
Our much-loved former columnist Jeff Bober returns to explain how to check and reset the bias of your amp’s output tubes—and delivers some potentially shocking warnings about a few common but dangerous techniques.
Hello again, Premier Guitar readers! It’s your old bud Jeff here, author of the once popular Ask Amp Man column. Editorial Director Ted Drozdowski asked me if I would be interested in writing about bias, and, of course, I said, “Sure, I know a thing or two about that!” So here I am, temporarily returning to these pages. Now, let’s get started.
What exactly is bias? Bias is prejudice in favor of or against … oh wait, wrong kind of bias. I think he wanted me to write about bias in a tube amplifier, which is far less polarizing.
Bias, as defined in the RCA Radiotron Designers Handbook, is “voltage applied to the grid [of a tube] to obtain a desired operating point.” Well, that is the most basic explanation, but for the most part it is good enough and pertains to the majority of tube output stages in our favorite tube guitar amps.
Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased.
Besides “applying” a voltage to a vacuum tube, however, biasing can occur in another way as well. There are quite a few amplifiers, such as a Vox AC15 and AC30, any of my Budda and EAST designs, and even most of the early, low-wattage amplifiers of the tweed era that use what’s known as a “cathode bias” design. This is where the current flowing through the tube (which attains the aforementioned “desired operating point”) is not set by the voltage “applied” to the grid of the tube, but is instead set by the resistor in the cathode leg of the tube. It’s a bit more complicated than that, but the result is an amplifier whose output stage is “self-biasing.”
Most amplification devices, including transistors and even preamp tubes, need to be “biased” in order to perform properly, but this type of biasing is fixed in the design parameters of the circuit. In the case of the preamp tubes in your guitar amp, bias is based on the value of the cathode resistor, among other things. But that’s enough design theory for today. Let’s get back to the core task of biasing the output tubes in most guitar amplifiers.
First, the bias voltages you see listed on many schematics, such as 52V on a black-panel Fender Twin Reverb or 51V on a Marshall 100W Super Lead schematic, are merely approximations of the voltages that should be expected in that area of the circuit. Setting the bias adjustment controls to these listed voltages in no way guarantees that your amp is properly biased. Tube bias is also dependent on the high voltage (or B+) applied to the plate of the output tube, which can vary within tolerances of the transformers as well as in the AC line voltage fed to the amp. (This is why amps can sometimes sound better in one room or club than others.)
But even more important to understand is that tubes produced in different factories across the globe will bias up differently! What I mean by this is, if you properly bias a set of output tubes—let’s say 6L6s made in Russia—and then you swap them out with a set made in China, in the same amplifier without changing the setting of the bias control, the end result will almost always be a different bias reading. This is why it’s always best to have checked and reset the bias whenever output tubes are replaced. Now, how do we do that?
The Preferred Method
Fig. 1
There are several different ways to measure output-tube bias current at idle. The safest method is to use what is commonly called a bias probe (Fig. 1). This is a device that is inserted between an output tube and its socket. (I typically make my own bias probes, but if you simply search “bias probe” online, you’ll find plenty to choose from. If you already own a multimeter, you can simply purchase the probes, but there are also options to purchase a full system with either a digital or analog meter, should you need it.) This device breaks the connection between the cathode (which is the metallic electrode from which electrons are emitted into the tube) of the tube and its ground connection, and inserts a small value resistor in between. It then allows the voltage across the resistor to be read. The resistor is typically 1 ohm and the resulting voltage drop across it is in millivolts (mV), so no chance of shock here. This provides a true and accurate measurement of the actual current flowing through one tube. Then, you set your bias and you’re done!
But even more important to understand is that tubes produced in different factories across the globe will bias up differently!
Ah, but wait! How do you set your bias? Let’s learn a bit more. Most tube amplifiers, if they are not cathode-biased designs, have some way to adjust the output-tube bias. One longstanding exception to this are most Mesa/Boogie amps. The bias voltage in these amps is not adjustable, which is why Mesa suggests only purchasing their tubes for their amps, because they are designed to fall within the acceptable bias range for their amps. This adds a certain degree of confidence for owner servicing, although, of course, it limits your options.
Let’s take a look, however, at a typical Fender or Marshall bias control. Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board. (You might want to go online to look at schematics for your amp to help you find it.) Either way, this is where you’ll make your adjustment.
To get started, you’ll most likely need to pull the chassis and place it in a stable work environment. Insert the bias probe device between one of the tubes and the socket (Fig. 2). Make sure all the volume controls are set to zero, turn the amp on, and let the tubes warm up. It’s also good to try to have a load on the speaker jack—whether a speaker or an appropriate resistor or load box. This is not 100 percent necessary for just setting the bias to a particular number, but sound checking is one of the ways I like to make the final adjustments, so being able to connect the speaker to the chassis while it’s on the bench is certainly a necessity for me.
Now, where to set the numbers? There are certainly more than a few opinions floating around on the interwebs about what optimal bias settings are. Some engineering types will tout 50 percent maximum plate dissipation or 70 percent maximum dissipation, and while it may look good or make sense on paper, I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal. It may work in the hi-fi world, where perfect sound reproduction is the goal, but guitar amplifiers are in the sound production business, so it’s a bit different. (In the most basic terms, maximum plate dissipation is the amount of power the plate of the tube is designed to deliver.)
Different types of output tubes have their own acceptable range of bias current. There are so many variables at play that there is no “correct” number. The plate voltage in the amplifier, the output transformer’s primary impedance, and the country of origin of a tube all factor into how it interacts with the voltage and output transformer to define what the optimal bias current will be. Below are the average ranges for some typical octal output tubes:
• 6L6: 25–35 mA
• EL34: 30–40 mA
• 6V6: 18–25 mA
• 6550: 35–45 mA
• KT66: 30–40 mA
Fig. 3
These should be the ranges in which these tubes will perform and sound the best, and they can be accurately measured with a digital multimeter. The best way for you to decide what setting is best for you is a combination of the reading on the meter and your ears! Using the bias control, set the bias to somewhere in the ranges given above (Fig. 3) and play the amp. Note: Some amps will act funny and develop horrible noises (parasitic oscillations) when a bias probe is in place while the amp is being played. If this is the case, you’ll need to remove the bias probe each time you play the amp.)
Move the setting a couple mA in one direction or the other and play again. Don’t expect extreme changes; that’s not what we’re looking for. Listen for subtle differences. Is one setting a little more or less harsh? Is the bottom end too soft or flubby? Is the amp as clean as you want it? Sometimes these little subtleties are what make one amp sound and feel better than another!
Most older Fenders have a pot with a slot for a screwdriver mounted to the chassis in the area of the power or mains transformer, while most older Marshalls have their bias pot mounted on the circuit board.
Also, you should be doing this at the volume you would typically use onstage or in the studio. You may not notice much change if your volume is at 1, but you want to optimize the amp for the way you will be using it.
Eyes Wide Open
Fig. 4
Knowing the ballpark bias numbers is good, and adding your ears is even better, but I also like to see what I’m hearing, so I always incorporate an oscilloscope when I’m setting the bias on an amp. I mentioned crossover distortion above, and when it comes to setting up amps for today’s pedal-hungry players, I find that setting the bias to where there is just a hint of crossover distortion at full output is what works best. Fig. 4 is what that looks like on the oscilloscope. This keeps the amp very clean and makes most pedal users happy.
By the way, here’s a mini primer in crossover distortion. In a push-pull output stage, which is found in most amplifiers with two or more output tubes, each tube (or pair of tubes) is responsible for amplifying at least half of the audio signal. If the tubes are not biased properly, one tube (or pair) will stop amplifying before the other tube (or pair) start amplifying. This will create crossover distortion. Proper biasing will allow the two halves to interact correctly. It’s like a nice firm handshake between both halves.
Beware These Old-School Methods
Let’s look at a couple popular methods that I do not recommend, but are worth discussing because they are, nonetheless, common. The first is: With the amp off and output tubes removed, use a multimeter to measure the resistance of each half of the primary side of the output transformer. This would typically be from the center tap to each side of the primary winding.
In the most basic terms, a transformer is a bunch of wire wound around a steel core. On the primary side of an output transformer, the center tap is the electrical “middle” of this long length of wire. This is typically where the high voltage is applied. The ends of this length of wire are connected to the plates of the tube, thereby applying the high voltage to the tubes. As an example, typically in most Fender amps, the center tap is red, and the ends of the primary windings are blue and brown.
Fig. 5
Next, install the output tubes, turn the amp on, and measure the voltage drop across each half of the output transformer with the amp at idle in operational mode (Fig. 5). Voltage divided by resistance will give you the DC current through the tubes. For example, 1.17V / 15.8R = 0.074, or 74 mA. The numbers I used here were actual measurements in one side (one half) of a 100W amp using four output tubes (two per side). So, divide the 74 mA by two, and you get an average of 37 mA per tube.
Next, you can try the shunt method. This requires a multimeter that can read DC current in milliamps (mA). Connect one meter lead to the center tap of the output transformer and the other lead to the output transformer’s primary side. Typically, in most amps using octal tubes (6L6, 6V6, EL34, 6550, KT88, etc.), this will be pin 3 on any output tube socket. Turn the amp on and, in operating mode at idle (i.e., volume off), measure the current across that half of the output transformer. For example, if your measurement is 72 mA and it’s an amp that utilizes four output tubes, the current measured is for two of those tubes, so once again divide by two to arrive at 36 mA per tube.
I’ve heard the result of guitar amplifiers designed by the book to optimal specifications … and to me they sound, well, less than optimal.
Both of those methods are very old school and still in practice, but I wouldn’t use either for two reasons: 1) I don’t believe they’re very accurate, and 2) they’re dangerous! You’re probing around inside the high voltage area of the amp, and one slip will either take out a fuse, take out a tube, take out your meter, or, worse case, let you know exactly what 450V DC feels like! So, although these methods are used, let’s just say no here.
Some Personal Insights
I’d also like to add a little personal experience to this procedure, based on decades in the biz. Back in the day, when I began servicing and modifying gear, guitarists were regularly playing 50- and 100-watt amps. (Everybody looked at me like I had three heads when I came out with the 18-watt Budda Twinmaster, but that’s a whole other story.) There were some overdrive and distortion pedals around (now all vintage), but certainly not the pedal proliferation we have now, so players were pretty much guitar, cable, amp … go! In these situations, I would most times run the tubes with a pretty hot bias so the amp would be fatter and overdrive a bit earlier and easier, as a decent percentage of the overdrive was developed by pushing the output tubes. As time went on, output attenuators became more popular, so amps could be pushed hard, but at more manageable volume levels. That was still a good scenario for a hotter bias of the output tubes in high-power amps. Eventually, players started playing lower-power amps, so they could open them up and get great output-tube distortion at lesser volumes. The problem is that hotter-biased low-power amps tend to get mushy and have less definition when pushed hard, so a more moderate bias setting is preferred here—just enough so there is no crossover distortion. Move up to today’s scenario and you’ll find that almost all overdrive and/or distortion is typically coming from a pedal. In that case, an amp is nothing more than an amplification device for pedals.
So, that’s what I’ve learned about tube-biasing from my decades of experience. But the bottom line is, there is no absolute right or wrong settings when it comes to biasing an amp. Keep your ears open and go with what sounds best to you.
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See and hear Taylor’s Legacy Collection guitars played by his successor, Andy Powers.
Last year, Taylor Guitars capped its 50th Anniversary by introducing a new guitar collection celebrating the contributions of co-founders Bob Taylor and Kurt Listug to the guitar world. The Legacy Collection revives five of Bob Taylor’s classic acoustic models, curated by the legendary luthier and innovator himself. “To imagine that we’re doing guitars that harken to our past, our present and our future all at the same time,” Bob says, “I really like that.”
In developing the collection, Bob preserved the essence of his originals while integrating performance and playability upgrades introduced during his tenure as designer-in-chief. “It’s an up-to-date version of what those guitars would be,” Bob explains, “but with the same sound.”
Visually, these guitars feel classic—clean, understated and unmistakably Taylor. While Bob’s original aesthetic preferences are showcased in his Legacy models, the nod to the past runs deeper than trade dress.
From his earliest builds, Bob favored slim-profile necks because he found them easier to play. That preference set a design precedent that established Taylor’s reputation for smooth-playing, comfortable necks. Legacy models feature slim mahogany necks built with Taylor's patented New Technology (NT) design. “My first neck was a bolted-on neck but not an NT neck,” Bob says. “These are NT necks because it’s a better neck.” Introduced in 1999, the NT neck allowed for unprecedented micro-adjustability while offering a consistent, hand-friendly Taylor playing experience.
What makes this collection unique within the Taylor line is Bob’s use of his X-bracing architecture, favoring his time-tested internal voicing framework over more recent Taylor bracing innovations to evoke a distinctive tone profile. Since Andy Powers—Taylor’s current Chief Guitar Designer, President and CEO—debuted his patented V-Class bracing in 2018, V-Class has become a staple in Taylor’s premium-performance guitars. Still, Bob’s X-bracing pattern produces a richly textured sound with pleasing volume, balance and clarity that long defined the Taylor voice. All Legacy models feature LR Baggs VTC Element electronics, which Bob says “harkens back to those days.”
The team at Taylor thought the best way to demonstrate the sound of the Legacy guitars was to ask Andy Powers, Bob’s successor, to play them. A world-class luthier and musician, Andy has spent the past 14 years leading Taylor’s guitar innovation. In addition to V-Class bracing, his contributions include the Grand Pacific body style, the ultra-refined Builder’s Edition Collection, and most recently, the stunning Gold Label Collection.
Below you’ll find a series of videos that feature Powers playing each Legacy model along with information about the guitars.
Legacy 800 Series Models
First launched in 1975, the 800 Series was Taylor’s first official guitar series. Today, it remains home to some of the brand’s most acclaimed instruments, including the flagship 814ce, Builder’s Edition 814ce and new Gold Label 814e.
The Legacy 800 Series features the 810e Dreadnought and two Jumbos: the 6-string 815e and 12-string 855e. Each model serves up a refined version of the Dreadnought and Jumbo body shapes Bob inherited from Sam Radding—the original owner of the American Dream music shop where Bob and Kurt first met. “I was making my guitars in the molds that Sam had made at American Dream,” Bob recalls. “There was a Jumbo and a Dreadnought. That’s all we had.”
All three Legacy 800 Series guitars feature one of Bob’s favorite tonewood combos. Solid Indian rosewood back and sides are paired with a Sitka spruce top, yielding warm lows, clear trebles and a scooped midrange.
Aesthetic appointments include a three-ring abalone rosette, mother-of-pearl Large Diamond inlays, white binding around the body and fretboard, and Bob’s “straight-ear” peghead design. Both Jumbo models also showcase a mustache-style ebony bridge—a nod to Bob’s early Jumbo builds.
Legacy 810e
The 810 Dreadnought holds a special place in Bob Taylor’s heart. “My first 810, the one I made for myself, was a thrilling guitar for me to make,” he says. “It’s the one and only guitar I played. It didn’t matter how many guitars we made at Taylor, that’s the one I took out and played.” The Legacy 810e brings back that bold, room-filling Dreadnought voice along with the easy playability expected from a Taylor.
Taylor Guitars | Legacy 810e | Playthrough Demo
Legacy 855e
Taylor’s first 12-strings found an audience in 1970s Los Angeles. “I was making guitars that would find their way to McCabe’s in Santa Monica and Westwood Music,” Bob says, “and these guitars were easy to play. Twelve-strings were a popular sound in that music. It was a modern country/folk/rock music genre that was accepting our guitars because they were easy to play. They also liked the sound of them because our guitars were easier to record.” The Legacy 855e, with its resonant Jumbo body, slim neck and gorgeous octave sparkle, carries that tradition forward.
Taylor Guitars | Legacy 855e | Playthrough Demo
Legacy 815e
The Legacy 815e revives Taylor’s original Jumbo 6-string, delivering a big, lush sound with beautifully blooming overtones.
Legacy Grand Auditoriums
In the early 1990s, Bob Taylor heard a consistent refrain from dealers: “Not everybody wants a dreadnought guitar anymore.” Players were asking for something with comparable volume but different proportions—something more comfortable, yet still powerful. This feedback inspired Bob to design a new body style with more elegant curves, more accommodating proportions and a balanced tonal response. The result was the Grand Auditorium, which Taylor introduced in 1994 to celebrate its 20th anniversary.
Thanks to its musical versatility and easy playability, Bob’s Grand Auditorium attracted a wide variety of players. “We came into our own with our Grand Auditorium,” he says. “People were describing it as ‘all around.’ It’s a good strummer and good for fingerstyle, but it’s not totally geared toward strumming or totally geared toward fingerstyle.” Also referred to as the “Swiss-Army Knife” of guitars or the “Goldilocks” guitar, the GA quickly became a favorite among guitarists across playing styles, musical genres and different playing applications including recording and live performance. “That guitar made studio work successful,” Bob says. It gained a wider fanbase with the debut of the “ce” version, which introduced a Venetian cutaway and onboard electronics. “That became one of our hallmarks,” says Bob. “If you want to plug in your guitar, buy a Taylor.”
Today, the Grand Auditorium is Taylor’s best-selling body shape.
The Legacy Collection features two cedar-top Grand Auditoriums inspired by past favorites: the mahogany/cedar 514ce and rosewood/cedar 714ce. Both models incorporate Bob’s original X-bracing pattern for a tonal character reminiscent of their 1990s and 2000s counterparts. Shared aesthetic details include a green abalone three-ring rosette, ebony bridge pins with green abalone dots, a faux-tortoiseshell pickguard and Taylor gold tuning machines.
Taylor Guitars | Legacy 815e | Playthrough Demo
Legacy 514ce
The Legacy 514ce features solid mahogany back and sides paired with a Western Red cedar top, yielding a punchy midrange and dry, woody sonic personality that pairs beautifully with cedar’s soft-touch sensitivity and warmth. It’s a standout choice for fingerstyle players and light strummers who crave nuance and depth. Distinct visual details include faux-tortoise body and fretboard binding, black-and-white top trim, and mother-of-pearl small diamond fretboard inlays.
Taylor Guitars | Legacy 514ce | Playthrough Demo
Legacy 714ce
The Legacy 714ce also features a cedar top, this time matched with solid Indian rosewood back and sides. The result is a richly textured sound with deep lows, clear trebles and a warm, mellow response. Inspiring as it is, this specific wood pairing isn’t currently offered in any other standard Taylor model. Additional aesthetic details include green abalone dot fretboard inlays, black body and fretboard binding, and black-and-white “pinstripe” body purfling.
While the Legacy Collection spotlights Taylor’s past, newer models from the Gold Label, Builder’s Edition and Somos Collections show the company’s legacy is always evolving. Explore the Legacy Collection at taylorguitars.com or visit your local authorized Taylor dealer.
Taylor Guitars | Legacy 714ce | Playthrough Demo
Detail of Ted’s 1997 National resonator tricone.
What instruments should you bring to an acoustic performance? These days, with sonic innovations and the shifting definition of just what an acoustic performance is, anything goes.
I believe it was Shakespeare who wrote: “To unplug, or not to unplug, that is the question. Whether ’tis nobler in the mind to suffer the slings and arrows of acoustic purists, or to take thy electric guitar in hand to navigate the sea of solo performing.”
Four-hundred-and-twenty-four years later, many of us still sometimes face the dilemma of good William when it comes to playing solo gigs. In a stripped-down setting, where it’s just us and our songs, do we opt to play an acoustic instrument, which might seem more fitting—or at least more common, in the folksinger/troubadour tradition—or do we bring a comfy electric for accompaniment?
For me, and likely many of you, it depends. If I’m playing one or two songs in a coffeehouse-like atmosphere, I’m likely to bring an acoustic. But if I’m doing a quick solo pop up, say, as a buffer between bands in a rock room, I’m bringing my electric. And when I’m doing a solo concert, where I’ll be stretching out for at least an hour, it’s a hybrid rig. I’ll bring my battered old Guild D25C, a National tricone resonator, and my faithful Zuzu electric with coil-splitting, and likely my gig pedalboard, or at least a digital delay. And each guitar is in a different tuning. Be prepared, as the Boy Scouts motto states. (For the record, I never made it past Webelos.)
My point is, the definition of the “acoustic” or “coffeehouse” performance has changed. Sure, there are still a few Alan Lomax types out there who will complain that an electric guitar or band is too loud, but they are the last vestiges of the folk police. And, well, acoustic guitar amplification is so good these days that I’ve been at shows where each strum of a flattop box has threatened to take my head off. My band Coyote Motel even plays Nashville’s hallowed songwriter room the Bluebird Café as a fully electric five-piece. What’s key, besides a smart, flexible sound engineer, is controlling volume, and with a Cali76 compressor or an MXR Duke of Tone, I can get the drive and sustain I need at a low level.
“My point is, the definition of the ‘acoustic’ or ‘coffeehouse’ performance has changed.”
So, today I think the instruments that are right for “acoustic” gigs are whatever makes you happiest. Left to my own devices, I like my Guild for songs that have a strong basis in folk or country writing, my National for blues and slide, and my electric for whenever I feel like adding a little sonic sauce or showing off a bit, since I have a fluid fingerpicking hand that can add some flash to accompaniment and solos. It’s really a matter of what instrument or instruments make you most comfortable because we should all be happy and comfortable onstage—whether that stage is in an arena or theater, a club or coffeehouse, or a church basement.
At this point, with instruments like Fender’s Acoustasonic line, or piezo-equipped models from Godin, PRS, and others, and the innovative L.R. Baggs AEG-1, it’s worth considering just what exactly makes a guitar acoustic. Is it sound? In which case there’s a wide-open playing field. Or is it a variation on the classic open-bodied instrument that uses a soundhole to move air? And if we arrive at the same end, do the means matter? There is excellent craftsmanship available today throughout the entire guitar spectrum, including foreign-built models, so maybe we can finally put the concerns of Shakespeare to rest and accept that “acoustic” has simply come to mean “low volume.”
Another reason I’m thinking out loud about this is because this is our annual acoustic issue. And so we’re featuring Jason Isbell, on the heels of his solo acoustic album, a piece on how acoustic guitars do their work authored by none other than Lloyd Baggs, and Andy Fairweather Low, whose new solo album—and illustrious career—includes exceptional acoustic performances. If you’re not familiar with his work, and you are, even if you don’t know it, he was the gent sitting next to Clapton for the historic 1992 Unplugged concert—and lots more. There are also reviews of new instruments from Taylor, Martin, and Godin that fit the classic acoustic profile, so dig in, and to heck with the slings and arrows!Ernie Ball, the world’s leading manufacturer of premium guitar strings and accessories, proudly announces the launch of the all-new Earthwood Bell Bronze acoustic guitar strings. Developed in close collaboration with Grammy Award-winning guitarist JohnMayer, Bell Bronze strings are engineered to meet Mayer’s exacting performance standards, offering players a bold new voice for their acoustic guitars.Crafted using a proprietary alloy inspired by the metals traditionally found in bells and cymbals, Earthwood Bell Bronze strings deliver a uniquely rich, full-bodied tone with enhanced clarity, harmonic content, and projection—making them the most sonically complex acoustic strings in the Ernie Ball lineup to date.
“Earthwood Bell Bronze strings are a giant leap forward in tone, playability, and durability. They’re great in any musical setting but really shine when played solo. There’s an orchestral quality to them.” -John Mayer
Product Features:
- Developed in collaboration with John Mayer
- Big, bold sound
- Inspired by alloys used for bells and cymbals
- Increased resonance with improved projection and sustain
- Patent-pending alloy unique to Ernie Ball stringsHow is Bell Bronze different?
- Richer and fuller sound than 80/20 and Phosphor Bronze without sounding dark
- Similar top end to 80/20 Bronze with richer low end than Phosphor Bronze
Brent Mason is, of course, on of the most recorded guitarists in history, who helped define the sound of most ’90s country superstars. So, whether you know it or not, you’ve likely heard Mason’s playing.
Professional transcriber Levi Clay has done the deepest of dives into Brent Mason’s hotshot licks. At one point, he undertook the massive project of transcribing and sharing one of Mason’s solos every day for 85 or so days. Mason is, of course, on of the most recorded guitarists in history, who helped define the sound of most ’90s country superstars. So, whether you know it or not, you’ve likely heard Mason’s playing. Levi shares the insight he gleaned from digging deep, and he tells us what it was like when they shared a stage last year. Plus, Levi plays us some great examples of Mason’s playing.