Sunday, January 31, 2010

A simple vacuum tube amp

I've owned and serviced quite a number of pieces of tube audio gear over the years. I've even owned more than a few bits of tube gear in the past that are quite sought after these days. Many were were a mixed bag to own and feed. Especially when the output stage becomes unstable, and eats an expensive set of tubes. My goal was to create something simple and musical. Hopefully, it wouldn't cost an arm and a leg.

Anyone who has built just about anything from scratch will tell you that the cost of materials and labor far outweigh the cost of an assembled product from a store shelf. It certainly turned out to be the case here. A pretty decent power amp could be purchased just for the cost of just the resistors and capacitors in this particular amp. Holco precision resistors, and Sidereal poly capacitors did add up. Throw in teflon coated silver stranded wire, and silver plated ceramic tube sockets, and cost added up even more.

The start of this project began with a set of transformers from an old Sansui unit. They were in great physical condition, and well suited for a project like this one. The output stage takes cues from both the Sansui they originated from, but from some Fisher influence as well. Unlike many common cathode designs of the 60s, I chose a simple self bias approach used in a smaller Fisher 6L6 based amplifier designed many moons ago. No tweaky bias adjustments needed at all.

Looking to cut costs somewhere, I discovered that the Pilot company used a simple 12AX7 tube for each channel of a power amp delivered in the 1960s. Half the dual channel tube used for the input stage, and the second half for the driver/phase splitter. The credit belongs to these trailblazers, and not me. The result is a musical amp that won't cost your entire rent check to feed with tubes. The cathode bias approach definitely squeezes a long lifespan from the power output tubes. Common cathode proponents might claim tighter bass, and a few more watts of power, but I say the trade-off is well worth it.

This amplifier is currently on display at Austin Stereo in Austin Texas.


Anonymous said...

Nice post Nat, informative and helpful. Now tell the folks this amp is primarily suited to Jazz, vocals, acoustic and chamber music. Rock and rollers and Reggae types need not bother. ;-) Bill

Nat said...

Bill: as much as I would like to take credit, the credit is due our new contributor 'Pixelriffic', otherwise known as Joe. Joe is a long time audiophile and formerly one of the talented techs at Austin Stereo. Many years ago, long before I learned to love the smell of multicore solder under the gun, Joe restored my Mac 225, Fisher 500c, and more.