[NEW] vacuum nonsolidstate distortion...

[amz-fx]

i think i remember that incandescent bulbs start out very low resistance (couple of ohms)

Depends on the bulb...  the ones in my low watt practice amp are about 100 ohms and change very little as the circuit warms up.  The bulb never lights even when playing it hard.

There has to be enough current flow through the filament to cause it to get hot, which doesn't happen at guitar or line levels.  Drive a bulb with a big power amp and it will light up!

regards, Jack

[GFR]

In the RDH there's a chapter about compressors/expanders with some light bulb based designs, ranging from simply connecting them in parallel or in series with the speaker (not recommended) or in a "bridge" circuit (so that the impedance seen by the amp is fixed). Car ligth bulbs (12V) do light in this application

[puretube]

probably similar to this:
http://www.pat2pdf.org/patents/pat3397285.pdf