|Although I have built this project and I am currently using this preamplifier in my current system, and it is definitely the best preamp I have ever owned, I make no claim that the drawings and schematics are accurate. I accept no responsibility for injury to you or your property that may be the result of you attempting to build this device. I did a lot of experimenting with values to get the sound that I was after. You may need to do the same, depending upon your tastes and setup. I have noted a few components in the Schematic that I know will cause you trouble if you try to change them. Before attempting to build this device, please read through the following precautions. They will save you many, headaches and money, and possibly your life.
1. Practice good safety when working around high voltages. Never stick both of your hands into a high voltage circuit at the same time. Use only one hand at a time to probe a live circuit. The output capacitors on this circuit can cause extreme pain and even death if they discharge from one hand to the other through your heart. I was bitten several times while making adjustments to this circuit and it was not a pleasant experience. Discharge the power supply electrolytic capacitors with a 10K 5 Watt resistor before you attempt to work on the circuit. You can wire the 10K resistor through a push button switch for convenience.
2. Solid state devices, especially LSIC’s (large scale integrated circuits) do not get along with tubes. The typical working voltage of a modern Pentium class processor is around 1.5 to 2 Volts: Compare that with the 300 Volts DC applied to the plate of the second stage triode in this circuit. If at any point your tube bias voltages get into your computer, things are going to pop and smoke. It is imperative that you not under-rate the working voltage on the audio output coupling capacitors, as these are the devices that are isolating the tube bias supplies from the rest of your system. If you don’t believe me, ask my dead SoundBlaster cards. The 450 Volt rating is to account for a failure in the linear voltage regulator which will cause the tube plate bias to jump to close to 400 Volts. Under-rating any other capacitors in this circuit is not a good idea either, but a failure will probably only result in a failure of the preamp and it’s components, and not take out your entire computer system and attached peripheral devices.
3. Breadboard the entire circuit and test it before you build it for real. Guitar preamps are all about flavor, and I have tweaked the parts in this circuit to suite my taste. This is perhaps the biggest payoff for the do-it-yourselfer.
4. Get an old system, (even a 486 will do as long as it has a working sound card) to test the preamp on (Packard Bell computers really are good for something). Make sure you are happy with the preamp’s sound and function before you install it into something that you really care about. If you make any changes to the circuit, even minor ones, test the preamp on an old system before installing back into a system you really care about.
5. After you’ve made any changes to the circuit, measure the ac and dc voltages on the outputs and the voltage from the chassis of your preamp to the chassis of your computer, before connecting to a sound card. Make sure there are no voltages that are going to wreak havoc with your system.