„BR stood for Barnes & Reinecke, a Chicago firm that came up with the new designs for Gibson. Gibson licensed the basic circuits for its amplifiers from Western Electric, a predecessor to Bell Labs, nowadays known as Lucent Technologies. The BR line of amplifiers were all fitted with electro-dynamic speakers like the pre-war amplifers. A Jensen speaker was specified for the BR-1. The BR-1, BR-3 and BR-4 features tone controls as well.The restriction of material such as metals during the war forced the Gibson factory to suspend its line of electric guitars and amplifiers. The production at Gibson did not start again until 1946.
A completely new line of hawaiian guitar/amplifier sets were now introduced with model designations Ultratone/BR-1, BR-3, BR-4, BR-6, and a couple of years later, BR-9.
The amplifiers used the 6SJ7 (sharp cutoff pentode) for microphone inputs, 6SL7 (dual high mu triode) for instrument inputs and second gain stage, 6SC7 (high mu triode) as second gain stage, 6SN7 (dual medium mu triode) for phase splitting, two 6V6 or 6L6 power tubes, and 5U4 or 5Y3 rectifier tubes. This was a basic set of tubes that Gibson would be using until the mid ’50s.“