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The Ampeg SVT is a bass guitar amplifier made by Ampeg. The SVT, which stands for Super Valve Technology, 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 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. The SVT head was initially used 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 is called the "black line" SVT. 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. 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 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 round, 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 "Harmonic growl" 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 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 tend to blow the tubes.
Some users have an electronics technician re-wire the 12BH7 feed same as on the later 6550 heads. By adding a 1K and filter cap feeding from the 220V screen supply to the front voltage amp of the 12BH7, this keeps the 6146B tube running reliably with far fewer issues. 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. As such, 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. This will make for a very powerful 400W SVT. 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. Some amp technicians prefer to disassemble to PT and tap the windings from the side of the bobbin to create a lower voltage taps. Other methods are to use voltage regulation.
The pre-amp is notorious for ground loop hum. Re-wiring and separating the audio ground shield from the power return ground lead in the MOLEX connector is one solution. The pre-amp can sound quicker if the circuit is rebuilt with Hi-End Audio Grade coupling caps. Diodes were later used to by-pass the 22ohm screen resistors. 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 30mA and beyond, preventing further burning of the PCBA. The diode will clamp the current in the 22ohm screen resistor to 30mA, so preferably the plate resistor will blow...since the plate resistor is acting as a "fuse".
Plate resistors should be kept off the circuit board by approx 1/2" min to prevent PCBA burning. The diode is taking on the current surge to protect the 22ohm resistor, however the peak over current in the diode can only sustain for a short duration. The 6550A version usually idles with roughly 700V DC on the plate and 350V DC on the Screens...during full clean sine wave 300W power output, operating voltages will dip to roughly 650V DC on the Plate and 325V on the Screens. The Output Transformer plate load is 1.6K at 4 Ohm tap and 1.75K at the 2 Ohm tap.
In the early 1980s, Ampeg was bought by MTI in Japan, and the SVT was produced there. Functionally identical, its primary cosmetic differences consisting of: white lettering on a black faceplate, black grill cloth, "elephant hide" or rougher textured tolex, and rack case-style spring-loaded handles, updated from the previous (and painful) rubber covered metal strap handles. 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. Original SVT Transformers were made by ETC...