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The Guitar Signal
We all know your guitar’s signal comes from your pickups, but to understand the amplified signal, let’s start at electrical ground. In practice, ground in a guitar amp means a connection to the chassis. (In the AC4 schematic, the ground connections look like upside-down Christmas trees.) Electrons flowing through a tube originate from ground. The cathodes of the EF86 and the EL84 each have a resistor attached to ground. This creates the small DC voltage on their cathodes to prevent the electrons from flowing. When the guitar signal reaches the grid, the electrons then flow. However, the cathode resistor alone would also affect electron flow when the guitar is played. A bypass capacitor is put in parallel with the resistor to increase gain and allow AC electrons to effortlessly get through. The electrons released by the guitar signal flow from ground to the EL86 cathode, then to the plate, through a .047μF signal capacitor, and through the volume potentiometer to the grid of the EL84. At the EL84, a similar electron flow takes place, but this time it’s more powerful. Enough electrons will travel from the EL84’s plate to the output transformer to drive the speaker.
The electrons don’t stop at the output transformer, though. If you look at the schematic, you’ll note that they pass through it and cycle back to ground. In a way, you can think of an amplifier as an electron circulator whose ultimate goal is to send electrons through the output transformer. Our job as guitarists is simply to get those electrons to do that in tune and with reasonable timing.
Here we see a view of the AC4’s chassis with the back panel removed (above), and with the chassis removed from the amp (below)—a design that makes it a bit of a chore to try out tubes from various manufacturers, both old and new stock.
The “Vibrato Oscillator” Circuit
You’re probably familiar with the mix up in terminology between “vibrato” and “tremolo.” The 1960s Vox AC4 schematic used “vibrato” to refer to the oscillation in volume that is more commonly referred to as “tremolo.” Because some of you may want to refer to the original AC4 schematic, we’ll stick with the company’s terminology here.
The AC4’s ECC83 (12AX7) vibrato tube creates a low-frequency oscillation. That oscillating voltage is connected to the cathode of the EF86 tube, which affects the bias. Think of it as sending a very low-sound signal to the EF86’s cathode—maybe 2–10 Hz (cycles per second). These frequencies are way too low for the human ear to detect, but they do affect electron flow in the EF86 from 2 to 10 times per second.
Components in More Detail
Now that we’ve got our quick overview of how an amp works out of the way, let’s get into some more detailed descriptions, component by component.
Power transformer – The power transformer is the amp’s larger transformer. It converts 120V wall voltage (240V in many countries) to a high AC voltage entering the rectifier (EZ80 in the case of the Vox AC4) tube. The transformer also supplies 6.3V AC to the filaments (heating elements) of the tubes. (Some rectifier tubes require 5V for the filaments, but not the AC4’s EZ80 tube.)
Capacitors (aka caps) – Capacitors are shown in the schematic as two parallel lines perpendicular to the wiring. In some schematics, one of the lines may be curved. There are three types of capacitors in a guitar amp—filter, bypass, and signal—and their values are measured in microfarads, which are designated by the symbol μF.
Filter capacitors are large metal cylinders that, like batteries, hold a charge—even long after the amp has been unplugged. Unlike batteries for household items like flashlights and smoke detectors, they hold potentially lethal voltages. These are why you don’t mess around inside your amp unless you know how to do so safely. The rectifier tube’s purpose is to convert the AC voltage (a sine wave) into a constant DC voltage to power the tubes. The rectifier tube does a good but not perfect job. What emerges is actually a ripple-like DC voltage, so the filter capacitors help reduce the ripple by storing and releasing high voltages. Filter caps typically have values in the range of 8–50 μF, sometimes higher. The AC4 uses two 32 μF caps and one 8 μF cap. The two 32s are actually both inside one cylinder—i.e., they are a single component in the amp. The 8 μF cap is a separate component.
As previously mentioned, in an AC4 a resistor and a bypass capacitor are connected to the cathodes of the preamp tube and the power tube, wired in parallel—meaning, side-by-side. (In the AC4 schematic, the cathode is the lower element in the tube diagram.) Current flowing through a resistor causes a change in voltage. Cathode resistors are used to add DC voltage to the cathodes (2.7V for the EF86 and 8.5V for the EL84). The purpose is to make the cathode positive in relation to the grid. That cathode resistor, however, also resists the guitar signal’s current flow. Hence, the parallel addition of a bypass capacitor. Since a capacitor will block DC but allow AC to freely pass through, the bypass cap does what its name implies—it allows the electrons needed for amplifying the guitar signal to bypass the resistor and flow freely through the cathode. In an AC4, the EF86 and EL84 bypass capacitors are both 25 μF. Larger values would let more bass through, while smaller values would reduce it.
Signal capacitors, meanwhile, are the small caps inside the amp, and they perform two critical functions. First, they block DC voltage while allowing AC voltages (like the guitar signal) to pass through. They also determine, according to their value, which guitar frequencies will pass through. In other words, signal caps define the tone of the amp. AC4 signal-cap values range from .1 μF–.001 μF. Smaller values (like the .001 μF cap on the AC4’s tone control) allow only treble frequencies to pass through. Put another way, the tone control sends high frequencies to ground instead of letting them reach the power tube.