Ampeg SVP-Pro Bass Preamp and SVT-3-Pro Bass Amplifier

svppro overview
svt3pro overview
svppro front
svt3pro front

Last Update 01-24-2010

My first taste in tube bass preamps was my Tubeworks Blue Tube.  While it was a good preamp, it is pretty much a one trick pony and doesn't get much variety in tones.  But the Bluetube convinced me that a tube preamp went a long way to shaping the tone of a bass guitar.  So I found this used SVP-Pro preamp in a store and really liked the tone variety.  When I got home, some internet searching revealed that not only did it have a tube front end, but the tone controls were based around tubes and it also had a tube that was dedicated to overdrive.  The Blue Tube is nice but the SVP-Pro is a bass players' preamp.  So the next day I brought my Alvarez six string bass for the acid test (low B) and the SVP-Pro soon became another fixture in my ever expanding studio.

The SVT3-Pro came later when I needed for a bass amp for gigging.  I have a modular system with the SVP and other components depending on my gig situation,
IE it is the system I use if I am playing keyboards and bass.  But for gigs when I am only playing bass guitar, the modular system is overkill.  If I matched the tone controls and graphic EQ (hint the settings in the pictures are NOT what I use) then both sound the same.  When the overdrive is used, the similarities end there.

Both overdrive circuits employ a 12AU7 low gain tube.  The 12AU7 is better suited for overdrive than the 12AX7 because you don't need all that distortion for bass guitar.  The SVT3 overdrive tube is placed in the power amp section.  Since the SVP lacks a power amp, the overdrive tube is placed before the EQ section, right after the input preamp.  The timbre is different between the two when overdrive is active.  The SVP overdrive is more effective for dynamic touch - when I slap-n-pop the 12AU7 pushed into overdrive which adds harmonics that help the tone cut through.  The SVT3 overdrive gets a really nice punch on a stage when I play fingerstyle.

Both units are descendents of the original SVT bass amp, a landmark Ampeg design that was released in 1969.  The SVT was the first real bass amp designed for large venues and they were very popular with bass players.  A complete SVT system included the head and two speaker cablinets each loaded with eight ten inch speakers.  I have heard (and felt) a complete system in full flight at a large bar I used to frequent, and the sound radiated everywhere even all the way in the back.  It was also a heavy system to cart around and the SVT bass head was a very heavy head, packing in two large transformers (power and OT) and a sextet of 6550 output tubes.  Ampeg designed some failsafe features into the head that prevented a tube fault from disabling the entire amp - if a tube went faulty in the middle of a gig you could still play the amp, albiet at reduced output power.  The complete SVT system was practically indestructable.

The closest modern equivalent is the SVT2-Pro which included the output tube sextet.  The SVP-Pro preamp is lifted right out of the SVT2-Pro.  The SVT3-Pro uses a solid state amplifier - no output tubes.

A comparision of both SVP-Pro and SVT3-Pro units:

ampeg-controls-compare ampeg-compare-internals

The front panel controls are almost identical.

Both units use a 12AX7 tube for the input preamp and the EQs are based around 12AX7 tubes - like the classic SVT bass head.  The graphic EQ is solid state and was not a feature in the SVT however it is a practical addition.  You can get aggressive sounds by pushing the input preamp into distortion.  The EQ is really effective (better than the Bluetube) and the sweepable mid really helps dial in your tone.  While the original SVT had three selectable frequency ranges for the mid EQ, the SVP/SVT3 has five (220hz, 450hz, 800hz, 1.6Khz, 3Khz).  The EQs have +/-12dB boost/cut ranges, while the graphic EQ has +8/-10dB boost/cut ranges.  A gain makeup is provided in the graphic EQ.

Pushbuttons engage various tone shaping options.  "Ultra Lo" emphasizes the low end spectrum with a +3dB boost at 40hz and and -12dB cut at 500hz, quite effective.  "Ultra High" gives you a +8dB boost at 8Khz that is effective for playing with picks.  "Bright" provides a +7dB boost at 2Khz, I have yet to find an application for this as it is a bit harsh.  "Mute" silences all the outputs with the exception of the tuner output for silent tuning onstage.

Very important: many Ampeg owners complain of excess noise (Bloody whiners. Just Kidding).  The Ampeg units left the factory with cheap Chinese tubes installed, the worst made on the planet (which were also the cheapest, and the beancounters are always emphasizing cheapest components to keep the retail reasonable and their bonus checks viable).  My units were not immune from this.  You can drastically reduce the noise by replacing the inferior Chinese tubes with better ones from Groove Tubes, Rubys, JJ, or Electro-Harmonix.  In fact the signal-to-noise ratio not only improved on my units with the better tubes, but the output signal was also hotter!

As you can see from the pic showing the inside of both units, the circuit boards are definitely not the same.  Whether they are the same EQ and preamp circuits I cannot ascertain without a schematic.  Certainly the SVT3 preamp tubes are not going to fit in a 1U rack standing upright, so no surprise that the boards are different.  But they do show good workmanship.  Both these units were made in the US, later units were made in Asia and are reported to be inferior to the US-made Ampegs.

ampeg-rear-panel-svt3 ampeg-compare-rear-panel

Both units sport identical outputs, with the exception of speaker outputs on the preamp-only SVP-Pro.  Tuner out is an unprocessed direct output to your tuner, which is unaffected by the MUTE button for silent tuning onstage.  An effects loop gives you access to your favorite processors, I have a JBL/UREI 7110 compressor patched into mine.  There is a transformer isolated balanced output for DI to FOH or recording station, and it is switchable to pre or post EQ and the level is variable (the SVT3-Pro has an additional unballanced DI output and ground can be lifted by pulling the Level potentiometer).  Preamp Out gives you the final processed signal, the SVT3 adds a power amp input jack.

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