Bass guitar preamplifier in the Ampeg SVT tradition
These pages describe the design and construction of a 1U rack mount bass preamplifier. The design was adapted from the preamplifier section of the classic Ampeg SVT.
|This project is my own re-implementation of
the Ampeg SVP-CL, a 1U rack preamp that Ampeg
manufactured between 2004 and ~2009. I chose this as
the starting point for an SVT-like preamp, rather than
starting from the original SVT design for many reasons:
My version of the rack mount SVT preamp differs from
Ampeg's in the following ways:
Design and construction issues
There were several design and construction challenges:
Wiring for the switches, jacks, potentiometers, and tube
sockets is documented here..
This is the most challenging project I have attempted since I started scratch-building ten years ago. My objective was to create a functionally complete and sonically faithful adaptation of the classic Ampeg SVT preamp, and to publish the design and build process for the do-it-yourself constructor. It is by far the quietest musical instrument circuit I have built (>74 dB S/N), probably because I carefully followed proper grounding, RF suppression, and isolation practice for all the jacks, switches, and pots. Audio measurements confirm the preamp gain, frequency response, tone control function, and clipping characteristics predicted by simulation.
If you decide to build the project from my documentation
I believe that this preamplifier can be
constructed by intermediate to advanced builders.
However, it is quite a tight fit for the 1U box I
used. The mechanical locations and dimensions shown
in the full-size layout drawings have not been carefully
checked against the as-built unit. Builders should
physically check the mechanical fit and clearance for all
components before drilling and machining. I have
checked the circuit schematic / layout drawings pretty
well. However, it is still possible that a few
wiring connections may be missing, or incorrect on the
documentation. I (or preferably several other
builders) will have to actually build the project using
the documentation to verify that it is correct. It
is likely that you will want to or need to use some
different components in your build due to availability or
preference. You will have to determine if the
substitute will work electrically or mechanically, though
I may be able to help you with this.
I have provided email help to builders from
all over the world who have built my prior projects.
I hope my documentation will prove useful, and I will
attempt to correct any errors you find, or to add missing
information you suggest. You should have already
successfully built several vacuum tube projects before
attempting this one, and to understand and to follow safe
design and work practices for building and repairing mains
operated electrical equipment.
Other builders have completed this project!
As of March 2016, two other builders have successfully built this project from my documentation. Electromik (in French) made a nearly exact duplicate of my build that worked perfectly on the first power-up! Klaus in Germany finished his build at nearly the same time, and got it working on the second try. So I think it's safe to conclude that my documentation is correct. They both found European sources for parts (like enclosures and power transformers) that were uneconomical to ship from the US. Thanks to both of them for acid-testing the documentation, and congratulations on finishing your epic builds!
My construction survived its trip across the
Atlantic Ocean, and is now happily employed by the client,
who reports that it sounds wonderful, better than an SVT.
During my research for this project, I did
not find any well-documented DIY builds of a
feature-complete SVT or its preamp. There was this
thread on GroupDIY, where Okabass built what looks
to be a complete SVT-like preamp and amp. I found
another thread on a forum I can't seem to find at the
moment that showed a few pictures of an SVT preamp that
someone built in the 70's or 80's, also with no
So I present this project's documentation to
those wanting to make your own version of this 1970's
icon. This is not an inexpensive project to make,
and will take considerable effort to construct. If
you are not DIY building for the challenge and
satisfaction of improving on the original, then it will be
cheaper and easier to buy a used Ampeg SVP-CL.
I might attempt to design an updated version
of the 300 watt RMS SVT power amp. Kevin O'Connor
presents a design for a SVT-like preamp and amplifier in The
Ultimate Tone Volume 3: Generations of Tone,
which I purchased while doing the research for this
project. The preamp was a very simplified design,
but the power amp seemed well thought-out for the
time. I believe a modern 300W RMS tube power amp
would benefit from a switch mode power supply (SMPS) with
a microcontroller to monitor and control safe
operation. The dumb linear supply has 20 kg's of
iron and copper.transformer, and things melt and/or
explode when something goes wrong with a tube or other
component. However, designing a smart 1 kilowatt
SMPS is not something I am ready to jump into right
away. If you have some technical expertise to
contribute in this area, feel free to contact me.