Help!I'maRock!
Mediocringly Derivative
And a good article on the amp from Premier Guitar: http://www.premierguitar.com/articles/Differences_in_Fender_Concert_4x10_Amps
Mine is the G612-A version described as follows:
The other version of the Concert during this period was produced between late 1960 and 1963. This version utilized the 6G12-A circuit. As with all of the Concerts during this time, it was also a two-channel amp, but the vibrato circuit had been substantially changed and now required the addition of another preamp tube. Looking at the schematic of this amp, it appears the greatest change to the vibrato circuitry is in the oscillator area of the circuit. This part of the circuit became much more simplified than its predecessor, and by the looks of it became the basis for the oscillator found in the popular opto-isolator version used today. While there indeed was another tube added to this new design, the tube was used to take the output of the oscillator circuit and produce two out-of-phase oscillator signals. These were then fed to the modulation part of the circuit, which would modulate the audio signal. The audio signal was split in order to feed two different amplifiers in the modulation circuit, but because the signal was split through different RC (resistor/ capacitor) networks, there was a slightly different signal being fed to each of the two amplifiers. Once combined together again, these slightly different signals being modulated by two out-of-phase oscillator signals provided the resulting pitch shifting aspect of this vibrato circuit. If you see six preamp tubes across the back of the amp, you have this 6G12-A version.
Your amp actually has vibrato, not tremolo. Don't sell it.