Miniblok Single Ended Triode
This project was my entry into tube amplifiers, and the first project I actually completed to any satisfaction in quite some time. As such, it has a fond place in my heart, even if in retrospect it isn't really that impressive. I spent countless hours pouring over various online sources learning how tube amps worked, and I kept coming back to this design. Eventually, I just knew I was going to have to build one.
The amp follows the MiniBlok design from Fred Nachbaur, although my version uses a beefed up power supply and provides two channels (stereo) rather than monoblock (mono) output. As a simple and inexpensive way to get started with tubes, including a lot of good introductory theory on Fred's website, this isn't a bad way to go. Another route I should mention for beginners, which undoubtedly gives a much higher quality finished product, is the TubeLab Simple SE kit. This is a printed circuit board with tremendous flexibility to run just about any tube and transformer you could come across.
The MiniBlok design has several unique features which makes for a simple and inexpensive build. First, the dual triode 13EM7 (originally designed for TV sets) provides a preamp and power stage in the same glass package, so that each channel only requires one tube. With a very modest power requirement (at least for tubes) the MiniBlok design uses a standard 120V to 12.6V transformer to provide the 12.6V heater voltage, then a second identical transformer and frequency doubler to provide the 200 V plate voltage. A second frequency doubler on the heater circuit also provides -35 V of fixed bias for the cathode. It looks like a crazy scheme, but it actually works quite well. I beefed up the power supply capacitors using some I found for cheap at AllElectronics and removed any trace of hum compared to the original supply. I also used higher current transformers to be able to power both tubes (rather than one in the original design).
For output coupling, I used XSE 15-8-5k transformers from Edcor Electronics, which are probably complete overkill on an amp which puts out 1 W/channel at full output power, but they are about the best inexpensive transformer option out there.
I didn't want to spend much money on a cabinet, and I wanted an easy material to drill through, so I decided to use a thin aluminum sheet (for grounding mostly) covered with a plexiglas plate to give it enough stiffness to support the transformers. I was looking for a nice opaque white (to match my Mac Mini) but could only find clear. The sides are just clear pine boards screwed in place. The whole thing was extremely easy to assemble just using hand tools, but has an interesting look which I happen to like.
The sound of the amp is really not bad considering how cheap most of the components are. It is plenty loud for my kids' room, although there is considerable distortion when the amp is turned up to full power. For casual listening, it really has a nice warm tone and does not at all sound bad.
Picture Gallery
From behind.
From underneath, showing my really terrible wiring job. I learned a lot (mostly what not to do) on this project, and frankly several things here are probably unsafe. I haven't had the energy (or the inclination for that matter) to go back and fix it. One of the biggest lessons learned is to lay out the components (at least virtually) before you start soldering things in place. I think I ended up with more wire than components.
A few other photos from directly above, and showing some details of the cheap plexiglass plate over the even cheaper sheet aluminum frame.
