Ampeg SVT

The Ampeg SVT is a bass amplifier made by Ampeg. The SVT, which stands for Super Valve Technology, was designed by Bill Hughes and introduced in 1969. The SVT bass head produced 300 watts at a time when most amplifiers made less than 100. The SVT has been through many design changes over the years but is still in production today. The SVT head was initially coupled with a pair of sealed 8x10" speaker enclosures because one cabinet could not handle the power of the SVT. Later on Ampeg updated the speakers in the enclosures so that one cabinet was sufficient.

There are three types of original SVT amps. The first are the "blue line" SVTs. These are called that because of the engraved blue text and lines that surround the tone controls. 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. The second version of a vintage SVT is what many call the "black line" SVT. The lines and text are black instead of blue, and like the later run "blue lines" use 6550 power 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. They also came with 3-prong power cables, and no polarity switch.

The SVT amps with 6146B tubes tend to put out a bit more watts of power as well as have a more pronounced grind in low mids that makes these amps desirable for some bass players. 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..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... another reason the 6146 amps have more growl than its 6550 counterpart is because the pre-amp is turned up a bit higher to achieve the equivalent power output swing, thus more "grind" (fuzz bass) can be obtained from the preamp.

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. The reason most 6146 SVT amps blow tubes and resistors is due to front end 12BH7 voltage amplifier is being fed from the 430V node. During loud transients and overloads this will produce an AC signal that far exceed the 12BH7 follower that has 220V on the plates. So when the follower grid is driven far over its own plate voltage it saturates on the positive half of the signal and thus takes over the BIAS voltage forcing it very positive. The time for this voltage to come back to normal is based on the time constant of the 150K mixer resistors and the coupling cap...by this time it is too late, the BIAS is pushed way positive and the current gets slammed through the 6146 and they blow.

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