I’ve always wanted to build a one-knob guitar amp, so I did. It’s another Champ/Princeton clone without the tone control. 1 x 12AX7, 1 x 6L6 (not 6V6!), ‘canonical’ 5Y3 tube rectifier. The main lesson I learned was that a one-knob guitar amp is cooler in theory than it is in practice, and that adding minimal tone-shaping (in other words, a ‘High’ or ‘Bright’ rolloff) is worth the (minimal) effort.
On the other hand, it was interesting to try to balance the output for a range of instruments (guitars). I used a Telecaster, an SG, a Strat, and a Les Paul with custom pickups when I was deciding what values to ‘hard-code’ for bypass capacitors and whatnot (see C2 and C5 in the schematic below). The video was done with just a Tele because I was too lazy to include anything else.
For questions and comments, email email@example.com
Here is an action-packed video of the ‘Uno Knobbo’:
And here is the schematic:
Uno Knobbo schematic
Note: the schematic shows the feedback resistors connected between the output jacks and the cathode of the second preamp stage. I thought the result was muddy so I disconnected them for the video above.
Also, the schematic specifies a 6V6 as the power tube but I used a 6L6 instead, because I thought it was a little more articulate at high gain levels. As I recall, I used the 5k tap of the output transformer for the 6L6 (brown wire), instead of the 8k tap (red wire).
What I came out with this time was fiddly and awkward because I didn’t think the layout through as well as I could have. A better layout for this sort of 5-Watt clone would be the the previous project referenced above. If I do anything like this again and I use one of those Hammond Enclosures again, I’ll probably use that layout, with separate, smaller turret boards. Here is the layout for good measure:
Uno Knobbo layout: not my best effort
I tried the Uno Knobbo with several speakers and I like the Eminence Lil’ Texas the best, maybe because they rolled off the piercing high end and the two Jensen speaker I tried didn’t. All the speakers I tried were 12″ because I don’t have any 10’s.
By the way, if the enclosure looks weird, it’s because I tried doing my own powder coating on it, with limited success. That was fun though.
Also, I stuck a TH Audio badge to it because I had a few lying around – I’m not planning to sell these (yet) under my side business.
(Public service announcement: comment are disabled because I got tired of the endless spam. For questions and comments, email firstname.lastname@example.org.)
This is my first single-ended amp build. It puts out about 5 Watts, which is plenty loud, but not bonecrushing. It’s based on the Fender 5f2 “Princeton” circuit, which is quite similar to the 5f1 “Champ,” the main difference being that the 5f2 has a tone control. This is not a kit – the chassis, cage, and filter choke are from Hammond and I kind of winged it with the drilling. The chassis is aluminum and the cage is steel. The transformers are from Classictone and the tubes are JJ.
Now that I’ve done my own Princeton/Champ clone, I can see and hear why so many people like having at least one of these around.
If you see anything I’ve said here that might be inaccurate or wrong, please let me know so I can correct it.
Proof that this doodad actually works:
Original schematic… or at least, the one I see all over the place.
Update 8/30/2023: It still works, after ten years. I extracted it from a closet yesterday and hooked it up to my Dr. Z 1-12 cabinet, and it sounds fine.
Another update 8/20/2023: I’m really sad that Classictone went out of business because I used to get most of my transformers from them.
Below are the schematic and a possible layout for this build. The layout diagram is close to what I actually built but not exact, and it’s not to scale – it just shows where things can go for minimal wire-crossing and decent ground distribution.
Take the layout diagram with a grain of salt – the resistors that connect directly to the grid inputs of the tubes (R2, R5, and R10) really should be soldered directly to the socket pins, with as little exposed lead as possible. Also, the cathode resistor for the power tube (R11) probably should be rated > 3W.
Since I’ve never gotten my grubby hands on an original 5f2 or 5e2, everything I’ve learned has come from schematics, sound samples, video clips, other people’s blog posts, and discussion boards. But from what I do know, the main differences between this build and a ‘canonical’ 5f2/5e2 are:
1. Different configuration and location for the ‘tone’ control. I put ‘tone’ in quotes because in my version, it’s just a low-pass filter pretty much like the one in a Rat pedal. For this build, ‘tone’ is situated after the second triode just before the power stage, as a 10n capacitor to ground and a 250k pot.
2. Larger values for the B+ filter capacitors. The 5f2 used ~= 8uF whereas this build uses a 10uF after the rectifier, and 47uF for B+2 and B+3.
3. 6L6 power tube instead of a 6V6. I connected pin 3 of the octal socket to the 5k tap (brown lead) to make it more 6L6-friendly (closer to 4-5k impedance). Before that, I used the red lead of the OT for a 6V6, for about 8k impedance. Personally, I like the sound of the 6L6 a little bit better – it was more… smooth, maybe? But I realize that using a 6L6 for a power tube in a 5-Watt amp is severe overkill, since it draws a lot more heater current than a 6V6.
4. The 5f2 had a feedback resistor between the secondary side of the output transformer and the cathode of the second triode stage. This build doesn’t. Originally I had one but I thought it made single-coil pickups sound too harsh so I got rid of it.
5. This build has a (relatively) large bypass capacitor in parallel with the plate resistor of the first triode stage. It rolls off high frequencies and helps to alleviate the ‘icepick’ effect at high volumes.
6. This build has a 220k grid-stop resistor at the input to the second triode stage of the preamp. Like the cap mentioned in #5, it helps to roll off high frequencies.
7. This build uses an elevated heater supply (see schematic) with a DC offset of about 45 Volts for the filaments, to reduce AC hum.
8. This build has separate outputs for 16, 8, and 4 Ohms. The 5f2 usually had a single output.
I’m sure there are other differences I’ve forgotten to list. There is a little AC hum but it’s quickly overpowered as the volume knob goes up. Overall, the build is a little messy – I didn’t really know how it would go together when I started it, and I made a zillion little tweaks to it. I left the transformer leads a little bit long in case I reuse the transformers for other projects. Next time, I’ll use a chassis that’s higher than 2 inches, and a little wider for better component spacing – the choke just barely fits. Near as I can tell, I’m getting about 5 Watts at the output (assuming 50% loss for a single-eneded output transformer). That’s plenty of cowbell for just sitting around playing.
Like the original Champ, this build does not use a master volume control. That means the power stage is always running near full boil. As the Gain knob goes clockwise, the volume increases, but so does the clipping.
The Classictone output transformer I used (40-18031) has two different leads for 5k and 8k primary impedances. I’m using 5k here for a 6L6. 5k would also work for an EL84. 8k would be good for a 6V6.
Above, from left: 12AX7 preamp tube, 6L6 power tube, 5Y3 rectifier. All the tubes are JJ. I’ve been getting good results with JJ tubes lately, especially the JJ 6L6. The ‘sag’ from the 5Y3 rectifier is really obvious in the sound of the amp: to me, it’s like the signal is hitting a rubber wall when I lay into the strings. The knobs, from left, are volume and tone. There is no ‘master’ level control. The volume pot controls the strength of the signal from the plate of the first tridoe stage in the preamp to the grid of the second triode stage.
Below are photos of the interior. It’s pretty obvious that I had some issues with the layout.
For this build, I tried ‘floating’ the filament wiring (green twisted wires) instead of having them hug the chassis.