“You gotta remember, I wasn’t really shit until about a year-and-a-half ago,” Bartees Strange reminds the crowd at Nashville’s Basement East just before performing his song, “Hennessy.” “I was just in my basement playing guitar. And my wife was like, ‘Do the dishes ... Do something other than play guitar.’ Now all I do is play guitar again [laughs].”
Strange (born Bartees Leon Cox, Jr.), is a sponge and synthesis of everywhere he’s been and everything he’s seen or heard. Born in England and raised in Tulsa, Oklahoma, his experience performing with Brooklyn-based post-hardcore outfit Stay Inside and a later move to Washington D.C. have all contributed to his singular cosmic-slop sound. He notes during the Rundown that, as an adolescent, his guitar heroes were Thomas Erak of the Fall of Troy and Omar Rodríguez-López of At the Drive-In and the Mars Volta. But in the next sentence, he confesses his love for Nelly.
“I always thought people aren’t really honest all the time with what they’re listening to,” asserts Strange. “I think a lot of people like a lot of things. I grew up in a pretty country town, and everyone would say they just like country music. But I was like, ‘You like the Nelly record, dog. You like Get Rich or Die Tryin', man, and you also like LeAnn Rimes and Toby Keith songs, and Brad Paisley’s guitar playing. But you also jam B2K and pop songs, too.’ I’ve never been afraid or ashamed of what I like, so it all goes into my own music.”
What he’s been saying through 2020’s Live Forever and 2022’s Farm to Table has been connecting with fans and critics alike. The magnetism is Strange’s smooth synergy. This allows him to touchpoint influences from albums like Nelly’s Country Grammar, At the Drive-In’s Relationship of Command, the National’s Boxer, and Phoebe Bridger’s Punisher into one harmonious, original package that has landed him on dozens of year-end lists and earned him an 83/100 rating from Metacritic for both of his full-length releases. [Editor’s Note: The Metacritic website uses their proprietary Metascore to distill the opinions of the most respected critics’ writing online and in print to a single number.]
Finishing his earlier thought to the Nashville crowd, he summarized: “‘Hennessy’ is a song I wrote when I was a kid, and growing up I thought there was all these weird stereotypes I had to get over to become who I am … [The hook of the song is meant] to kind of say, I know there’s all these expectations of what a black person does … but I just want you to see me for who I am and for what I’m trying to say.”
He might not have been “shit” 18 months ago, but he’s certainly on his way to becoming the something of the sort in the coming years. We’ll be here listening and appreciating.
Ahead of Strange’s final 2022 tour date supporting Farm to Table, Bartees and his guitar-playing compatriots welcomed PG’s Chris Kies onstage at Nashville’s Basement East to talk shop. During the interview, the trio explained how their “guitar wars” create a compatibly melodic arms race and structure their cohesive sound. We get introduced to a collection of oddball axes and go through their collective setups—which Strange fondly refers to as “Tone Capital”—assisted by a store’s-worth inventory of pedals. Plus, stick around after the Rundown to check out a heartfelt message from Bartees and the band’s wonderful performance of “Hennessy.”
Brought to you by D’Addario XPND Pedalboard.
Bartees’ Battle Axe
Strange’s main axe for much of 2022 was this 1959 Gibson Custom Shop ES-335 Reissue “Chicago Music Exchange Spec” that features the delicate deterioration of the Murphy Lab treatment. It has a maple body (with a maple center block and red-spruce bracing), a 1-piece mahogany neck, an Indian rosewood fretboard, Kluson tuners, and custom CME-spec “S” Gibson humbuckers.
“Honestly, it’s pretty sick. It’s the dopest 335 I’ve ever played,” contends Strange. “It has a very versatile sound, and with its low-output humbuckers I can get it to chirp a little bit, but I also can go off on it.”
It has replaced touring duties for his beloved 1967 Epiphone Casino and a 1963 Gibson ES-125T. This and the rest of his riders take D’Addario EPN115 Pure Nickels (.011–.048). The 335 will stay in either standard or D-A-D-A-A-E tunings.
Strange Baraniks
After Bartees’ 2020 debut, Live Forever, came out, luthier Mike Baranik built Strange this Baranik RE-1 that boasts a reflective pickguard with the words “Never Die” emblazed on it. Its standout specs include a Baranik handwound gold-foil-style pickup that slides, a groovy, give-it-a-rip Göldo DG Tremolo in Shorty-Design, an illuminated control pod, and wooden saddles. It comes in at a feathery 6 pounds. Strange busts it out for his song “Heavy Heart” because of the guitar’s jangly grind.
“The RE-1s were designed to simplify the manufacturing without losing the most critical parts of a guitar, playability and tone,” says Baranik. “Almost every single one of the RE-1’s parts are made here in the shop from repurposed materials.”
Goldilocks
Another one of Strange’s treasures is a 1959 Fender Jazzmaster. That classic stays at home, but he needs the instrument’s sonic flair for his nightly set, so he contacted Fender’s Jason Klein and sent over a request to recreate his ’59 with a few slight cosmetic changes. He wanted an Aztec gold finish with a matching headstock, complete with an anodized pickguard. Strange often starts the set with this golden goose on songs “Escape the Circus” and “In a Cab.”
A Low-Price Highball
Like his other touring guitars, this Gretsch G9520E Gin Rickey acoustic/electric fills in for his pricier, vintage flattops. The price was right at under $300, and Strange really loves its darker, boxier sound that meshes well with Graham’s brighter Orangewood acoustic. Another plus was that it came stock with a Gretsch Deltoluxe soundhole pickup that enables Bartees to run this into his Vox AC30.
A Voxy Solution
“In Tone Capital, U.S.A., things can change. The weather, all kinds of things … but honestly, the three of us are always kind of looking at each other like, ‘What is not right? Is it an amp? Is it a guitar?’ There’s dysfunction in Tone Capital, so after spending a lot of time with Fender amps, I’ve returned to AC30s for its crisp highs that match really well with the dark, mellower vibe of the 335,” says Strange. He plugs his guitars into the Vox AC30C2X’s low-input/top-boost section. This particular 30-watt combo comes with a pair of 8-ohm Celestion Alnico Blue speakers.
Bartees Strange’s Pedalboard
As the governor of Tone Capital, Strange has the most advisors on his board. Breaking them down by function, Bartees’ dirt and filth comes from a Land Devices HP-2, Fowl Sounds Obsidian Fuzzstortion (the unmarked black box), Bondi Effects Breakers Overdrive, and a ZVEX Box of Rock. Time-based effects include an Alexander Pedals Rewind Programmable Echo, a Boss DM-3 Delay, and a Source Audio Ventris. Bartees’ modulation machines are a Farm Pedals Tombstone Tremolo and a Fairfield Circuitry Shallow Water K-Field Modulator. Two other noise manipulators include a Chase Bliss Blooper and a G-Lab BC-1 Boosting Compressor. Other boxes are a Radial SGI guitar interface (upside-down at the top), a Radial HotShot DM-1, and a TC Electronic PolyTune 3 Noir Mini.
A V for G
“I just got this for this tour. I kind of bought it because I thought it’d be the most ridiculous guitar that I could bring onstage, but I’ve slowly discovered it’s the most the comfortable instrument I’ve ever had,” admits multi-instrumentalist Graham Richman. The 2022 Gibson Flying V in antique natural has stayed the same since he bought it, except for the fresh set of D’Addario EPN110 Pure Nickel strings (.010–.045).
Les Paul, More Gristle
This one used to be Richman’s number one, but only gets action for one or two songs, like “Kelly Rowland.” He still enjoys playing the Gibson Les Paul Standard ’50s P-90 because it has “more gristle and cuts in an interesting way,” compared to his V.
Orange You Glad to Play Me
For “Black Gold,” Graham puts on this Orangewood Sierra Live, that’s equipped with a L.R. Baggs Anthem pickup.
Deluxe Bassman
Richman runs all of his electrics into the above Fender ’68 Custom Deluxe Reverb. He landed on this combo because of the punchier Bassman circuit inside the Custom channel.
Graham Richman’s Pedalboard
When you’re a touring musician, cartage costs for gear are always a concern, and it’s no different for Richman. He downsized his setup to a Pedaltrain Metro 16 thanks in big part to the Boss MS-3 Multi Effects Switcher that not only can control MIDI pedals on his board, but also offers 112 internal effects, too. Graham relies on the MS-3 for all his delays, reverbs, and modulation. His gain stages come from nearby standalone pedals: Black Mass Electronics The First Herald, Black Mass Electronics 1312 Distortion V3, Walrus Audio 385, and a JHS Double Barrel.
Surface-Level SG
“Honestly, it was an aesthetic-first purchase,” concedes guitarist Dan Kleederman. “It’d be really cool to play a SG Junior in this band—I hoped I’d like it … and I did!” This sweet surprise is a 2021 Gibson SG Junior that appears to be all stock, but he added a Bigsby B7 vibrato and a push-pull switch on the tone that cuts higher frequencies when pulled out. He said he was sold on its sound once the band made the move to in-ear monitors, because it sits in its own lane within the three-guitar attack. And because of that, this one sees the most action of Kleederman’s trio.
Hand-Me-Down Tele
This 1998 Fender USA Thinline Telecaster is from Dan’s father, who bought the Tele in the early 2000s and recently loaned it to him. He gave Dan his blessing to customize it as he saw fit—so it now has a 4-way selector unlocking a series circuit that combines the bridge and neck pickup for a beefier, hotter signal. You’ll also notice that tone and volume control knobs are pulled from a Gretsch. “I’m in a phase where I like messing with the guitars and their looks,” says Dan. He uses this guitar every night for “Heavy Heart.”
Speaker of the House
“This is a very special situation here. Part of what makes this guitar unique is the fact that it has a built-in amplifier that you can turn on and off,” details Kleederman. The 1960s Silvertone 1487 TG-1’s gold-foil pickup is original, and was the initial allure for Kleederman to make the purchase.
And for “Hold the Line,” where Dan plays slide—to give the song a rustic, back-porch, AM-radio vibe—he engages the tiny amplifier and sends a signal to FOH via a Shure Beta 98H/C.
Foxy Voxy
Kleederman puts all three of his electrics through a hand-wired Vox AC15HW1X that comes with a single 12" Celestion Alnico Blue Speaker. He borrowed the combo from Bartees’ FOH, Kitzy. He uses the low input of the top-boost circuit and says it works well for cutting through and providing some defined power to his sound.
His board starts with an always-on JHS Morning Glory. The next level of grime is the Matthews Effects Architect. He chose this one because it includes a boost, three different clipping modes, and a 3-band EQ, all in a small footprint. A Wampler Mini Ego handles compression, while an Xotic EP Booster gives him another intensifier of volume and gain. The ZVEX Fuzzolo helps Dan double bassist John Daise’s parts in a song like “In a Cab,” or give him a super-gated attack during “Boomer.” Then we enter the section of Dan’s crazier colors that get painted on with a Walrus Audio Mako M1, a Source Audio Collider delay/reverb, and a Boss DD-8 Digital Delay. And, stealing a page out of Bartees’ playbook, Dan slots a distortion (Animals Pedal I Was a Wolf in the Forest Distortion) at the end of his chain to “make everything messy and fun.” Off to the side of his board sits a Dunlop DVP4 Volume Mini Pedal, and a Sonic Research ST-300 Mini Stomp Box Strobe Tuner keeps his instruments steady.
Once we plug in our guitars, all sorts of electrical processes happen as our signal makes its way from the input jack on through the unique set of electrical components that give each amp its signature sound and on through to the speaker. What goes on inside of our amp once we've plugged in our guitar? And what makes one amp louder than the next?
Although there's much, much more to cool amp tones than could possibly be discussed in an introductory piece like this, there are a lot of basics in common between the various brands and types of circuits, particularly with regard to how tubes (preamp and power), watt ratings, and speakers work. Because of this, we can learn a lot from a more specific example. To that end, let me tell you a little story about one of my favorite amps.
Dan Formosa found his 1960 Vox AC15's international voltage selector was incorrectly rated, and avoided overloading the amp's original tubes after doing an extensive online search and calculations.
I recently had a revelation about a beautiful, fawn-Tolex-covered, circa 1960 Vox AC15 that I bought from a dealer in the U.K. (full disclosure: many years ago) and finally got around to restoring. That meant replacing the electrolytic capacitors before daring to turn it on, since they have a life span. The AC15's international voltage selector on the far right of the control panel has settings for 115, 160, 205, 225 and 245 volts. I expected my U.S. wall voltage to be a few volts higher than its nominal 120, but still within reason for powering the amp at the 115 setting. However, the readings I got when checking the internal voltages were sky high. Its original Mullard EL84 power tubes were being overloaded at almost 17 watts, while 12 watts is the designated maximum and 14 watts would be pushing my luck. A few Variac voltage experiments over the next few days, along with some obsessively created Excel calculations and charts, verified that a wall voltage of 105 would be more appropriate. A week of deep Google searches and an eventual exclamation of "Thank you online discussion boards!" uncovered the problem. While there were no markings on my AC15's power transformer, chassis photos of two exact same amps and transformers showed the power transformer input terminals labeled as 105, 145 (not connected, like on mine), 160, 205 and 245. Despite the control panel's graphics, the amp never had a 115 volt option. That setting connects to the power transformer's 105 volt terminal. Furthermore, the 225 and 245 selections were both connected to the 245 terminal. Apparently when Vox printed that panel in 1960, they were just kidding.
My near-miss chance of seeing the power tubes glow like it's Christmas led me to think about the journey electrons take through an amp, combining forces emanating from your wall and your guitar to power the speaker. And what it means to overload a tube, as I came close to doing. Did you ever wonder why a single EL84 tube is rated at 12 watts, but powers a 5 watt amp? Or why two EL84s power a 15 watt amp? And why, when adding two more to the set, four will produce 30 watts? Let's explore watts and electrons, and investigate how exactly they travel in your amp, from power tube to speaker.
Identifying the limit of a tube or a speaker in watts means defining the maximum amount of energy per second it can safely handle.
Power In Vs. Power Out
When discussing power and watts, keep in mind that your tube amp isn't primarily functioning as a guitar amplifier. It's more of a space heater that produces sound. Here's a question that Steven Fryette, of Fryette Amplification and Sound City Amps, is frequently asked: "How is this a 30-watt amp when it says 100 watts on the back?" The short answer: An amplifier is filled with components that consume power that never gets to the speaker. Power transformers get warm, the pilot light and heating filaments within the tubes suck up a lot of juice—the preamp tubes and power tubes are approximately only 50 percent efficient— and there's heat being produced by the output transformer. Power-wise, the speaker operates mostly as a heat sink. A tube amp is therefore far less efficient than you might guess. More than 99 percent of the incoming power ends up as heat. Less than 1 percent exits as sound. To help understand how all that power turns into hardly any sound, we'll discuss EL84 tubes—although any power tube could serve as an example, since all are guided by the same physics.
At the center of the tube, preamp tubes included, is a cathode, a small tube that emits a cloud of electrons when heated. The plate—that's the gray or silver metal wall that you see when looking through the tube's glass—contains a high-voltage, electron-attracting DC charge. The signal from your pickups is sent to the preamp tube's grid, and eventually to the power tube's grid. The grid is a wrap of wires within the tube surrounding the cathode. The grid regulates the flow of electrons traveling from the cloud to the plate. In a class A or class AB amplifier (more on that to come), the grid allows electrons to flow even when at rest, or "idle," meaning electrons are on the move even with no guitar signal on the grid. Start to play and an increase and decrease of electron flow perfectly mirrors the guitar's signal. Electron flow is also known as current.
An RCA 6BQ5, aka EL84, tube consumes 12 watts, but like all power tubes it produces about half of that in power. The EL84 is a staple in the world of power tubes, typically associated with Vox and Marshall amps.
So, what's a watt? A watt is a rate of power—one joule per second, with a joule being a unit of energy—and can be calculated by multiplying volts times amps. Therefore, a watt is a measure of energy per second. Identifying the limit of a tube or a speaker in watts means defining the maximum amount of energy per second it can safely handle. Given the calculation for wattage (volts x amps = watts), you can see that increasing voltage, amps, or both will increase wattage.
Defining that power relationship one step further, what's an amp? It's short for "ampere" (not, in this case, "amplifier"). An amp holds the "per second" dimension of time seen in watts. In a classic plumbing analogy, volts are equivalent to water pressure, while amps measure the flow rate of that water. Too much of either will electrically flood your tube or speaker.
Water flow and pressure may not be a great analogy, because what really results when a tube or speaker becomes overloaded with watts is too much heat. But to complete the water analogy, resistance (or the related term "impedance" … we'll get to that, too) is like reducing the diameter of the water pipe. It's therefore fair to think of a tube as an electron pump, continually circulating electrons.
The Secret Life of Watts and Tubes
Electrons bombarding the plate too quickly will cause it to glow red and radically shorten the life of your tubes.
Receiving the up-and-down voltage waves of a guitar signal, the grid controls the flow of electrons, holding some back or unleashing them in accordance with whether you're delicately picking or bashing. The high level of positive, electron-attracting DC voltage on the screen grid and plate elements determines the amount of electrons pulled from the cathode. (Essentially determining how loud your amp gets.) Tubes, however, have limits, both on the rate at which the cathode can produce electrons and on the rate at which the plate will accept them.
Try to attract more electrons than the cathode can emit and you'll reach saturation. Flood the plate with too many electrons and you'll exceed its maximum dissipation level, overheating the tube. Set the grid's bias voltage too negative and you'll reach cutoff, a point where the negative swing of the guitar signal's sine wave will suddenly prevent any further electron flow from the cathode.
Picture your guitar's signal as a simple sine wave—a pure A440, for instance. Turning the volume up high can produce too much voltage swing on the tube's grid, and then on the plate, to be handled cleanly. The result you hear will be the sound of a sine wave being abruptly flattened at the high and low points of the wave. You may be perfectly happy with that level of distortion. But what if we overload a tube in a less friendly manner?
Class Acts
Amplifier circuits are designed to use tubes in different ways. The circuits we are primarily concerned with in tube amplifiers are class A and class AB. However understanding classes A and B helps to explain class AB, a hybrid of the two. So….
How Class A Circuits Catch a Wave
In a class A amp circuit, the power tube constantly carries the entire signal. So, a tube operating in a class A design is always conducting at maximum dissipation—full on—whether you're playing guitar or not.
Amplifiers with one power tube—single-ended amplifiers—operate in class A. That one power tube carries the entire 360-degree span of the sine wave, measured along a horizontal axis in degrees. The bias is set so that the amp idles along the vertical (Y-axis) center of the sine wave, evenly positioned between the peaks and valleys. That means the tube is always conducting at maximum dissipation—that it's always on full whether you're playing or not. When playing, the guitar signal creates peaks and valleys in the sine wave. Many, actually. The peak of the sine wave increases current flow; the valley of the wave reduces it.
This flow diagram shows how an EL84's power comes from electrons flowing from ground, through the tube, through the output transformer, and back to ground. It's a cycle.
An EL84 power tube can produce approximately 5 watts in a single-ended amp. Therefore, you would think two EL84 tubes would produce 10 watts. And that's true: Power tubes can be configured in parallel to double the output power. Consider, for instance a Gibson GA-9 amp, which puts two 6V6 tubes in parallel. It's done, but not often. Why? Because a class AB configuration can produce more than double the power output from two power tubes. But before we get to that….
Make Some Noise, Class B
In a class B amp, each tube carries exactly half of the signal. Because the transfer of the signal from one tube to the other is never perfect, it creates crossover distortion.
In a class B amp, two power tubes share the sine wave. One conducts the first 180 degrees of the wave, and the other conducts the second. It's a push-pull arrangement. Unlike in a class A amp, each tube is at work only half the time. This allows each tube to be pushed further, into higher amplification, during the time it's conducting. To take advantage of that rest time, voltages at the plates can be higher, as can the signals going into the power tubes' grids. If a single EL84 tube can deliver 5 watts in class A, it can deliver twice that in class B during its half of the sine wave. Two tubes, therefore, will deliver four times the power, in theory. In practice, it may be less. Another advantage of a class B circuit is that at idle, neither tube is conducting, so it's a very efficient configuration for power consumption and for tube life.
All of that would be great for a guitar amplifier if the transition from one tube to the other occurred instantaneously. It doesn't. As the sine wave moves from positive to negative and back to positive, there's a delay—a misalignment in the transition between the tubes. The delay creates crossover distortion. Steven Fryette's description: "Crossover distortion can create a fizzy sound in the amplifier, [because] one tube is turned off before the other is fully turned on." And that, in a nutshell, is why class B isn't a common option for guitar amps. Enter class AB.
Class AB—Double the Fun
A class AB circuit solves the crossover distortion problem by having two (or four) tubes overlap responsibilities. Each tube, or each pair of tubes, carries more than half of the 360-degree signal of the sine wave.
In a class AB circuit, two power tubes share the responsibility of conducting the sine wave, similar to class B, but with some overlap. The tubes are set up so that one starts conducting before the other finishes, so each tube conducts for more than 180 degrees of the sine wave. This eliminates issues with the transition from one tube to the other. While not as powerful or efficient as a class B circuit, it's close—and the reason two EL84 tubes can deliver 15 watts in class AB amplifiers.
But if one EL84 delivers 5 watts and two can boost that to 15 watts, why do four only deliver 30 watts? Because in an AB amplifier with four power tubes, the tubes work together in two pairs, with each set delivering exactly twice the power of one tube. In a Vox AC30, for example, each pair of parallel EL84s creates 10 watts. It then puts the pairs in class AB configuration, doubling the output of a two-power-tube-amplifier, like the Vox AC15, from 15 to 30 watts. The diagram here explains that in greater detail.
In a class AB circuit, each power tube get a chance to rest half the time an amp is operating. Because of that, power tubes can be pushed harder when they are conducting.
The Output Transformer Takes Sides
The output transformer converts high voltage and low current on the primary side—which is to say, the tube side—of the circuit to enough low voltage and high current on the secondary—or speaker—side to drive a speaker. An output transformer's primary side is rated in ohms, but ohms in impedance, not resistance. The difference is that impedance takes into account that an AC signal is involved, since resistance will vary significantly depending on the frequency. (Frequency is the number of oscillations per second in the AC signal.) The impedance determines the rate of flow of electrons, with higher impedance being more restrictive.
The Alliance: Speakers and Transformers
It's important to match a speaker's impedance rating with the output transformer, because, interestingly (and maybe somewhat surprising), the impedance on the primary side of the output transformer will change based on the impedance of the speaker you connect on the secondary side. If you connect a speaker rated at half the impedance—for example, put a 4-ohm speaker in place of an 8-ohm speaker—the impedance seen by the tubes will be cut in half. Twice the current will flow on both the tube side and the primary side. The 4-ohm speaker will be louder but can lead to trouble. Your power tubes or output transformer can overheat. It's not risky, however, to put a 16-ohm speaker in place of an 8-ohm speaker, although it won't sound as loud. In discussing this with John Paice at speaker manufacturer Celestion in Ipswich England, he had some simple advice: "Don't do it." Best practice is to match the speaker with the output transformer.
Doubling the wattage of a 15-watt amplifier will increase perceived loudness by 23 percent, not double it. And so, a 5-watt amp would sound 71 percent as loud as a 15-watt amp.
In terms of guitar amplification, we measure—and hear—power and loudness along a logarithmic curve. Doubling the wattage going into a speaker results in a 3 dB increase. At 3 dB more, we're not doubling loudness. It's approximately a 23 percent increase in volume. You can therefore expect a 30-watt amplifier to sound 23 percent louder than a 15 watt amplifier. And a 5-watt amplifier will be 71 percent as loud as a 15-watter.
If mixing speakers in a multi-speaker cabinet, be conscious of each speaker's impedance rating (they should match) and also of each speaker's sensitivity rating, found on its spec sheet. (Sensitivity is usually determined with a microphone connected to a sound level meter placed one meter in front of the speaker. The result is expressed in dB.) Advice from Celestion's Paice: "If mixing speakers, try to keep their sensitivity rating within 3 dB of each other, because any more than that will become noticeable. The more sensitive speaker will dominate the blend."
What’s with Speaker Wattage
A large speaker magnet does double-duty. It will hold the voice coil more firmly, producing more bass. It also acts as a larger heat sink. A Celestion G12M rated at 25 watts incorporates a 35-ounce magnet. A G12H at 30 watts incorporates a 50-ounce magnet. "A bigger lump of metal is better at dissipating heat, so you can put more power into it," explains Paice. In addition to heat, too much power into a speaker can potentially result in too much cone movement, damaging the cone and its surround, and possibly resulting in failure. Nonetheless, a 50- or 100-watt Marshall amp pushing a set of four Celestion 25-watt speakers is a classic sound, employed by Hendrix, Clapton, Page, Slash, and many other guitar heroes. Running multiple speakers in a cab reduces the punishment any single speaker must take. And, of course, using a high-power-rated speaker with a low-power amp can also net good sonic results. "Some people think that you have to put as much power into a speaker as it will take," says Paice, "but you can get lots of breakup with a high-power speaker using just a lunchbox-size amp."
Bactrian Amps, Anyone?
You may be thinking, okay, if doubling watts into a speaker doesn't double the loudness, I'll just use two amplifiers. No, no, no—the same principles apply. Since we hear logarithmically, two 15-watt amplifiers will give you the same output as a single 30-watt amplifier. It's an increase, but not double.
I like going back to the classic 1959 publication on sound and amplification, Basic Audio, Vol 1. by Norman H. Crowhurst. He shows an illustration of two crying babies in a twin stroller, comparing their loudness with one crying baby in a stroller. Two babies are louder, but not twice as loud. So while that physics phenomenon may not work to your advantage as a guitar player, think of how grateful you would be if you were the parent of twins.
Peeling the Onion
Let's take a deeper look inside tubes, output transformers, and speakers.
This diagram shows the ve components within an EL84 tube. Note the minute distance between the grid and cathode. That's the open range for negative-charged electrons.
Under the Glass
Ever wonder what's behind the glass of your amp's tubes? Well, there's a lot going on in your average pentode or triode—electrons charging around, hitting walls, held at bay. Let's examine an EL84, which is a pentode, as is an EL34 and many other power tubes. That means five elements are at work within the tube (not counting the filament, the heating element tucked inside the cathode). Schematic diagrams like the one below portray tubes as if the cathode is on one side of the glass and electrons flow in a straight line through the tube, with all elements evenly spaced.
In reality, the cathode sits vertically in the center of the tube, and its electrons flow outward. When the cathode is heated, a "space charge" of electrons—a cloud of negative-charged particles—form around it like swarming microscopic bees. Because opposites attract, they are instantly drawn to the high positive-DC voltage of the plate. But the grid stops them. The grid is a wrap of thin wires encircling the cathode that carry your guitar's signal. The grid's at-rest charge appears negative to the cathode, slowing the electron flow. There are two ways for the grid to assume that negative appearance, depending on an amplifier's design: Either the grid is connected to a small negative charge or the cathode has a small positive charge. Electrons don't care which method is used. Just ask 'em.
The cathode, grid, and plate are elements common to triodes (three-element preamp tubes, like a 12AX7) and pentodes. The two additional elements inside the pentode are the screen grid and the suppressor grid. Like the guitar-signal grid, they are wraps of thin wire with mostly open areas that allow flying electrons to reach the plate without being blocked. And like the plate, the screen grid carries a high electron-attracting DC voltage, but its voltage, unlike the plate, is consistent, whereas plate voltage will vary with the signal.
The suppressor grid, the outermost wrap of wire closest to the plate, is connected to the cathode and its job is to repel electrons, which hit the plate and bounce off. The suppressor grid sends them back to the plate to avoid power loss. Beam tetrode tubes like the 6V6, which have four elements, incorporate metal plates that serve a function similar to a pentode's suppressor grid, working to keep the electrons in place.
This illustration shows the three grids plus the cathode and plate in a typical pentode tube.
Are Your Tubes Biased?
Sure, you've heard the term bias, but what is it and what does it do for your amplifier? Bias refers to the amount of negative charge the cathode detects on the grid, and it is set to keep the electron flow in check at a happy, medium level. Too negative and not enough electrons will flow when you're playing, so your amp won't produce enough volume and will sound anemic. Too positive you'll be bombarding the plate with too many electrons and overheating it, producing a warm red glow that you don't ever want to see in a tube. At that point, its lifespan could be measured in minutes.
The wattage a tube's plate receives can be determined by multiplying the rate at which electrons flow from the cathode to the plate times the voltage at the plate. The former is measured in amps, and in a cathode-biased amplifier can be calculated by knowing the value of the resistor connected between the cathode and ground, and the voltage drop across the resistor (the "drop" is the voltage measured between one end of the resistor and the other). An EL84 is designed to receive up to 12 watts maximum, and this or just below becomes the target when adjusting the tube's bias. So there you go.
The Many Tasks of Output Transformers
In the main story, we talked about how the output transformer wrangles voltage and works to impede and control the flow of electrons toward the speaker. That's not all it does, but in the process of doing that, it also blocks high voltage DC from streaming through the circuit, which is why you won't get electrocuted touching your speaker connections.
On the primary, or tube, side, the output transformer's impedance rating should more or less match the required impedance for the power tube or tubes being used. That impedance is measured in ohms, on the order of 4,500 ohms for a single EL84 tube, and 8,000 for two in class AB. An output transformer designed for an impedance lower than what the tubes want will lead to too much current flow, overloading the transformer, the tubes, or both. And soon they're kaput.
High voltage on the power tubes' plates also comes from the output transformer, via the rectifier tube or circuit. And that DC voltage is regulated by a large filter capacitor to help smooth out any ripples in voltage.
Yes, Speakers Are Sensitive
There's a rating for how reactive a speaker is to a signal that's typically called sensitivity. Awwww…. A speaker's sensitivity is measured by sending a 1-watt, 1-kHz signal into the speaker and measuring the loudness at 1 meter away.
If 1 watt sounds low, remember that power efficiency of a speaker is also surprisingly low. Most of the power going into a speaker is dissipated as heat. According to Celestion's John Paige, 97 percent of input power becomes heat, and only 2 to 3 percent converts to sound. Years ago, regulations required that speaker voice coils include a fire retardant, because occasionally they'd ignite onstage.
Since speaker sensitivity varies, an easy way to increase or decrease the loudness of an amplifier is to simply change speakers. But here's a quick lesson in sound physics. We measure loudness in decibels, or dB, a unit of sound pressure level, or SPL. Similar to the way we rate the magnitude of earthquakes, decibels are based on a logarithmic scale. So, check out this chart. It illustrates the perceived loudness you might expect for speakers of varying decibels.
And remember, our ears work in a surprising way. To perceive sound as being twice as loud requires an increase of 10 times the sound pressure, or 10 dB. Therefore 70 dB will sound twice as loud as 60 dB, and 80 dB will sound four times as loud as 60 dB. For reference, casual conversation is around 60 dB and 120 dB is jackhammer painful.
Here we're going to take a look at the inner workings of standard amplifier circuits—the tubes, transformers, resistors, and capacitors that work together to create the amazing tones that have powered countless songs for the past 60+ years. While this stuff may be daunting to some of you, take heart—this is century-old technology. The basic concepts really are not too difficult to grasp.
We'll discuss amplifier circuits by looking at my absolute favorite small amp, a 1960s Vox AC4. While it's small and simple, an AC4 actually is not the simplest guitar amp out there. Unlike Fender's earliest tweed Champs, the AC4 has a tone control and tremolo, which gives us a bit more to talk about.
But before we get started, let's make it clear that this article is not in any way encouraging or equipping you to open up the back of your amp and start poking around. Make no mistake: Amplifier circuits, even when unplugged, contain voltages that can kill you. And if you're an amp tech, please excuse any oversimplification in the discussion—this is a primer for general consumption, not a compendium of possible exceptions and anomalous phenomena.
The Vacuum Tube
First, let's talk about some basic principles of electricity. An electron—the heartbeat of electric energy—is a negatively charged subatomic particle. In a vacuum (i.e., in the absence of air and matter), an electron will, in fact, fly through space if attracted by a sufficient positive charge—because opposites attract. Experiments conducted well over a century ago demonstrated that electrons will not only fly through space, but they can also be controlled. Scientists showed that, in a vacuum, electrons flowing from a heated metal element—the cathode—and being pulled toward a positively charged element—the anode—can be deflected by a magnetic field.
Cathode vs. Fixed Bias
A Vox AC4, like many amps, is designed to make the power tube's cathode slightly positive—a state that is commonly referred to in the guitar universe as cathode biased. Other amps, instead, put a negative charge on the power tube's grid. That's called fixed bias, and it has a similar effect. Either method causes electrons to stay put on the cathode until needed.
Learn how to control that magnetic field accurately and, as RCA did, you can display an image of Felix the Cat on a phosphorescent surface at the far end of the tube. The tube used in that case was the cathode ray tube (aka CRT)—better known today as an old, pre-LCD/LED/plasma television.
In guitar amps, we're not that interested in displaying images with our tubes, but we're still very interested in controlling those electrons—and we can use a guitar to do it. Picture this: In the center of a tube's glass envelope is a cathode. It carries just a slight positive charge, and it's ready to release a gazillion electrons. It's especially ready if it's been heated. Surrounding the cathode is the anode—although in the guitar universe we typically call it the plate. The plate carries a high positive charge that's ready to pull those negative electrons toward it. To the highly positive plate, the cathode's slight positive charge still makes the cathode seem negative (we'll talk more about this slight positive charge later). If you place these two elements in a vacuum and power them up, electrons will fly relentlessly toward the plate. When you add a third element—the grid—between the two, you can control the flow of electrons. And when you position the grid close to the cathode and connect the grid to the relatively tiny voltages coming from your guitar pickups, something interesting happens: The tiny signal unleashes a flood of electrons, allowing them to fly freely to the plate. That rush of electrons from the cathode to the plate mirrors the signal from the guitar, amplifying its signal many times over.
Okay, so let's get back to that earlier mention of the slight positive charge. The reason we want the cathode to carry a slight positive charge is that it makes the grid, with no charge yet applied, seem negative. Voltages are relative. And while opposites attract, like charges repel. The apparently negative grid close to the cathode will keep those negatively charged electrons in place until the guitar signal is ready to swing the grid positive to release them.
One other useful electron-related fact to know is the difference between voltage and current. Think of current as the amount of water flowing through a pipe. More current means more water being delivered. Voltage, on the other hand, is like water pressure— it's the force behind that water. Increase the voltage (pressure) and you'll increase the current (amount of flow). A resistor acts like a constriction in the pipe, with more resistance being analogous to a tighter constriction. So it follows that placing a different resistor in a circuit will affect both the voltage and the current.
What actually goes on inside a guitar amplifier is obviously a bit more complex than just the flow of electrons in tubes, though. Next we'll do a quick overview of the additional parts involved, followed by more detailed, part-by-part descriptions.
The Voltages
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.)
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 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.
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.
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.
The Guitar Signal
We all know your guitar's signal comes from your pickups, but to understand the amplified signal, let's start at electrical ground. In practice, ground in a guitar amp means a connection to the chassis. (In the AC4 schematic, the ground connections look like upside-down Christmas trees.) Electrons flowing through a tube originate from ground. The cathodes of the EF86 and the EL84 each have a resistor attached to ground. This creates the small DC voltage on their cathodes to prevent the electrons from flowing. When the guitar signal reaches the grid, the electrons then flow. However, the cathode resistor alone would also affect electron flow when the guitar is played. A bypass capacitor is put in parallel with the resistor to increase gain and allow AC electrons to effortlessly get through. The electrons released by the guitar signal flow from ground to the EL86 cathode, then to the plate, through a .047μF signal capacitor, and through the volume potentiometer to the grid of the EL84. At the EL84, a similar electron flow takes place, but this time it's more powerful. Enough electrons will travel from the EL84's plate to the output transformer to drive the speaker.
Here we see a view of the AC4's chassis with the back panel removed (above), and with the chassis removed from the amp (below)—a design that makes it a bit of a chore to try out tubes from various manufacturers, both old and new stock.
The electrons don't stop at the output transformer, though. If you look at the schematic, you'll note that they pass through it and cycle back to ground. In a way, you can think of an amplifier as an electron circulator whose ultimate goal is to send electrons through the output transformer. Our job as guitarists is simply to get those electrons to do that in tune and with reasonable timing.
The “Vibrato Oscillator" Circuit
You're probably familiar with the mix up in terminology between "vibrato" and "tremolo." The 1960s Vox AC4 schematic used "vibrato" to refer to the oscillation in volume that is more commonly referred to as "tremolo." Because some of you may want to refer to the original AC4 schematic, we'll stick with the company's terminology here.
The AC4's ECC83 (12AX7) vibrato tube creates a low-frequency oscillation. That oscillating voltage is connected to the cathode of the EF86 tube, which affects the bias. Think of it as sending a very low-sound signal to the EF86's cathode—maybe 2–10 Hz (cycles per second). These frequencies are way too low for the human ear to detect, but they do affect electron flow in the EF86 from 2 to 10 times per second.
Components in More Detail
Now that we've got our quick overview of how an amp works out of the way, let's get into some more detailed descriptions, component by component.
Power transformer
The power transformer is the amp's larger transformer. It converts 120V wall voltage (240V in many countries) to a high AC voltage entering the rectifier (EZ80 in the case of the Vox AC4) tube. The transformer also supplies 6.3V AC to the filaments (heating elements) of the tubes. (Some rectifier tubes require 5V for the filaments, but not the AC4's EZ80 tube.)
Capacitors (aka caps)
Capacitors are shown in the schematic as two parallel lines perpendicular to the wiring. In some schematics, one of the lines may be curved. There are three types of capacitors in a guitar amp—filter, bypass, and signal—and their values are measured in microfarads, which are designated by the symbol μF.
Filter capacitors are large metal cylinders that, like batteries, hold a charge—even long after the amp has been unplugged. Unlike batteries for household items like flashlights and smoke detectors, they hold potentially lethal voltages. These are why you don't mess around inside your amp unless you know how to do so safely. The rectifier tube's purpose is to convert the AC voltage (a sine wave) into a constant DC voltage to power the tubes. The rectifier tube does a good but not perfect job. What emerges is actually a ripple-like DC voltage, so the filter capacitors help reduce the ripple by storing and releasing high voltages. Filter caps typically have values in the range of 8–50 μF, sometimes higher. The AC4 uses two 32 μF caps and one 8 μF cap. The two 32s are actually both inside one cylinder—i.e., they are a single component in the amp. The 8 μF cap is a separate component.
As previously mentioned, in an AC4 a resistor and a bypass capacitor are connected to the cathodes of the preamp tube and the power tube, wired in parallel—meaning, side-by-side. (In the AC4 schematic, the cathode is the lower element in the tube diagram.) Current flowing through a resistor causes a change in voltage. Cathode resistors are used to add DC voltage to the cathodes (2.7V for the EF86 and 8.5V for the EL84). The purpose is to make the cathode positive in relation to the grid. That cathode resistor, however, also resists the guitar signal's current flow. Hence, the parallel addition of a bypass capacitor. Since a capacitor will block DC but allow AC to freely pass through, the bypass cap does what its name implies—it allows the electrons needed for amplifying the guitar signal to bypass the resistor and flow freely through the cathode. In an AC4, the EF86 and EL84 bypass capacitors are both 25 μF. Larger values would let more bass through, while smaller values would reduce it.
Signal capacitors, meanwhile, are the small caps inside the amp, and they perform two critical functions. First, they block DC voltage while allowing AC voltages (like the guitar signal) to pass through. They also determine, according to their value, which guitar frequencies will pass through. In other words, signal caps define the tone of the amp. AC4 signal-cap values range from .1 μF–.001 μF. Smaller values (like the .001 μF cap on the AC4's tone control) allow only treble frequencies to pass through. Put another way, the tone control sends high frequencies to ground instead of letting them reach the power tube.
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.
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!