Steve's DIY Guitar Amp Pages

Click on the thumbnail to open the Adobe Acrobat .pdf file in your browser if you have the Acrobat Reader installed. The file will open in a new window and will take a while to download if using a modem. Right click to save it to your hard drive. Click on the icon below to get Adobe Acrobat Reader if you do not have it. The first icon is a .pdf file that contains the full color cover. The second file is a black & white .zip file that contains .pdf files of the article.

* Reprinted with permission, from Glass Audio, Issue 2, 1992, p.p. 1-13. © 1992 by Audio Amateur Corporation. P.O. Box 876, Petersborough, NH 03458, USA. All rights reserved. Glass Audio is no longer published. It has been combined with Speaker Builder and The Audio Amateur into AudioXpress Magazine, please see the AudioXpress web site for details.

Background: This project was originally published in Glass Audio in 1992 (Vol. 4, No. 2). The author, Kirk Elliot, converted a Fender® "Black Face" Champ into a high-quality studio amplifier. He added a second gain stage (the Champ only has two), a fully regulated DC filament supply, replaced the standard Fender® tone stack with a Baxandall tone circuit, replaced the 6V6GT tube with a 6L6GC tube and the output transformer from the Champ 12 amplifier, and replaced the stock speaker with a Celestion G8S-50 8" speaker. The article also mentions the trials and tribulations the author went through in performing this mod.
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Building: I built this amp in April of 2000 from scratch, opting not to deface and devalue a vintage amplifier. (I had purchased a Silver Face Champ for this project, but couldn't bring myself to mod it.) I also made several changes: I used a Hammond 125E universal output transformer which is physically larger than the Champ 12 OT as specified in the article. I put in a tube rectifier to cut down on the excessive B+ voltage I was getting, I left out the regulated DC filament supply. I've found that a lot of the noise can be eliminated by tightly twisting the filament wires and referencing the filament winding center tap to the power tube cathode. My amp is very quiet. Additionally, I built the amp in a head configuration rather than a combo. I changed several of the component values to make the amp work better with my gear.
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Sounds: How does it sound? Pretty good I think. It definitely has a Marshall® vibe to it. It is higher gain than a stock Champ, but not overly saturated like a Boogie® or Soldano®. For those kinds of sounds, I was able to drive the front end harder using a Boss® Blues Driver™ Pedal set as a volume boost: gain all the way down, volume maxed out. It also sounds different due to the fact that it is single ended. Tubes make a big difference in the circuit. The two best sounding tubes were the RCA Black Plate 6L6GC, and a GE 6EZ5 (similar to a 6V6GT). EL-34s sounded okay, and most surprising of all was that a Sovtek® 5881, Tung-Sol 5881, Mesa® branded Chinese 6L6GC, Sylvania STR-387, and GE 6L6GCs all sounded pretty much the same.

Overall, this amp has only moderately more gain than a stock BF Champ. This amp definitely needs to be cranked up to get it to sing. In other words, it has to be turned up very loud, like many good sounding tube amps. Don't let the 12 watt rating fool you, it is deafening in a small room.
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Disclaimer: As a side note: if you want to build this amp, please scratch build it as I did. Altering a vintage Fender® Champ will devalue its worth a great deal and make it less desirable should you ever decide to sell it.
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Here are some pictures of the amp:

	This picture shows the front of the amp. Controls from left to right are: pilot light, on/off, standby, master volume, volume 2, treble, bass, volume 1, input jack. Notice the large power transformer and the Hammond 125E universal output transformer.
	This picture shows the amp from the back. The tubes from left to right are: 12AX7, 12AX7, Sylvania STR-387, 5U4GB. The STR-387 is a rugged version of the 6L6GC meant to hang upside down from an amp chassis. Notice the extra hole next to the output jack, this was originally for the output jack but I screwed up :)
	This photo is of the wiring. The amp is very cleanly laid out, and consequently there is very little noise or hum.
	This is a drawing that I did for the layout. I originally laid it out on an 8.5" x 11" sheet from a legal pad. I find that this works very well for me. The drawing was scanned at 100% so it will be big, and will not fit in your browser window. Please note: there is a mistake on the drawing. The resistors connected to the outside lugs of the bass control are marked 22k on the layout, but should be 220k resistors. Sorry for any inconvenience.
	This is eyelet board layout that I used to drill the circuit board. It is 11" long, and is scanned at 100%. You will have to use an 8.5" x 14" piece of paper to get the board to the correct dimensions. To use it, load the large drawing, right click on it and save it. Open it up in your viewing software and play with the print settings until the board is the dimensions listed on the drawing.
	This is the pre amp and power amp schematic. The pre amp has three stages, and is high gain. Note the Baxandall tone stack, and the .02uF caps from the B+ to ground to decouple the pre amp stages. The power amp is similar to a Fender® Champ, but is about 12 watts.
	Here is the schematic for the power supply. If you look carefully, there is a lot of filtering. This was a decrease from the amount in the original article.