A patent showing low voltage operation of vacuum tubes, valves

It is US5022305


https://worldwide.espacenet.com/publicationDetails/biblio?II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19910611&CC=US&NR=5022305A&KC=A


By Butler Brent K


Some more information on Brent Butler' history:


https://www.butleraudio.com/about.php


See also the commercial history of the guitar distortion pedal:


http://www.kitrae.net/music/Music_mp3_Tube_Driver_Versions.html