Tim finishes is AD200 project with component selection and installation, then fires it up to hear his work
Selection of the parts to be used in a design with a given purpose is an important task. Almost every part in an amplifier makes a difference in the tone—starting with the quality of the AC coming out of the wall, to the power cord, all the way through to the speaker and into the room where the natural acoustics can make or break all of your hard work and tone. Some are pronounced differences, some a bit more subtle. If you add a dozen subtle nuances together, you get a pretty big nuance.
So, how does one wade through the endless amounts of caps, resistors, transformers and other “magical components” available on the market these days? Well, if you’re lucky enough to have the time and financial windfall that we all dream of having, then you can spend your life experimenting, buying and selling great gear without a care in the world. Hopefully, you’re able to develop a relationship with an artist or shop that you can trust and use them as a sounding board. Ultimately, you need to learn to trust your ears. Some of the best sounding amps that I’ve heard have been made with the cheapest, crummiest components. If it sounds right to you, than it is right!
A word of caution: experimenting can be very fun and rewarding, but it’s easy to get too caught up in the bells and whistles. Don’t forget about the music. I found that when testing some different capacitors in one of my favorite amps that one of the caps sounded much “better” then the other. When I put the "better" sounding cap in and played, I found myself analyzing the sound in every way, from the bass response to the singing highs and, of course, how it felt. When I put the “inferior” cap in, I would start to play, get lost, look at the clock and realize that a half hour had blown by and I didn’t even listen to the cap because I was lost in the music! The “inferior” cap has lived in my amp ever since.
Figure 1: F&T electrolytics power supply caps
For the coupling caps, I decided on Sprague Orange Drops. I use type 716 as they have a higher quality lead and are a bit more accurate they their 715 brethren. The Orange Drops, while perhaps not as sweet sounding as other choices, have a nice tight bass response with a slightly pinched top end that seems to help with harmonic content and punch. A client described their feel as “walking on packed sand instead of dry loose sand”—perfect for this bass amp.
Figure 2: Components installed
The pots for this project were a mix of Clarostat Mil Spec. and CTS (CTS because I didn’t have all the values in the Clarostat). I use different types of wire throughout the amps that I build depending on the wire’s purpose. As signal wire, I use certain types of hook-up wire made for the audiophile market. For the B+ and power supply, along with the grounds on the star, I used a high-quality copper multi-strand.
Once all of the parts are selected, they’re laid out and mounted to thick glass component boards (fig 2) with very sturdy standoffs for ease of mounting. Silicone is applied between caps to create a cushion and help to dampen any unwanted smearing of the signal or microphonics due to rogue vibrations. Paul from Mercury Magnetics sent me a set of beefy transformers that looked quite handsome dressed in their orange end bells (fig 3).
With the boards made and everything in its place and wired, it was time to fire this creature up and check the voltages at key locations. Using a variac, voltages are brought up to full power slowly. This is done for a couple of reasons. The first reason is so the electrolytic capacitors in the power supply can form their dielectrics. The second, and perhaps more important, reason is so I can keep an eye on the voltage ratios to make certain that everything is hooked correctly before I install the tubes and risk their lives (and my wallet).
One thing that I appreciate about this design is that Orange took the time (and expense) to design a power transformer and power supply that would deliver 600+ volts to the plates of the power tubes with roughly half of that on the screens. This allows you to get the benefits of high plate voltage (headroom and power) while maintaining a safe operating environment for the power tubes. Now that we’re certain everything is safe and sound, the power tubes (Svetlana “winged c” KT88s) and preamp tubes (mix of NOS RCA and Sylvania) are installed and biased.
Though I’ve been building and modding amps daily for a few decades now, every so often I’m surprised by an end result. With the first strum of the strings, I was floored by the increased headroom, clarity and bell like tones coming out of this powerful amplifier. The change and added versatility in preamp/overdrive section was a pleasant surprise as well. While originally designed as a bass amp, it is fantastic for guitar and pedal steel as well. In retrospect, I’m very glad that I went through the trouble and expense to rebuild this amplifier. It has become my “go to” for almost every application. Long live headroom and power!
Tim Schroeder is the owner, master luthier and chief designer of Schroeder Guitar and Amplifier Repair in Chicago Illinois. There he oversees the daily repair operations of the shop as well as designs the amplifiers and effects that they manufacture in house.