Let’s just assume that someone forgot to update a few numbers here. If that was truly the case in a (semi) functioning amplifier, my initial diagnosis would be a bad output transformer. Also, the stated plate voltage of 342V DC indicates a voltage drop of 78V across the primary of the output transformer. Keep in mind that this is supposedly while using the same 5Y3 rectifier and same 125P1B mains transformer as the CBS-era Vibro Champ, though if everything else in the amp is the same, this is theoretically impossible. Meanwhile the Bronco schematic shows the voltage at the first power supply filter cap as 420V DC. Perhaps the engineers at Fender decided at some point to design a double-duty transformer, one that could supply the needed voltage for a Champ with a 5Y3, but with sufficient current to supply a Princeton and the increased voltage output from its GZ34 rectifier. The pre-CBS AA764 Champ schematic shows a 5Y3 and P1B mains transformer, with a B+ voltage of 360V DC and plate voltage of 350V DC-very similar to the CBS Vibro Champ. The plate voltage of the 6V6 output tube is stated as 342V DC, giving us a 13V drop across the output transformer primary, which is certainly possible. This is supposedly achieved with a 5Y3 rectifier tube using a mains transformer with part number 125P1B. The schematic for the CBS-era Vibro Champ shows the main B+ voltage at the first power supply filter cap as 355V DC. That said, let’s see if there are any explanations for the differences between these schematics. The Bronco name appeared on the front panel in red rather than the usual blue, and the amp was packaged with the Bronco guitar as a beginner’s set sometime around 1967. The Bronco was essentially a Vibro Champ with a different name. The other schematic you reference, AB764, was actually for the Bronco amp. I searched extensively but could not locate a schematic with the number AB764 specific to the Vibro Champ-all I found was the usual AA764. Do you know if all the 1972 and later Vibro Champs used the AB764 schematic? Is there any way to tell whether a Vibro Champ uses the AA or AB version of the circuit without opening it up? The AB764 circuit contains changes to the (higher) voltages used in the amp, and also a cap can change from a 20-20-20 can to a 40-20-20 can. I believe most people think that only the AA764 circuit was used in these amps, but from the schematic, it seems the AB764 circuit appeared around 03/24/71. I recently acquired a 1972 Fender Vibro Champ, and after trolling the web discovered that there were actually two circuits used in the Vibro Champ: AA764 and AB764.
0 kommentar(er)
