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May 2014
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How Tube Amps Work

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How Tube Amps Work

Resistors – These are the small, cylindrical components with color-coded stripes indicating their value. If you haven’t already guessed by their name, they resist the flow of electricity. They are represented in the schematic as a peaks-and-valleys shape, like a seismograph reading or a few capital V’s strung together. Higher values resist the flow more than lower values. In doing so, they decrease voltage as electrons try to travel through.

Resistance is measured in ohms, often using the symbol Ω. A “k” after a number indicates thousands (i.e., 220k Ω = 220,000 Ω). “M” or “meg” indicates millions. The lowest value seen in an AC4 is 150 Ω, while the highest is 10M Ω (10 million ohms). In addition to ohms, resistors have a wattage rating. Most resistors in amps are rated at 1/2 watt. Wattage needs to be higher if the resistor is in the power section. In an AC4, the 1k Ω resistor located between the first two filter caps is rated at 5 watts. (Note: some amps will use a component called a “choke” here rather than a resistor. A choke is an inductor that looks like a small transformer. Inductors don’t like changes in current flow, which means they will help choke out some of the ripple we spoke about, reducing amp hum.)

Preamp tubes – The first tube that your guitar pickups’ signal will get to is the first preamp tube. In many amps, it’s a 12AX7 (ECC83 in Brit parlance), but in the Vox AC4 it’s an EF86. Remember the three elements inside a tube—the cathode, plate, and grid? The presence of those three elements define the tube as a triode tube. An EF86 adds two more elements, making it a pentode (from the Greek term “penta,” meaning “five”).

The two additional elements within a pentode are the screen and the suppressor. Like the grid, the screen and suppressor are wire wraps inside the tube, not continuous metal. This allows them to impose charges that affect the electrons while still allowing the majority of electrons to pass through. The presence of the cathode and plate within the tube makes the tube itself something of a capacitor. To reduce that unwanted capacitance, the screen is placed between the cathode and the plate, with a DC voltage applied. The suppressor is the wire wrap closest to the plate, and it is connected to the cathode. (In an EL84, this connection is made within the base of the tube.) Because the suppressor has large gaps in it, it has virtually no effect on electron flow from the cathode. Still, some electrons will hit the plate and bounce off. The suppressor sends the electrons from these “secondary emissions” back to the plate.

Power tubes – Just as the guitar signal is amplified by the preamp tube, the signal from the preamp tube is amplified by the power tube. In an AC4, it’s an EL84. The five elements in this pentode tube perform the same functions as the triode EF86’s elements, only with greater current passing through.

‘‘Vibrato oscillator” – Besides preamp and power tubes, you’ll see another tube in our AC4 and most other amps with a tremolo and/or reverb circuit. Often, as is the case with the AC4, it’s a 12AX7 (ECC83).

Looking at the schematic, you’ll notice something different about the 12AX7 relative to the EF86. It’s a dual triode, meaning it has two separate triodes in a single tube. As used in the AC4 vibrato circuit, the two halves work closely together.

Unlike some other amps’ tremolo circuits, which let you control the speed and intensity of the effect, the AC4’s only offers a knob to govern speed. When the AC4’s footswitch is open (i.e., when its internal contacts don’t make a connection), the vibrato circuit is heard. It sends a voltage to the cathode of the EF86 preamp tube in pulses, while an array of capacitors and resistors along with the speed control determine the rate. Closing the footswitch sends the oscillation to ground, deactivating the vibrato effect.

The two halves of the 12AX7 are wired to invert the AC sine wave. Electron flow in the two halves works 180 degrees apart— completely opposite. There are three signal capacitors in the vibrato circuit, and each one offsets the sine wave 60 degrees. The vibrato speed control affects that offset. As mentioned, think of the vibrato circuit as outputting a low-frequency oscillation, 2–10 cycles per second—too low to hear as a sound, but affecting the EF86’s cathode bias that many times a second.

If you look at the schematic, you’ll see that the oscillation originates on the right side of the 12AX7, sending it to the grid on the left side. The cathode (pin 3) sends the oscillating voltage to the EF86. The result is a variation in the preamp tube’s ability to allow electrons to flow, 2 to10 times per second.

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