Paul Reed Smith on Where Amp History–and Tone–Begins

Let’s talk about guitar amp history. I think it’s important for guitar players to have a general overview of amplifiers, so the sound makes more sense when they plug in. As far as I can figure out, guitar amps originally came from radios—although I’ve never had the opportunity to interview the inventors of the original amps. Early tube amps looked like radio boxes, and once there was an AM signal, it needed to be amplified through a speaker so you could hear it. I’m reasonably certain that other people know more about this than I do.

For me, the story of guitar amps picks up with early Fenders and Marshalls. If you look at the schematics, amplifier input, and tone control layout of an early tweed Fender Bassman, it’s clear that’s where the original Marshall JTM45 amps came from. Also, I’ve heard secondhand that the early Marshall cabinets were 8x12s, and the roadies requested that Marshall cut them in half so they became 4x12s. Similarly, 8x10 SVT cabinets were cut in half to make the now-industry-standard 4x10 bass cabinets. Our amp designer Doug Sewell and I understand that, for the early Fender amps we love, the design directed the guitar signal into half a tube, into a tone stack, into another half a tube, and the reverb would join it with another half a tube, and then there would be a phase splitter and output tubes and a transformer. (All 12AX7 tubes are really two tubes in one, so when I say a half-tube, I’m saying we’re using only the first half.) The tone stack and layout of these amps is an industry standard and have a beautiful, clean way of removing low midrange to clear up the sound of the guitar. I believe all but the first Marshalls came from a high-powered tweed Twin preamp (which was a 80-watt combo amp) and a Bassman power amp. The schematic was a little different. It was one half-tube into a full-tube cathode follower, into a more midrange-y tone stack, into the phase splitter and power tubes and output transformer. Both of these circuits have different kinds of sounds. What’s interesting is Marshall kept modifying their amps for less bass, more high midrange and treble, and more gain. In addition, master volume controls started being added by Fender and Marshall around 1976. The goal was to give more gain at less volume. Understanding these circuits has been a lifelong event for Doug and me.

Then, another designer came along by the name of Alexander Dumble. He modified the tone stack in Fender amps so you could get more bass and a different kind of midrange. Then, after the preamp, he put in a distortion circuit in a switchable in and out “loop.” In this arrangement, the distortion was like putting a distortion pedal in a loop after the tone controls. In a Fender amp, most of the distortion comes from the output section, so turning the tone controls changes the sound of the guitar, not the distortion. In a Marshall, the distortion comes before the tone controls, so when you turn the tone controls, the distortion changes. The way these amps compress and add harmonics as you turn up the gain is the game. All of these designs have real merit and are the basis of our modern tube–and then modeling—amplifiers.

Everything in these amps makes a difference. The circuits, the capacitor values and types, the resistor values and types, the power and output transformers, and the power supplies—including all those capacitor values and capacitor manufacturers.

I give you this truncated, general history to let you know that the amp business is just as complicated as the guitar business. I didn’t even mention the speakers or speaker cabinets and the artform behind those. But what’s most important is: When you plug into the amp, do you like it? And how much do you like it? Most guitar players have not played through a real Dumble or even a real blackface Deluxe Reverb or a 1966 Marshall plexi head. In a way, you’re trusting the amp designers to understand all the highly complex variations from this history, and then make a product that you love playing through. It’s daunting, but I love it. There is a complicated, deep, and rich history that has influenced and shaped how amps are made today.