Since I’m teaching this for my Basic Electronics Class at Shenandoah University, I’m going to include you all in the fun of learning the signal path of a basic tube guitar amplifier. And it doesn’t get much more basic or cooler than a Fender Champ guitar amp. This little screamer makes 5 watts of gritty tube power, and sounds great for lap steel, blues harmonica, and even hot leads if you push it a bit.
If you’ve never read a schematic before this looks pretty complicated, so lets break it down into its basic parts and consider each one separately. First of all, most schematics follow a signal path from left to right, so the input of the amp should be on the left side of the page, and the output of the amp should be on the right side of the page.
On the far left are the 1/4″ input jacks marked 1 and 2. There’s a pair of 68,000 ohm resistors in series with the signal (68K), plus a 1 million ohm (1 Meg) grid resistor to “ground” (it looks like an upside down Christmas Tree). The little pointy arrows are little switches that are opened up when a plug from a guitar cord is inserted.
Next we have two sides of a single 12AX7 signal tube. The left side is the initial gain stage and buffer, and the right side picks up the signal from 1 Meg potentiometer (volume control) in the middle of the diagram. Each tube has 3 elements, the plate at the top, grid in the middle, and cathode at the bottom. We call it a “triode” because of these 3 active elements.
Here’s the output section, with a single 6V6GT tube powering the speaker with around 5 watts. Note that there are more than 3 elements because there’s an extra “screen” to help direct the electron flow from the cathode on the bottom to the plate at the top. So this tube is called a power “tetrode”.
Below is the power supply. On the left side is the plug that plugs into the 120-volt AC receptacle in your house or stage. There’s a switch to turn it on and off plus a 2 ampere fuse to keep everything from catching on fire in case of an internal short circuit (no fooling). That little .05 mF (microfarad) 600 volt capacitor going to ground is what we used to call the “death cap” on these older amplifiers, since if it shorted out and you had the power plug the wrong way, the chassis of your guitar amp was connected directly to the incoming 120-volt line. That can kill you, so grounded power cords have been around since the late 60’s.
You’ll also see the power transformer in the middle, which steps up the incoming 120 volts on the left to 350 volts DC or so plate voltage, plus steps it down to 5 volts AC for the 5Y3GT rectifier tube, and 6.3 volts for all the other tube “heaters” (the things that glow in all tubes).
More on this in Part II where we go into detail as to exactly what’s happening inside those tubes (valves). See you then….
Mike (tube head) Sokol