August 25 2008

Projects Micro Champ Amp

A tiny rendition of a Fender Champ with 7-pin miniature tubes.

Overview

Revision 1, Fall 2001

The Micro Champ was my second amp build, after the Moonlight. Its initial goal was to build a 1W amp with a basic preamp and a power section using the 6AK6 miniature power pentode. I succesfully built an initial version with a blackface Fender Champ preamp andv a single-ended 6AK6. That’s where the name came from.

That first version was really just a Fender Champ with a small output tube. The one odd thing about it was the power supply rail was “backwards” from the normal configuration. The first voltage node supplied the preamp, then the second node supplied the plate of the 6AK6, and the third supplied the screen. I built it that way because the 6AK6 has a maximum plate voltage of 180V (and it’s not hardy enough that you can go way above it—I fried 3 tubes while prototyping it). So the preamp gets about 250V, while the output stage got 180V.

I soon replaced the 6AK6 with a 6AQ5, and found I liked that quite a bit more. Especially when triode-connected, the 6AQ5 sounds very sweet. I also pulled out the tone stack. The 6AQ5 is essentially a 6V6 in a 7-pin bottle with lower voltage ratings. At 180V B+ it produces about 2W of output power.

The initial build eventually got taken apart—I needed the output transformer for theLunchBox.

Revision 2, Spring 2003

The chassis sat empty for more than a year, until I started building the (as yet still in the initial stages, after 3 years) Micro Demon. When I built the turret board for that, I also built one to revive the Micro Champ (they use the same chassis size). For this one I’ve flipped the power supply to the traditional configuration, supplying more voltage to the 6AQ5 for a little more output power. It sounds a little dirtier and more aggressive at 250V than it did at 200. That may also be related to the lack of negative feedback too. It also has more clean headroom, which was one of my goals this time around.

The final thing is basically a 5E1 tweed champ, with slightly more gain (2nd stage bypassed), no negative feedback, and of course the 6AQ5 instead of 6V6.

When I rebuilt it, I also took the opportunity to build a spiffy turret board (seen in the featured image, above) out of blue G10, because blue sounds better ;).

Debugging Notes

I had some really nasty sounds that sounded like a combination of low-frequency motorboating and high-frequency parasitic oscillation when I first fired it up. After going over all my lead dress and not solving it, I checked the input jack. At this point I noticed that the noise went up and down in volume with the guitar volume control—not something you would expect with amp-induced noise! I took a look at the input jack—I had just reused the jack I used more than a year ago, with the same 1M resistor soldered onto it. Took it out, replaced the 1M resistor to ground and resoldered all the connections and voila! Noise gone.

Attenuator

Of course, I had to add an L-Pad attenuator to this amp too. This is built right into the amp, at the speaker output jack. It gives 12dB and 15dB reductions. Tone definitely suffers at that low volume, but it helps keep up good relations with the downstairs tenants.

Schematics & Sound Samples