The PA1 DIY Tube Preamp: Efficiently Built With Salvaged Components

There are plenty of resources about building tube preamps on the web and in print, so I thought I would share something a little different. This instructable covers the construction of an open source tube preamp of my design and not only is this a unique design, but there is special emphasis on sourcing and salvaging old electronics for the largest components and saving energy during construction. This tutorial covers basic vacuum tube electronics theory and contains an analysis of the schematic as well as a high resolution virtual build that explains the entire preamp circuit layout. This preamp has some very unique features, first the plate voltage for the tubes is supplied by an SMPS to move most power supply noise out of human hearing range. This also helps to improve the efficiency of the preamp. Second it has a small power amplifier output section that is very similar to the buss amplifiers found in old mixing consoles and recording devices such as record cutters or reel to reel machines. Third this preamp has no select-able filters or tone controls, this allows maximum signal to pass with minimum noise. Instead the tone is set by the selection of the coupling capacitors and other circuit design choices. I hope this tutorial is helpful and gives insights to beginners and experienced makers alike.

1 12 volt 2 amp DC power supply

1 12ax7 tube

1 12at7 tube

1 reverb transformer

1 Taylor Electronics 1364 smps (200 volt model)

1 potentiometer 1meg audio taper

1 fuse holder

1 2 amp fuse

1 power switch

1/2 watt resistor assortment

2 tube sockets 9 pin

2 terminal strips

3 1/4 jacks

2 5 watt 100 ohm resistors

2 22uf 450 volt electrolytic capacitors

2 22uf 25 volt electrolytic capacitors

1 .02uf 600 volt film capacitor

1 .047uf 600 volt film capacitor

Assortment of wire

Assorted small nuts and bolts

Drill with assorted bits

Screwdriver assortment

Pliers or ratchet and socket set

Step drill and or metal files

Soldering iron and solder

This demo is of a Republic tricone resonator guitar recorded with the PA1 DIY tube preamp and an old mxl2001 condenser microphone.

The voice at the beginning of the video is recorded with the same setup.

I use this preamp for all of the voice-over work on all of my videos.

You may have something collecting dust that is perfect, if not I recommend using old rack mount equipment.
Ideally it will be audio equipment so you can reuse pots and jacks, but you can use old computer equipment as well. A metal case is best as it will help with safety and the sound of the preamp by rejecting external EM noise. If you do not have anything on hand then checkout this video about sourcing used equipment for reuse and repurposing. It also contains some ideas about making money by repurposing and reusing.

All manufacturers are different, but here is a video that demonstrates how to dismantle a piece of rack mount equipment.

Plan the layout of the build in a way that reduces the amount of power you have to consume drilling.

If there are holes in the case that will fit the potentiometer and jacks use them.
If you can use pre drilled holes for pilot holes for the tube sockets then you can save even more power. You will most likely need to use a step drill and or metal files to enlarge the holes enough to fit the tube sockets. You will also need to drill the holes for the nuts and bolts that hold the tube sockets in place. Try to reuse the fuse and switch of the old rack mount unit, this saves parts, money and power. The only other holes you need to frill are for the terminal strips, SMPS board and transformer. For the SMPS board cut a piece of plastic to place between the board and chassis to act as an insulator. I used an old plastic soda pop bottle.

Planning is crucial to ease the assembly process.

You should sketch out where everything should go in this phase.

If you are experienced with CAD software a virtual construction could help the project go more smoothly.

Check the fit for all parts and ensure there is enough room not only for the parts but for your hands and tools during assembly.

The assembly layout is found in the layout and virtual build video.

The above video SMPS is all about the Taylor SMPS used in this project.

You will have determined the location in step 3.

Mount all of the large components such as transformer, jacks potentiometer, SMPS board and terminal strips.

Run the component leads through the eyelets of the terminal strips and bend them up to hold the components in place.

We will be using 12 volt DC heating so twisting is not absolutely necessary, but still recommended.

Cut the wires one at a time that go from the tube sockets to the terminal strips.
Trim the insulation from one end, feed it through the eyelet of the terminal strip and twist it with the component lead to hold it in place on the correct terminal. Rout the wire to the correct pin of the tube socket and cut it to length.

Run the ground wires from the terminal strip to the ground points the same way you ran the wires from the terminal strip to the tube sockets.

Run the audio signal wire from the terminal strips and tubes to the proper locations for jacks and the volume potentiometer.

Double check each lead goes from the proper point of the terminal strip to the correct point on the jack,
potentiometer or tube socket.
Solder the connections at the terminal strips. If you are not familiar with soldering check out this video about selecting solder and flux.

Finish wiring and adding components to jacks such as the grid leak resistor on the input jack.

Triple check all connections are in the right places.

The virtual build video lists a few checks to perform before installing tubes or powering on the preamp.

You can watch the build video 1 to learn more about the components used in this device.

Build video 2 takes you through the schematic, explains how the circuit works, some possible modifications, plus the signal and voltage path.