Single Ended Class A Tube Amp
This amp was largely motivated and built following the KT88 design shown at diyaudioprojects.com based on a design by Mikael Abdellah.
I used a 750V center tapped power transformer salvaged off Ebay and a 5H choke from an old HP instrument (also bought off eBay) for the power supply. I bought decent Solen capacitors for the input to the CLC filter, and limited the capacitance here to 15 uF due to worries about sparking the 5U4GB rectifier. A pair of inexpensive 1000 uF, 250 V electrolitic capacitors arranged in series (each bypassed with 220k bleeder resistors) gave 500 uF at 500V for the power supply output. Tests with and without additional filtering for the preamp stage showed that with this much capacitance, hum was already negligible. I probably should have added some additional snubbers, but in my quick bench tests I really couldn't tell any difference.
I wanted something that was flexible enough to take any 6L6/KT66/KT88 family tube, so I made the power stage cathode bias adjustable by adding a 200 ohm 2 W pot in series with a 470 ohm 10 W power resistor. Test points across 1 ohm resistors also in series allow the bias current to be directly measured with a DVM as the pots are adjusted.
The output transformers are GXSE 15-6-5k from Edcor Electronics. In the end, I am using Sovtek 6L6WXT+ tubes biased at 60 mA which dissapate about 20 W per tube at idle in class A. Due to the thick 1/4" plate I used for construction, the power tube sockets couldn't be undermounted, and KT88 tubes looked silly sitting up on top of the sockets. The price was a bit of a factor as well. I did include the anode/UL switch as shown in Alex Gendrano's design, although I would have to say the UL sounds much better driving my ancient Boston Acoustics A60 speakers.
I haven't had a chance to put together a test stand to measure power output or distortion, but I expect this thing barely puts out 5 W total. Even into relatively inefficient 90 dB speakers, however, this thing is plenty loud or casual listening. The blue glow from the 6L6 tubes provides a nice ambiance as well.
Picture Gallery
From behind, showing the almost professional-looking back panel.
From underneath, showing my decidedly unprofessional point-to-point wiring job. I had grand plans to use turret boards to make this look really nice, but then my cathode bias pots ended up right where the turret board was going to go. In the end it just seemed easier to just solder everything in place. Thankfully, I was careful enough with the layout that everything worked perfectly on the first try and I didn't need to resolder anything.
One last photo from directly above. I planned to build a cover for the output transformers using an aluminum plate for the top and MDF or birch for the sides, but I seem to have lost enthusiasm for doing this now.
