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Are 1960–63 Concerts all the same? What are the differences?
Good question on one of my favorite amps. You are speaking, of course, about the Fender brown-era 4x10 Concert amps. Here’s a little background on the amps, as best as I can put it together for you.
The Concert amp was first introduced into the Fender line in 1959. It was a two-channel amp and came in a 4x10 combo configuration. Its only effect was vibrato, as reverb had not yet found its way into the Fender amp line. I really don’t have too much information about the 1959 version of the Concert, except for the fact that it was produced using the 5G12 schematic, which seems to be a pretty elusive item. The 5G13 Vibrasonic schematic is available, however (the two amps were electrically the same in 1960), and there are very minimal differences between it and the Concert’s 6G12 schematic, so I would assume that the 1959 and 1960 versions of the Concert were very similar. The next versions of Concert amps, the amps in question here, produced from 1960 to 1963 actually differ in two ways. One was cosmetic and the other electrical.
Electrically, there were two different versions of the Concert produced between 1960 and 1963. The first version, produced in 1960, was based on the 6G12 schematic and was, as stated above, electrically the same as the Vibrasonic amp of the same era. The “12” in the schematic number is actually the model number of the amp. In comparison, the model number of the Vibrasonic is 13. This would indicate that design work on the Concert preceded that of the Vibrasonic, but according to those knowledgeable on the subject, the Vibrasonic was released earlier than the Concert. This version of the Concert can be quickly recognized by checking the tube compliment. If you’re seeing five preamp tubes across the back, the amp is an early- to mid-1960 Concert produced using the 6G12 circuit. Another recognizable and interesting attribute of these amps was the controls, which were backwards from what we’re used to seeing on the face of a traditional Fender amp. Following the input jacks, the controls were Bass, Treble then Volume, not the traditional Volume, Treble, Bass. One glaring error on the 6G12 schematic, as well as the Vibrasonic 5G13 schematic, is the fact that the 470-ohm output tube screen grid resistors are marked as 470K ohm. That’s a design parameter that definitely would not work.
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.
Between the two, the 6G12-A is my personal favorite. The vibrato in these amps is wonderful sounding. It’s one of the only circuits utilized in a Fender amp that actually lives up to the term vibrato, which by definition needs to include some form of pitch shifting. This circuit achieves that to some degree. The other versions of vibrato used in Fender amps, be it the opto-isolator version or the bias-varying method, are actually forms of tremolo, not vibrato. These circuits do nothing more than vary the amplitude of the circuit.
That’s my story on a couple of great Concerts. Enjoy.
Co-Founder and Senior Design Engineer Budda Amplification firstname.lastname@example.org or budda.com.