I play guitar, and I have wanted a tube amplifier for quite some time. However tube amps are expensive and complete amp DIY builds can be tricky if it’s your first time. So I decided to do the next best thing by building a standalone preamp. This takes advantage of the fact that I already have several high-wattage solid state power amps, if you don’t mind the lack of an all-tube signal path.

The chassis is a standard 1U rackmount chassis that used to house some old broken television equipment. I dug it out of someones garage and figured it would be a great starting point for this project. The power transformer is and EDCOR XPWR024-120 supplying 350V @ 60mA and 6.3V @ 2A. This is dropped down to the 250V DC that the tubes operate on by a two-diode center-tap rectifier. The design is based on the preamp section of the AX84 Hi-Octane amplifier, with some slight modifications I will note later.

preamp_finished

The control placements are rather non-standard, partially because this way was simpler to wire. I could’ve flipped the chassis and used the other side for a more usual layout, but I wanted to take advantage of the fact that there were numerous holes already drilled in one side from the previous components. Going from left to right the controls are: Power Switch, Mid, Bass, Treble, Master Volume, Gain 2, Bright Switch, Gain 1. In the end this preamp isn’t excellent for actual rackmount use since it takes up so much vertical space, but I didn’t want to hide away the tubes and lose that aesthetic.

The schematic for my design is in the group below. It is very similar to the Hi-Octane preamp, with some changes that are improvements in my opinion. Changes include power supply filter cap draining resistors (for safety) and larger filter caps, the insertion of a bright switch, a slightly differently biased second gain stage, a tone cap across the input jack, and a slight change to the tone stack. You will also notice several LED’s powered by the filament winding. One of these is the power light, the other two are placed under the tubes. You can see the tubes with the under-lighting pictured below, as well as a shot of the preamp circuitry as a whole.

With the original Hi-Octane design, I found the amount of high-frequency grit unpleasant. The 470p cap across the 1M resistor on the input jack alleviates this and improves the sound greatly. The 1.5K cathode resistor on the second gain stage decreases gain slightly but results in a clearer, more well defined sound when driven. The 1n cap connected to the bright switch exists in the Hi-Octane schematic, but it is not switched. I’ve found that I prefer to have the bright switch on when playing with a clean sound, and turn it off when playing with high distortion to cut out residual gritty treble sounds. The 1K cathode resistor on the third gain stage and 47K resistor in the tone stack were merely changes of convenience, I didn’t have the components the schematic requested on hand but these are adequate substitutes.

I used a hand drill with a 3/8 inch and 1/8 inch titanium drill bit to cut the necessary holes in the chassis. When cutting through metal it is important to keep the drill bit lubricated, so I would spray WD-40 on the tip while drilling. I cut the square holes for the tube sockets with a Dremel tool and heavy cutting wheel. The terminal strips are mounted to the top panel of the chassis by small nuts and bolts. I came up with the layout for the components on the terminals after a bit of brainstorming, and drew it on some graph paper. I went for a layout that would provide the most convenient and least cluttered wiring.

The project on a whole was not as difficult as I initially anticipated. I didn’t keep exact track, but I would estimate that it took less than 20 hours from start to finish. I am incredibly pleased with the way it turned out. It sounds absolutely fantastic, like a high gain monster straight out of the 1980’s. Though words can only describe so much, so watch the video below for a better idea of how this preamp turned out.

I took plenty photos while putting this together, but they are too many to display on this page. If you would like to see my photos of this preamp build from start to finish in all of their high resolution glory, you may download a zip here.