hum in the power supply and directly heated cathodes don't seem to fit with
other evidence at hand, including a schematic.