Guitars sound best when blasting out distorted tone through suffering speakers. Unfortunately this begins to happen at aproximately the same time the neighbors begin to complain and hearing damage begins. This amp should let me pump out the raunchy cranked amp tone at low volume levels. It is loosely based on a partial design published on the STF Electronics site. It uses a single ended 6s4a output tube and basically a Champ preamp section with a little extra power supply filtering and preamp stage decoupling. I omitted the tone control because I always dime them anyway so why bother with those cheezy little picofarad capacitors :) Output power is probably about a watt or two.

The cabinet is made of some sort of really colorful pine. I tried fingerjointing for the first time and I figured I might as well try it without a fancy jig just to see what happens. The results are fine as long as you ignore the wood filler :) I used Minwax Natural stain and Bullseye spray Lacquer for the finish. I like to keep it as simple as possible becaue I hate finishing. Let's just say that I only use 100 grit sandpaper and a palm sander. Anything else is just too much trouble :)

Pegboard and hardware store handle - how retro-minimalist!

I had these really cool G10 turret boards, but they are a little compact for a circuit like this, so I had to use the smallest filter caps I could find. I had a salvaged power transformer that was a little hotter than the ratings would indicate (it is so hard to tell from no-load measurements what you will get for B+ voltages) so I used a 5y3 recifier instead of the diode recification in the original design. Close inspection of the photos will reveal the uf4007 diodes still on the circuit board. At least the 5y3 gives the 5v winding something to do besides just run the pilot lamp.

I really wanted to go all out to keep the noise level down so I used a choke in the power supply just because I had one. The simple circuit allowed me to use a full-on star ground where every ground in the whole mess goes to a single lug on the chassis. Even the input and output jacks are insulated from the chassis. The lead from the input jack to the first triode is shielded (Kaschnerized!) and goes directly to the grid with no padding resistors. Also I twisted all AC leads as tightly as possible. The net result is a dead quiet amp even with AC filaments. You have to put your ear right up to it to hear anything even at full volume. Nice.

The original design used a 12ax7 preamp tube, which offers tone ranging from distorted to crazy-insane-distorted. A 12au7 gives mild to raunchy blues distortion, and a 12at7 is somewhere in between. You can turn the volume all the way up and control the tone with the guitar volume if you want - hey, maybe I should go volume-less! No knobs - that would be minimalist! It isn't as tonally versatile as a Champ for instance, but it is damn fun. Come to think of it, a linear volume might be nice to get more low volume control (or do I have that backward).

Interior view with baffle removed.

There is some sort of overly flatulent distortion that happens with loud chords sometimes that I think is either the speaker or the output transformer. I am using a $13 Radio Shack fullrange driver with the whizzer cone cut off so it would not suprize me if it was the speaker. I ordered a Jensen P8R-8 so that should be much better. The output transformer is from an old TV I think and is rated at 5W but it is tiny so it would not suprize me if it was saturating. One of those new Hammond 125BSEs would be nice, or something with closer to a 6 or 8K primary impedance that would match the 6s4 better.

Update: I installed the Jensen P8R and boy-oh-boy is it nice. As you can see from the above photos it cleared the chassis by less than 1/8". Like they say, it's better to be lucky than to be good :) The so-called flatulent raunch is still there for cranked chords, but with the Jensen it works :) Once this baby breaks in it should be *real* nice.

Update: It turns out that bypassing the cathode resistor on the 6s4 was causing it to have way too much gain and probably run way into clipping causing the nasty distortion that I had attributed to the output transformer. I removed the cap and now it is much more behaved. It gets pretty clean at low volume and still breaks up nicely when you dig in. It might be a little too clean with single coil pickups so I want to try bypassing the cathode resistor on the second preamp gain stage to get a little nastier. I have definitely learned a lot in trying to get this thing to sound just right. It is amazing how many different sounds you can get from this incredibly simple circuit.