Tube tremolo project from Popular Electronics Magazine 1957
http://api.ning.com/files/pip6g4jx0sfvBAWHy5wjjfaWonCWTtLbLnC0IbkgcBYmWn4yXhc8XfocqYQJbetDrQYZZNp66hKguFJ4-HYtuqXcJbHgLpJV/BuildYourOwnVibratoPopularElectronicsDecember1957ArchivedbyCigarBoxNation.pdf
I love the name: VIBRATOR :o ;D
Screenshot of the schematic is attached.
Vibrator..... nice !