Sunday, March 16, 2008

Tube amp - design

Months away from a soldering iron gave me the itch to start working on something. In my "past life", I had a hobby of building audio amplifiers and speakers, and I never got a chance to work on a tube project back at home. Now, I figured, is a good time to start.

Hi-fi tube designs are necessarily expensive, the tubes involved can cost quite a bit, and the designs aren't exactly for the beginner. The next best thing, IMO, is a guitar amp, so that's what I'm going to try. Given that I don't play (oh the irony!), I enlisted the help of a friend, S., who will also be building the speaker cab in addition to providing guitar input :)

So anyway, I started off by hunting for existing guitar amp designs, and there are literally thousands to find. I settled on the VOX AC-15, since it's a well-known and proven design. It's also relatively low-powered, which is good, since S. plans to play in his apartment, not a stadium. Also, the output tubes are EL84s, which are not terribly expensive. Finally, it is a push-pull design, which means that the output transformer isn't too expensive, and won't saturate easily.

I did do some tinkering with the design, though. First, I took out the tremolo and reverb parts of the AC-15. These are a bit difficult for me, as a beginner, to work with. Besides, S. has a bunch of fancy effects pedals which would do the job. I also decided to do away with the rectifier tube of the classic AC-15, and use silicon diodes instead, with a series resistor to allow the supply to sag a little when heavily loaded. Caps across the diodes kill some of the switching noise. There's also the usual filter cap and bleeder resistor.

There were several variants of the AC-15 design floating around. I'm using two 12AX7 dual-triode tubes as pre-amps, since they're cheap and plentiful. I might also try an EF86 input stage later. There's two sets of channels, a "normal" and "brilliant" channel, the difference being the "brilliant" channel uses smaller coupling caps between stages, which decreases bass response. Also, the tone controls only affect the "brilliant" channel. A third 12AX7 acts as a phase splitter to drive the power tubes. The power tubes are EL84 beam power tetrodes. Given the relatively low power involved, there is no power tube bias control.

Another change from the AC-15 design is the inclusion of a switchable 12-dB attenuator. This will permit S. to drive the amp really hard without too many complaints from the neighbors :)

Some more hunting around lead me to TubesAndMore, a nice online store that lets you buy, as the name suggests, tubes and a lot more stuff that you can use to build hi-fi and guitar amps.

So anyway, that's what I'm going to try. I'll post here when I get some results. Anyone with tube experience, feel free to give this beginner a hand :)