Just how cheaply could I build a useable and great sounding tube guitar amp? A good place to start would be the classic '50s Champ Amp circuit. It's simple, which is good for cheap, since the component count is low; and depending upon which speaker you run through, it can sound astoundingly good.
So, the goal for this project will be to build an amp, which hopefully works, and has a tone that is somewhat close to the legendary Champ sound, but as cheaply as possible. I guess the title of this post kind of gives away the total end cost of this project, and the photo above gives you some idea of the beauty factor, which is as low as the price.
Doing The Circuit
Let's talk tone. I've had the good luck to have played through most of the various Champ models that Fender made over the years, as well as a few kit clones and similar amps from other makers. They each have their own flavor, but the baseline tone comes from this basic preamp design:
Actually, the vast majority of all guitar amps ever made, no matter what make or model, have had variations of this same basic preamp, and through countless recordings and live performances, it has come to define the sound of the electric guitar. A notable exception was the Vox AC-4, which had an EF86 pentode front end; as well as some lower power US-made amps which used 6AU6/EF94 pentode first gain stages. But by and large, the sound we associate with electric guitar is a vacuum tube triode 1st gain stage, followed by a volume control, and then into a 2nd gain stage.
One reason Champs in particular sound so darn good is because their tone is just simply that almost universal guitar tone, but without refinements, effects, or cascaded gain stages. It is clean, sweet and chimey at lower volumes, and progressively becomes more raw and edgy as you turn up the knob, finally topping out at a roaring overdrive with a guitar equipped with low output single coils, or a flat-out slamming distortion with humbuckers or higher output singles, all at living room, bedroom, or studio control room levels.
Here's the schematic for the justly famed Fender 5F1 Champ Amp:
Simple! What we have here are two gain stages, each 1/2 of a 12AX7 tube, with a volume pot between them; the 2nd triode stage is also the driver into the 6V6GT power output tube, which connects to a speaker through the output transformer, which is an impedance matching device. Along with the circuitry that provides the power necessary to operate the amplifier, that's all there is.
And now here's a schematic of that same basic circuit, but with a few changes that were made along the way while putting this project together - it's kind of an "as built" circuit diagram, to use an obscure engineering term. I used to be an obscure engineer:
I may have been an engineer, but as you can see, I wasn't an artist - hey, it's freehand. The 5Z1 designation is just some bad humor. Moving on, here are the differences between this circuit and the Fender Tweed 5F1 Champ that it's based on:
• Only one input jack, and one 33K grid stopper resistor going into the first gain stage, which is very close to the usual two 68K in parallel = 34K.
• A 6.8uF cathode bypass capacitor on the first triode section of the 12AX7 reduces the "woof" (unneeded or unwanted low end that can make your guitar tone sound flabby, and sucks power while trying to reproduce that spectrum), relative to the response of the 25uF used in virtually all Fender amps. Also, you may have noticed that most Tweed Champ schematics show no bypass cap here at all, but if you look closely at those diagrams, you'll see two dots on either side of the 1.5K resistor, so someone in the past has erased that cap - and everyone since then has file-shared that schematic, and so it goes. No cathode bypass capacitor at the first gain stage results in less gain, less overall volume, and way less great gritty overdrive when you dime the volume pot - in other words, a Tame Champ - so, let's make sure that cap is in there.
• The .02uF cap going into the grid of the driver triode has been reduced to a .01, once again to reduce the woofiness a bit, especially when running any speaker larger than an 8-incher.
• In a nod to the Blackface/Silverface era, I've added a 1.5K grid stopper going into the 6V6, a 470K on the screen grid, and subbed a choke at the top of the main power supply Pi filter, instead of a 10K resistor. The 1.5K and 470K may help out when you run a 6L6 instead of a 6V6 (good to have the choice), and the choke simply reduces hum - always a good thing.
• A 10K resistor instead of a 22K at the second Pi filter ups the voltage at the plates of the 12AX7, which may add headroom. Or it might not, but I like a little more clean before the onset of dirt, and this seems to do the trick.
• The filter cap values are just what I happened to have in the bin, and the higher values may add to hum reduction; or they might not, depending on which "expert" you listen to. Those same experts also say that higher filter cap values ruin tone, but according to that logic, Bassmans and Marshalls should sound horrible, which of course they don't.
• There are two diodes for rectification instead of a 5Y3 tube - the power transformer I have doesn't have the 5V heater filament tap needed to run a tube type rectifier, such as a 5Y3. We could have used a rectifier tube with a 6V filament, but the few extra volts gained by diodes would have been nice in any case.
• The 6.3VAC heater circuit is balanced, with both legs referenced to ground via 100 ohm resistors, instead of the single ended filament supply circuit in the original Champ. Per conventional wisdom, this also reduces hum; I happened to have a couple 100 ohmers on hand, so why not?
• Finally, the AC power cable is a modern 3-conducter with ground, and there is no .05uF "Death Cap" referencing one leg of the AC to the chassis (and through the cable to your guitar and through the strings to your hands, and heart).
Parts Is Parts
After some years of messing around with tube amps, I've ended up with a couple small bins of old but still useful parts - resistors and capacitors in new or good used shape, some tubes and sockets, jacks, switches, left over wire, etc. I've also been lucky in knowing a couple of like-minded tube amp nuts, and I was able to rummage through their scrap piles and slip a few things into my pocket when they weren't looking, or actively distracted: "Whoa - is that an unopened beer over there?" "Where?!?"
After a period of scrounging around, I ended up with a small pile of cool old junk. The copper coated chassis came from a piece of old tube powered test equipment of some kind, as did the power transformer. The paper interleaved output transformer is from a 1950s organ amp that had four 6K6 output tubes in push pull parallel. I stared at the RCA Tube Manual until I got a headache, and finally decided that, with its center tap unconnected, it could handle one single ended 6V6, or even a 6L6, just fine.
I had some old 1/4" jacks, a fuse holder, some terminal strips and a switch laying around, and a good clean used octal socket, as well as (very important) a little rubber lobster. So far, everything's been free. Here's where the 99 cents in the title of this post comes from: Gene, Glenn, and Linda at Thompson's Electronics, the greatest used and vintage Hi-Fi store anywhere around here and maybe anywhere, had a brand new really old phenolic 9-pin socket for the 12AX7, and it cost... $.99.
In Part 2, we'll continue on, and try to make some kind of order out of this mess of old rusty parts, look at component placement and layout, and discuss how a schematic circuit diagram gets turned into a wired up amp that, hopefully, works. Stay tuned.
All photos in this post taken with either a Canon Elph SD-550, or a Lumix ZS-25
Click or tap on any photo to go to a higher resolution (larger) image