This is my first attempt at a DIY Tube-based Project. I wanted something that was inexpensive and fairly
easy to complete, as well as something that sounded good. This preamp/linestage is based on an article in AudioXpress, with a few
modifications.
I built this project as a two chassis affair in order to minimize any possible noise/hum from the power
supply. The boxes are Bud Aluminum Boxes, available at digikey.
The high-voltage supply is ~250VDC. It starts at the 25.2VAC radioshack transformer on the top. The AC from
the transformer is rectified bya full-wave bridge rectifier (the large one in the chassis) configured as a voltage doubler.
The doubling-capacitors are 2x 1000uF/200VDC electrolytic capacitors. The doubler is connected to a choke for filtering, followed
by another 470uF/300VDC capacitor. The heaters are wired in series, and are powered by a 24VDC supply from one of Tangent's TREAD
LM317 regulator boards. Heatsinking for the tread is provided by the chassis body.
The entire power supply is fed by a 24VAC 830ma wall-wart. The two chassis are connected together by a
custom umbilical made from 4-pin CB Radio Microphone connectors. The wire is wrapped in techflex to provide a better appearance.
The second chassis houses the actual linestage. There are two inputs, switching is done via the toggle switch
on the front of the chassis. The volume control is a 50k Alps "Blue Velvet." I used a 4.7uF metallized polypropylene + 0.47uF
metallized polyester to bypass the high-voltage supply, and a 680uF/16V Panasonic FM cap + 0.1uF cap to bypass the heater supply.
Each channel has a dedicated 5687 tube, with the triodes wired in parallel (parallel single-ended?). The remainder of the parts
are either 0.6 watt Vishay Draloric, or 1 watt Carbon Film resistors. The output coupling caps are 0.47uF Dayton Film and Foil
caps.
I think this linestage turned out pretty well for my first DIY Tube Project. The only issue I ran into was
the B+ supply being too high (250VDC versus the 215VDC that was specified in the article). This required me to replace the
470uF/250VDC post-choke smoothing cap that was originally installed. According to the tube gurus on
diyaudio.com, this is fairly normal in tube gear and shouldn't cause any adverse affects.
The preamp sounds very transparent as far as my experience with tube-gear goes. Definitely a good bang for the buck project. My
only regret is not painting the chassis -- I thought the aluminum would look good, but unfortunately it's built up quite a few
scratches.
