Info for "A Mighty Midget" from AudioXpress 05/08
The "Mighty Midget" is a simple single-ended pentode amp that uses inexpensive compactron TV tubes. It puts out about 3 watts per channel, and uses only one (two-section) tube per channel.
This project was published in audioXpress, May, 2008. Please support what's become about the only DIY audio magazine still published - buy a copy!
In summary, the amp uses 6T10 tubes, which contain dual-control pentode and power beam tube sections. The dual-control pentode is used as a regular pentode amp (with a trick of biasing up G3 to get linear characteristics). The beam tube section is used in pentode mode, with about 10% plate-to-grid feedback to get decent distortion and damping factor. No global feedback is used.
With a B+ voltage of about 300V, the amp puts out about 3.5W per channel, while dissipating under 10W on the plates of the output tubes. THD at 1W into 8 ohms is about 0.7%. You can see the FFT below...
The amp has a modest damping factor, with an output impedance of 3.5 ohms or so. It turns out that this sounds really good into my inexpensive Fostex FE164-based speakers, which tend to be a little brassy sounding on a low-output-Z amp.
I used small 7k SE OPT's that I found in Akihabara (Tokyo), but an Edcor XSE-10-8-8k should work well, as would a small Hammond SE OPT.
Anyway...
Since many of the photos and details are hard to read in the magazine, posted on this page are all the photos, schematic, and mechanical files for you to build this amp. Enjoy!
The schematic (144kB PDF file)
Mechanical layout:
PDF of the mechanics (128kB PDF file)
ZIP archive with AutoCAD DWG and R12 DXF files (144kB)
Photos
Click on the small photos for a full-size photo in grisly detail.
