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May 2014
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How Tube Amps Work

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How Tube Amps Work

The Voltages

AC4 Tubes
The AC4 uses four tubes—an EZ80 rectifier, an EF86 preamp tube, an EL84 power tube, and an ECC83 (12AX7)—to drive the tremolo circuit (which Vox calls a “vibrato” circuit.) The AC4 is designed to provide the plates of the latter three tubes with a different DC voltage that’s appropriate for that tube.
The first and largest component in an amp circuit, aside from the speaker, is the power transformer. It supplies electricity to the circuit, converting AC voltage from the wall to proper AC voltages for the amp. AC (aka alternating current) is a sine wave of electricity—an alternating positive and negative voltage coming from our electric sockets at 120 volts, 60 sine waves a second in the U.S. (These operating voltages vary around the world. Standard voltage can be 100, 120, or 230 volts, at 50 or 60 cycles per second.)

The AC4’s power transformer elevates the 120 AC volts to 250 volts AC, and then sends that voltage on to the rectifier tube, the first tube in the circuit. The rectifier tube’s job is to convert AC voltage to DC (aka direct current—a steady positive voltage rather than a sine wave). The power transformer’s other job is to supply low AC voltage to the filaments (the heaters) inside every tube in the amp—that’s what gets the cathodes hot. The filaments in the AC4’s tubes all work off of 6.3 volts.

Converting the power transformer's AC voltage to DC

voltage coming from it isn’t steady, it’s more of a ripple. Filter capacitors—the large, cylindrically shaped components that come next in the circuit—help smooth out the ripples in the DC voltage. Filter capacitors are similar in construction to batteries in that they store a charge—a potentially lethal charge—even after the amp is unplugged. This is why you should never poke around inside an amp unless you’ve been trained to safely discharge the caps.

The high and relatively steady DC voltage dispensed by filter capacitors goes to the tube plates—the elements that need that high, positive, electron-attracting charge. The amount of voltage on a tube’s plate is determined by the voltage coming off the filter caps, and also by resistors positioned along the DC line. With a high DC voltage, the plates are ready to start pulling electrons.

Note: AC and DC voltages can coexist on the same wire. In a guitar amp, the AC guitar signal is imposed on top of the high DC voltages. Fortunately, that AC signal can be separated: Capacitors in the circuit block DC voltages but allow the AC guitar signal to get through

To the uninitiated, circuit schematics can look like a rat’s nest of wires and components arranged in a way that saves space on paper—but that also needs to be mentally untangled in order to truly understand the circuit. Here is a 1960s Vox AC4 schematic, rearranged and color-coded to help you decipher what’s going on. The original Vox numbering system for the resistors and capacitors (R1, R2, C1, C2, etc.) is included, in case you want to follow along using the original Vox schematic.

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