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The 6550A version of SVT amps tends to have a more rounder deeper bass sound..
Mainly due to the output transformer primary inductance with respect to plate resistance..
Some users have an electronics technician re-wire the 12BH7 feed same as on the later 6550 heads.
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, as 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.
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.
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.