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was designed by Bill Hughes and introduced in 1969.
The word "valve" refers to the vacuum tubes (called "valves" in Britain and some other regions) used in the amp.
The lines and text are black instead of blue, and like the later run "blue lines" use 6550 power vacuum tubes instead of 6146Bs.
Later 1970s models have the same features as the "black line" SVTs except the lines around the tone controls have rounded corners and curve into the tone controls.
The 6146B tube in itself does not have instability issues and is a reliable tube.
The early driver circuit was not properly designed, therefore would blow the 6146B tubes on occasion.
Conversion of 6146B to 6550 tubes has a dramatic impact on the output power.
The amp will produce roughly 225 watts , due to the screen voltage being too low.
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 SVT is a stand-alone amplifier or "head" as opposed to a "combo" unit comprising amp and speaker(s) in one cabinet, and was capable of 300 watts output at a time when most amplifiers could not exceed 100 watts output, making the SVT an important amp for bands playing music festivals and other large venues.
The SVT has been through many design changes over the years but is still in production today.
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.
They also came with 3-prong power cables, and no polarity switch.