August 25 2008

Projects Micro Champ Amp

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

Table of Contents


    Revision 1a, 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, which is a 7-pin miniature power pentode. I succesfully built an initial version with a blackface Fender Champ preamp and 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.

    Sounded very good clean, and decent overdriven, but the 6AK6 doesn't really have the best overdriven sound.

    Revision 1b

    I eventually took apart that first version, as I needed the output transformer for the LunchBox.

    For the second revision, I switched to a 6AQ5 in the output stage. While still a 7-pin miniature tube, it's basically a 6V6 in a small package, so it's got that classic Fender Champ sound.

    As the 6AQ5 is a higher power tube than the 6AK6, the power supply in this version is in the more traditional configuration of supplying the output stage plates first, at higher voltage than the pre-amp. The output stage is getting around 250V, which gives it a lot more clean headroom. It probably delivers about 1.5W to 2W of clean headroom before breakup, and 2.5W to 3W of power overdriven.

    I also removed the tone stack, as it was just needless complication that made it harder to overdrive the power stage, and really didn't add anything sonically, leaving in just the volume pot and a bright switch for a little high-frequency bypass.

    The final thing is basically a 5E1 tweed champ, with slightly more gain, no negative feedback, and of course the 6AQ5 instead of the larger 6V6.

    Revision 2a and 2b, Spring 2003

    The chassis sat empty for more than a year, until I started on another project (one I never finished), and build a new, cleaner turret board for the Micro Champ.

    After the initial rebuild with the same circuit as R1B, I made a few tweaks to improve the tone, changing the pre-amp plate reistors, cathode bypass caps, and removing the "bright" switch.

    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.


    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 neighbors.

    Schematics & Sound Samples