BACKGROUND (or why we did what we did)
The Sound of Tubes - smoothness and detail, rich midtones, crystal clear highs.....soft clipping.....and astronomic prices! If you spend many hundreds (or thousands) of dollars (or Euros, Yens.....whatever is in your wallet) on a nice tube amplifier, you pretty much limit your potential for experimenting. You really do not want to open your new, shiny amplifier and start tweaking bias, changing negative feedback and risk blowing up tubes or burning transformers. Or worse, getting electrocuted! Once you spend that kind of money, it's unlikely that you will change the amplifier just to hear a difference between ultralinear and triode mode, or test pentode clipping. Most of us will be stuck with the amplifier for a while.
If you want to experiment and see what's all the hype about, you can try building your own amplifier from scratch. Do you have time to that? How about tools, test equipment, materials? May take a while to collect it all, build the amplifier and, finally, listen to it. It's all about the music, isn't it? We, too, need to be reminded of that from time to time.
Well, we also like to experiment. It is a little easier for us - after all, we have been in the electronics business for ages. We have benches, tools, test equipment, parts and a lot of experience. We are familiar with breadboarding, designing printed circuit boards, we know how to deal with high (read: lethal) voltages and protect ourselves (and our equipment) from being burned or zapped. In our work we design and build industrial, telecom and even military grade power supplies. We design equipment for high volume production and one-of-a-kind instruments.
We thought that it would be very interesting to use an old technology, apply modern design principles and come up with an inexpensive unit suited for experimenters who may not have all the resources that we have. The technology happens to be electron (a.k.a. vacuum or thermionic) tubes (or valves, as our friends across the pond like to say), and the unit is an audio amplifier.
First amplifier in what we anticipate is going to be a series of designs is a Single Ended (SE) amplifier based on the audio tube 16A8 (PCL82). There are "things" with this topology that are worth exploring - nobody can actually accuse it of being Hi-Fi in the typical sense and yet some of the most expensive amplifiers on the market are Single Ended type. The typical SE amplifier has a high content of second harmonic distortion, often 6% or even more. Damping factor is very low, we are talking low single digits or, sometimes, below 1. Transformers are big and heavy, and often lack deep bass. Maximum power is nothing to brag about, few Watts is what you typically get. Forget about using your speakers with complex crossovers, wide range speakers are what makes this kind of amplifier happy. And yet, many people are very happy with SE amplifier's sound signature - warm sound, detailed and rich midtones, music that comes to life and brutal exposure of poor recording techniques (close miked soloists in an orchestra come to mind). Definitely something very, very intriguing.
The PCL82 tube was designed for use in an audio amplifier applications, has excellent sound and is fairly inexpensive. It consists of two parts - input triode and output (power) pentode. That makes it little more difficult to design a good component layout, but it is less expensive than a separate triode and pentode and experimenting is simpler. PCB layout is not a problem for us, so we appreciate the fact that an experimenter's life is going to be easier and we get closer to our goal of offering inexpensive product. We opted for a "classic" design, probably one most commonly used, so there is a lot of information and experience, as you can easily see by visiting any of the audio discussion forums.
Now, when we say "inexpensive", we do not mean "cheap". No, you will find brand new quality components used throughout the unit. Professional, double-sided (two layer) PCB is used and base material is FR-4. The PCL82 tubes are Westinghouse brand and made in England. All assembled units are thoroughly tested and burned in for several hours.
We control our costs and we pass the savings to our customers. We use the same parts in some of our other projects (for industrial customers), so we have to buy good quality parts. But we buy a lot and we get decent volume discounts. Assembly errors take a long time to uncover and correct so we use printed circuit boards with top side silkscreen to tell us where each part belongs. We do not spend a lot of time on assembly, we prefer to spend it on testing. You will read that some people dislike PC boards in tube amplifiers, they will argue benefits of point-to-point wiring. They may be right, but it does not make sense to us, for several reasons:
1. Difference in sound, if there is any, will be minimal. Most people cannot hear it. Besides, this amplifier has no ambition of being high end. Just something that gives a lot of pleasure in a workshop and in a listening room.
2. Assembly with point-to-point wiring takes much longer time and costs much more.
3. PCB design is more robust (reliable), lends itself well to experimenting and is safer.
Below is the list of our design goals and we think we did a pretty good job in meeting them. Please read the list and send us your comments, your feedback is extremely valuable to us. It will help us tune up this product and release new meaningful designs in the future. Contact us at email@example.com
- Single ended design based on PCL82 / 16A8;
- 2.5W output power (pentode mode at clipping) - perfect for computer amplifier (Basic unit);
- Option to add tubes in parallel for 4W maximum power (Full-Pack configuration);
- Get as close as possible to the "dual mono" design for maximum channel separation;
- Two inputs - no selector switch required;
- Speaker and headphones outputs (switch selectable);
- Jumper selectable Pentode, Triode or Ultralinear mode;
- Jumper selectable global feedback or open loop operation;
- Easily replaceable output transformers;
- Modular power supply;
- ECL82 / 6BM8 can also be used (with filament supply jumpers reconfigured);
- Preassembled and tested printed circuit board available - just add a chassis and use it anywhere;
- Low cost (sub $200);
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