Paleo-Electronics

If you aren’t familiar with the real McTube, it’s a one tube foot pedal type pre-amp.  It was designed by a guy named Fred Nachbaur and is by all accounts a very cool device.

It uses a 12AX7 as a two stage pre-amp in much the same way that many other guitar amps do. The ouput of the first stage drives the output of the second stage with pot in between.

You can find the original article here and I recommend printing it or better yet storing a PDF locally.
As designed, it should be fairly cheap to build. That was a large part of the intention. Transformers are usually the most expensive part of any amp (outside of the cabinet) and this idea uses two standard ‘wall wart’ adapters (7-10 bucks) to power the heaters in a pretty clever two stage deal.

(Click on image for a better look) As you can see, the first transformer steps down from 120 VAC to 12 VAC. The heater power is tapped off and a second transformer steps back up from 12 to 120. A couple of bridges and some filtering and you get clean power. This ends up with about 140V or so at the tube plates. A pretty good value when you consider that the old GA-5 Only ran 165 to the plates of it’s 12AX7. Hotter is not more distorted, btw, in fact, the reverse may be true under some conditions.

So I went for the power section first. Cramming some components into a tag board and then dry testing. I had just about the voltages expected without load so I started soldering it all together. Unfortunately, I left the power on while I soldered the damn connection. Luckily for me, all I did was blow both of the cheap transformers. DOH! That’s a major fuckup though. I probably could have been hurt.

Forget the price. I’m gonna buy a hammond, maybe a 269BX (150 – 0 – 150) or EX (190 – 0 – 190) so that I can play around a little with higher plate values. Throw in a solid potentiometer and I could probably learn a lot about plate voltages and sound.

More as soon as I get the part.