Friday, November 23, 2007

Tubes! Boobs.

There are no boobs in this post. The title is from a lyric by the immortal Surf Punks. I was going to go into a digression on the formative effects of the Surf Punks on my musical tastes, but there is an entire wikipedia article on the subject. If you're looking for insightful commentary on early 80s California surfing culture, including the tense local-valley dynamic (My Beach. My Wave. Go Home!), property rights (Somebody Ripped My Stick) and sexual relations (Too Big for Her Top), you could do worse than to check them out. Not a lot worse, but somewhat.

Ok, moving on - the amp that I built out of tube organ parts is done, and it sounds wizard. Atfer I pulled the power amp section out, I kind of tried to figure out what the minimum connections I would need to make it work - input, volume, output and power. It had a hel of a lot more connections to the rest of the organ to accept input from all the stops, apply vibrato, and who knows what else. But, I got lucky and found the 4 wires I needed to make it work.

The power amp is a tried-and-true design from the early 60s - a pair of 6L6s in a "push-pull" configuration, which means each tube amplifies half the signal. It's efficient and clean, and is the basic circuit of venerable guitar amps like the Fender Bassman and Super Reverb, as well as plenty of hi-fi amps from the era. It has a preamp and phase splitter tube in front of the power tubes, and is good from anywhere from 25 to 40 watts, depending on configuration.

An amp built for guitar will have 2 or more gain stages in front of the power amp section to boost the weak signal from a guitar and put it through some tone controls before it hits the power amp section. Since this amp doesn't have that, you need to feed it a line-level signal, and do whatever processing you want before it hits the amp. Right now I have it running through my Roland Micro Cube, and out the headphone jack into the tube amp. I actually think this is a great combination. The Micro Cube has all kinds of digitally modeled amps, and by feeding that out into a real tube power stage and into vintage speakers, it really gives the amp modeling some life and lets it breathe. (I have a Behringer v-amp2 on the way to replace the micro cube).

So the cabinet is made from the original organ case, cut up and reassembled into a much smaller box. The speakers are the organ speakers, and are that early type that look incredibly fragile (paper cone, those weird rectangular magnets, thin stamped metal), but they are really rich sounding. I have an input jack, a volume knob, and a power switch (wired backwards). It sounds authentic. You can really hear the difference when you get it up loud enough to let the power tubes start breathing. I think I'm onto something with the digital front-end into the tube back-end. (And I think this is what Fender does with their CyberTwin series).

That last picture is of a smaller amp that I built from parts a few years ago. It's a single-EL84 practice amp that puts out 5 watts. That one is a lot of fun because it's so low-powered that you can make it distort nicely at volumes that won't split your head open. Interestingly, you can now buy that amp in 5 different guises at your local Guitar Center - (amp designs tend to be ripped off/copied across all the major manufacturers - there's only so many ways you can design a reasonable amplifier). When I built it, not so much - tubes have really enjoyed a comeback. The beautiful thing about them is the simplicity - look at the underside of a tube amp chassis and you really don't see that many parts - you can almost trace the schematic by hand if you want to.

Plus, the small one heats up and something shorts out inside. I have to set this small bottle of liqueur on top of it to clear the short and keep playing. You just don't get that kind of rock and roll character out of solid state.

1 comment:

Paul said...

nice clean looking set it sound great.