The Supro and Valco line

Valco was a contract shop that happened to own famous brands. It was established in Chicago in 1940 by Victor Smith, Al Frost and Louis Dopyera, all formerly of the National Dobro Company — the name is their three initials plus the abbreviation for company — and it built instruments and amplifiers under Supro, National, Airline and Oahu while also building amplifiers under contract for Gretsch, Harmony and Kay. One chassis therefore left the factory wearing whichever badge had ordered it, which is why the same circuit appears in the catalogues of companies that were nominally competitors, and why these amplifiers are catalogued here by the model number on the drawing rather than by the name on the cabinet. The house style is consistent across the badges and unlike anything from Fullerton: cathode bias on one shared resistor with no bias supply, paraphase phase inverters long after other makers had moved to the long-tailed pair, a single treble-cut tone control shared by both channels instead of a tone stack, tremolo that modulates a preamp cathode rather than gating the signal path, and — in the mid-power amplifiers — the 6973, a nine-pin beam power tube with only 12 W of plate dissipation that almost nobody else adopted. Valco merged with the Kay Musical Instrument Company in 1967 and the merged company folded the following year.