Category: Amplifiers

5.1 Tube Home Theater – Subwoofer

home sub amp layout 2.0

Originally I planned on building an integrated amp and subwoofer, and I even built the enclosure. But a few things happened that made me rethink that plan… My living room layout and furniture changed, making me realize how large and impractical the size and location of the sub would be. The preamp I ended up with didn’t put out as much voltage swing as I originally expected. The subwoofer I intended on using fell off a 6ft shelf onto a concrete floor, bending the frame possibly beyond repair. And some testing indicated that the speaker/cabinet resonance was likely to cause...

’76 Fender Vibrolux Reverb


This is the only brand-name amp I own. Even still, this amp came to me in heavily-modified form, mostly blackfaced with a multi-tap Bassman OT and a number of circuit changes including a solid-state rectifier and absurdly high B+ on the preamp. It hummed a lot. One speaker had a warped coil and the power tubes were microphonic. The reverb driver 12AT7 had run so hot it was misshapen! Goodness, it needed some help.

6BM8-Powered 5F6A Bassman


When I started this project, I had never heard of The Little Wing… I wish I had, it would’ve saved me a TON of work on calculations. Ironically I chose a lot of the same values (39k NFB, 5.1k grid stoppers, etc) that Geezer did; we must have similar tastes in power amp behavior! After tinkering around with the EF-Bomb for long enough, I decided to build something else out of the Frontman 15 chassis and cabinet that had been serving as guinea-pig for so long.

6BM8 Hi-Fi Amp

Final amp

This came up because I decided to start a little hifi project – something to push my bedroom TV’s audio to something better than the built-in speakers. So I thought, why not try to add adjustable compression? Warmly-biased triodes can compress one side of a soundwave, so what about sticking two in series to compress both sides?