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Output transformer – It may seem strange, but an amp’s output transformer doesn’t just provide power in any old way— it’s critical to shaping the amp’s sound. It does something interesting. Electrons flow through the power tubes’ plates at high voltages but low current. The output transformer converts that to a low-voltage, highcurrent signal that will drive the speaker.
The high DC voltage on the tube side of the output transformer will not pass to the speaker side—the output transformer blocks DC. But it will transfer the AC guitar signal to the speaker side.
Output transformers are rated in impedance (i.e., in ohms) on the tube side, and resistance (in ohms matching the speaker) and watts on the speaker side. Impedance for an EL84 is approximately 5K Ω. The AC4’s 8"speaker is rated at 3.2 Ω (basically 4 Ω). A single EL84 puts out 4 to 5 watts, so the speaker needs to be able to handle that (it shouldn’t be a problem for most speakers—that wattage is pretty low).
The ground connection plays a big role in understanding the flow of electrons through the power tube and to the output transformer. This simplified schematic shows the basic circuit. The amplified guitar signal pulls electrons from ground, through the bypass capacitor to the EL84 tube, through the output transformer, and through the filter capacitor back to ground.
Class-A operation – The designation of “class A” is often a topic of hot debate for some tube-amp enthusiasts. A guitar amp can run its tubes in class A, class AB, or class B. (Other classes exist, but not for audio applications.) Class A describes an amp in which a power tube conducts the entire sine wave of the guitar signal. Amps with two power tubes can divide that signal between the tubes, with one handling the “down” half of the guitar signal’s sine wave and the other handling the “up” half. It’s also referred to as “push-pull” operation. A perfect division between the halves is class B. In class AB operation—which is typical for many amps with two power tubes—each tube handles more than half, but not the full wave.
Any amp with a single power tube (aka “single-ended” amps) will always be class A—that single tube must handle the entire wave. That means our AC4 is class A, too. That said, amps with four power tubes typically pair two sets of class-AB-operating tubes, working much like a two-tube amp but adding power to each half of the sine wave. Similarly, amps with more than one power tube can still achieve single-ended, class-A operation by wiring two tubes in parallel. This allows them to essentially act as a single, more-powerful tube (the Gibson GA-8 is a good example of this).
Tube diagrams – Note that the arrangement of elements in a tube diagram is schematic, not actual. In the EL84, for instance, the cathode sits in the center of the tube, with the filament located inside the cathode. The other elements (grid, screen, suppressor, and plate) surround the cathode, in that order.
The cathode and plate are made from bent metal. The grid, screen, and suppressor, however, are wrapped wires. That’s how the electrons can travel almost unimpeded from the cathode to the plate—there’s space between the wire wraps.
Dotted lines in the tube diagram for the grid, screen, and suppressor reflect the fact that these elements are wire wraps, not solid metal.
Let the Electrons Flow
Now that you know the fundamentals of a tube amplifier, take some time to study the amp schematic. (The AC4 schematic shown here has been redrawn, color-coded, and notated to help clarify the concepts.) It’ll probably take several times of going over it to get things down, and you should always be very familiar with the schematic of any amp you’re working on. Again, keep in mind that the voltages stored in amplifier capacitors are lethal. If you’re not familiar with how to safely drain them of their charges, make sure you get a qualified amp technician to perform any mods or repairs.
If you’d like to start your journey toward being more proficient with amps, there are lots of great books and online sources that will help. Free PDFs of Navy Electricity and Electronics Training Series, Module 6—Introduction to Electronic Emission, Tubes, and Power Supplies are available online. Jack Darr’s Electric Guitar Amplifier Handbook, Norman Crowhurst’s Basic Audio, and Morgan Jones’ Valve Amplifiers are also great books to track down—or you can try to locate a vintage RCA Receiving Tube Manual. If not, then simply warm up those tubes, crank the volume, play a power chord, and listen to those electrons flow!