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:

The SVP-CL was a superset of the SVT preamp main channel, following the classic original's gain structure and tone shaping functions exactly.
It used the common 12AX7 and 12AU7 tubes, instead of the SVT's 12DW7's
It fit the entire preamp into a 1U rack chassis
It used normal toggle and rotary switches for various tone shaping controls, instead of the custom rocker switches found on the SVT.
It used a regulated DC filament supply
It incorporated a number of useful modern features not found in the original SVT

transformer balanced line output, switchable between pre- and post-eq
input pad
tuner output
two additional ultra-low tone settings and two more semi-parametric midrange frequency choices

A legible factory schematic was available.

My version of the rack mount SVT preamp differs from Ampeg's in the following ways:

My circuit is built boutique-style on 0.125" G10-FR4 (Garolite) using turret terminals; the Ampeg is built on a machine-made PCB.
I increased the high voltage power supply B+ by 20 volts, and inserted two more decoupling/filtering stages than the Ampeg.
Filaments are referenced to 77 VDC from a 1/4 voltage divider on the B+ supply to reduce heater to cathode voltage on the direct coupled stages
I added a 120V / 230V voltage selector as the client for whom I built this lives in Europe.
I omitted two features of the SVP-CL: footswitch muting and LED clip indicator. I left them out because these required transistors that I didn't have space for in my layout, and I did not think they contribute anything sonically to the circuit.

Design and construction issues

There were several design and construction challenges:

The turret board layout had to be developed from scratch, as the original SVT and SVP-CL circuits are built on PCB's. I describe how I did it here
The heart of the semi-parametric midrange control is a custom part, a multi-tapped toroidal inductor. The inductances are specified on the schematic, but:

The inductor is not readily available as an Ampeg service part, though they occasionally show up on ebay. You might be able to order one through a DIY friendly Ampeg authorized service technician.
One boutique transformer manufacturer (Mercury Magnetics) offers their version of this part for >$100 each
I hand-wound the inductor myself, using a winding recipe that I calculated by non-destructively reverse engineering an Ampeg service part that I purchased on ebay. I describe the process here.

Ampeg designed the high voltage B+ power supply and the regulated filament power supply around a custom power transformer. I adapted this design to use a readily available catalog toroidal power transformer. The details of the design and construction of the power supplies are here.

Details, details

Wiring for the switches, jacks, potentiometers, and tube sockets is documented here..
Schematic / Layout of the circuit except for the power supply is documented here.
Schematic / Layout of the power supply is here.
Mechanical layout is here.
This project required a well-finished front panel. I describe how I did that here.
LTSpice simulation of the circuit, and the schematic resulting from that process is here.
The bill of materials is documented here..

Results

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!

Final notes

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 documentation.

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.