A Look Under the Hood, part I

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I thought it might be kind of fun to take a walk through the innards of your People’s Amplifier. I’d like to do so in a way that isn’t too geeked out in terms of electronics-speak. I fully realize that most of us who use amplifiers don’t necessarily know about the precise electronics theory involved, and that’s ok.

When you plug your favorite guitar into the input jack on your People’s Amplifier, you are sending your signal (from the guitar’s pickups and through whatever pedals you have) onto the grid of the first preamp tube, a 12AX7. It passes a resistor that keeps the grid operating properly with the rest of the tube. That’s called a grid load resistor. The People’s Amp only has one input jack, and it is analogous to the “HI” input jacks found on vintage amps. Back in the 50s and 60s when these amps were first developed, there were significant output differences between single-coil (Fender) guitars and humbuckers, such as those found on Gibson Les Paul style guitars. A “LO” input actually sends much less of the guitar’s signal to the tube, where the entire signal of the “HI” input goes to the tube. I decided to go with the “HI” input, because it’s the one most of us use anyhow. Besides that, one of my building philosophies is to keep the signal chain as simple as possible. More components means more things to affect the sound and feel of your playing, more opportunities for noise to be introduced, and more opportunities for eventual failure.

Through the amazing magic of vacuum tube operation, the signal from your guitar is amplified within the tube, and the resulting signal, which is now much much louder, travels out of the tube from the plates. The 12AX7 preamp tube is actually two tubes in one. It’s called a dual triode. Triode, because it has three components (cathode, grid, and plate) and dual, because there are two of them. Many amps put a gain control in between the two triodes, so that the player can control how much signal gets passed from the first stage to the second. In the People’s Amplifier, both triodes are coupled together, and act as one big tube. The fully amplified signal is then sent along to it’s next destination.

If we may take a slight detour, I can point out that there are a few components connected to the tube that allow it to operate properly. There is a resistor connecting the plates to a voltage supply (referred to as B+). This resistor helps to insure that the proper amount of voltage is there in order for the tube to do its job. Connected to the cathode is a capacitor and a resistor. The other sides of these components are grounded. The capacitor is called a “cathode bypass capacitor,” and what it does is makes sure that any alternating current that travels to the capacitor gets grounded, rather than mucking up what should be going on in the tube. It also has a slight effect on distortion and tone shaping. The resistor helps set the bias of the tube (click here for more on bias). This type of design, used in all preamp stages, as well as in lower-wattage power amp stages (including the EL84s in The People’s Amp), is known as “cathode biasing.”

After your signal leaves the first 12AX7, it passes through the tone and volume network, which includes two pots (potentiometers or variable resistors) and a series of capacitors.  With the tone knob turned up to maximum, the entire signal is passed. As it is closed, the pot attenuates, or cuts the high frequency sounds, sending them to ground, passing only the lower frequency signals. The affected signal is then mixed with the output of the volume knob, which has some of the full signal passing through it. Because of the way these two pots are connected to one another, they interact in fun ways. One might think that there’s not much tonal control with just two controls, but there are sweet spots to be dialed in at different volume levels. As an aside, it would require another gain stage and preamp tube to drive a typical treble-mid-bass tonestack because of signal lost through a more complicated circuit. The volume knob on The People’s Amp acts as expected; when it is at maximum, all of the signal passes along.

The signal then travels through a coupling capacitor, which serves to block direct current on the signal path, and it’s headed off to the second 12AX7, which serves not to amplify our signal, but to split it and send each half to one of our power tubes. It’s pretty cool how all of this happens, but it’s also a little complicated. Each half of this particular dual triode sends the signal along, each a mirror image of each other. In other words, the phase of the signal passed from the first half of this tube is inverted, and the signal from the second half is not. This tube is referred to as a phase splitter or phase inverter. It really should be called a signal splitting phase inverter, but that’s a mouthful. Again, the additional resistors and capacitors here work to keep the tube operating within its specified parameters. In case you were wondering, The People’s Amplifier uses a long-tailed pair type of phase inverter.

So, now we have two signals—amplified by the first preamp tube, frequency and volume adjusted by the tone and volume network, split by the phase inverter (which also adds a little gain)—headed for the funhouse: THE POWER SECTION!

Come back next week to find out about how and where the magic really happens. Part II will discuss the power section, output transformer, and power supply!

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