In the early 1980s, Ampeg was bought by MTI in Japan, and the SVT was produced there.
Other notable differences of this era of SVT are found on the back panel, including a toggle switch for 2 or 4 ohm speaker impedance loads and a longer and thicker gauge 3-prong power cable.Apparently the transformers used are also of Japanese origin as opposed to the previous manufacturer in Chicago.Later on Ampeg updated the speakers in the enclosures so that one cabinet was sufficient. Early 1969-70 "blue lines" used 6146B beam power vacuum tubes.The 6146B proved to be unstable in the high-voltage amp, and was switched to the more robust and reliable 6550 tube around mid-1970.In the event of a tube short failure or simply a transient overload condition the diode will conduct once the current in the 22ohm screen resistor reaches 30m A and beyond, preventing further burning of the PCBA.
The diode will clamp the current in the 22ohm screen resistor to 30m A, so preferably the plate resistor will blow...since the plate resistor is acting as a "fuse".Mainly due to the output transformer primary inductance with respect to plate resistance..The 6146 is much lower in transconductance and is less sensitive to drive signal, thus needs a bigger voltage swing to drive than a 6550...Since the power transformer of a 6146B SVT will be about 220V DC on the screen supply at idle instead of the typical 350V idle screen voltage normally seen on 6550 amps.One solution is wire the screens in a VOLTAGE DOUBLE arrangement, which will end up at roughly 400V screen voltage at idle. Conversion from a 6550 tube to 6146B tube is a bit trickier..since the 6146B will not tolerate anything over 250V on the screens or else it will arc over. Re-wiring and separating the audio ground shield from the power return ground lead in the MOLEX connector is one solution.Some amp technicians prefer to disassemble to PT and tap the windings from the side of the bobbin to create a lower voltage taps. The pre-amp can sound quicker if the circuit is rebuilt with Hi-End Audio Grade coupling caps.