Jens’ two-microphone setup for resonator guitar and his Fender Super Reverb.
In a resonator, the metal cone underneath the front plate functions as a base for the bridge saddle and vibrates like a loudspeaker to project sound. Because of the instrument’s directional character and ease of feeding back, resonator guitars are difficult to use in live bands with loud stage volume. The goal is to get enough volume resonator onstage and in the room without feedback. Since it’s an important part of the instrument’s character, it’s necessary to capture some of that metal-cone voice within an overall balanced guitar tone that sits well in the mix. And you need to be able to achieve this in a way simple enough to reproduce the same tone night after night, and focus on your music, not technical problems.
The easiest way to mic a resonator guitar is to place a microphone on a stand about a foot in front of the guitar and run the signal into the mixer. But this pure acoustic route only works well in low volume situations—due to feedback—and requires you to play sitting down or standing still.
“Because of the instrument’s directional character and ease of feeding back, resonator guitars are difficult to use in live bands with loud stage volume.”
You can also use the type of clip-on microphone (often used for violin) made by Neumann and DPA, among others, to make you mobile, but if you get too close to the monitors or mains, feedback ensues. I recommend the microphones that come with a preamp and volume dial, to give you the most control. If you use one of these, point the mic directly toward the metal cone. Finding a nice tone will require some experimentation.
Another option is a piezo pickup. Some newer resonator guitars come with factory-installed piezos. It’s not very difficult to install a passive piezo yourself. You need to file and sand the bridge piece, detach the metal cone for wiring and soldering work, and, finally, drill a hole in the guitar where the jack plug goes in through the strap button. Getting the string tension correct over the entire length of the bridge saddle is the key to success. Lately, I have used hard oak as bridge material, which creates a smooth and mellow tone with bronze strings.
The author’s resonator “kit.”
Piezo pickups are less prone to feedback than acoustic microphones, so they are better for high-volume stage scenarios. But piezo pickups alone will not fully capture the tone of the resonator cone, so if you’re looking for more of that, add an acoustic clip-on microphone. That’s what I do. Both the piezo and the clip-on go to my Fender Super Reverb, and I use the normal channel for the feedback-sensitive acoustic microphone while the piezo goes to the vibrato channel with reverb. For dirty tones, I sometimes use an overdrive pedal for the piezo pickup. There are independent EQ and volume settings on both channels.
The third and last microphone technique I have experimented with is a humbucker. I bought one from 12 Bar Blues Pickups that is only 6 mm tall and built specifically for resonators. It fits nicely under the strings with enough clearance. The advantage with a passive magnetic pickup is obviously the simplicity. The installation process is easy if you simply tape or glue the pickup to your guitar and let the wire run externally on top of your resonator’s body. The kit I bought contains a small volume and tone box with jack input, and requires no battery. The result is, essentially, a hollow-body electric guitar suited for pedals and regular amps. It is also more resistant to feedback than both piezo and acoustic microphones. When I want to add some of the “bluegrass” flavor of the resonator cone, I add the acoustic microphone as mentioned earlier.
One drawback with conventional magnetic pickup technology is that bronze strings have a lower output, since bronze is less responsive to magnetic fields than nickel. I have kept the bronze strings on my resonators because of the great tone. To get even output on the bass strings, the bridge height measures a little higher on the bass side than the treble side.
So, there we have several different techniques for capturing modern resonator guitar tones, including a few options with guitar amps. Now, go experiment!
Now the world’s best-known 6-string duo, Gabriela Quintero and Rodrigo Sánchez have long-reaching roots that extend from metal to Irish folk music and distill into their unique take on nylon-string-acoustic-guitar music.
“It was just something to kill time,” admits Gabriela Quintero, one half of the Mexican guitar duo. “Just to be in the moment and not to think too much about it, even though here in Zihuatanejo it was more like the tropical version of the apocalypse [laughs].”
The other half of the duo, Rodrigo Sánchez, concurs that the pandemic presented a unique set of circumstances that allowed them to be creative without the added pressure of making a record, going on tour, or meeting a deadline. “Musically speaking, it was a very unusual process for us,” he says. “We weren’t really thinking about recording a new Rod and Gab record, and we didn’t really know what was going to happen. It was a really detailed process we never had done before, because we never had this amount of time to record an album.”
Rodrigo y Gabriela - True Nature (Official Audio)
"True Nature" is off Rodrigo y Gabriela's first album in 4 years. The album 'In Between Thoughts...A New World’ is available now on limited edition vinyl, CD...Guided by spiritual practices like Buddhism and nondualism, Rodrigo y Gabriela’s presence-of-mind approach to the guitar has led them on a fantastic, fulfilling journey from their humble heavy metal beginnings in Mexico City, to busking on the streets of Ireland, to performing in front of tens of thousands of people on the world’s biggest stages, opening for Muse and others.
Formed in 1998 out of the ashes of their heavy metal band, Tierra Ácida, Rodrigo y Gabriela left their hometown of Mexico City to pursue their musical ambitions in Dublin, Ireland, where they first began busking with their acoustic guitars on tourist-heavy Grafton Street, mixing elements of flamenco, rock, and heavy metal. In 2002, they released re-Foc, showcasing their virtuosity on guitar and their unique fusion of musical styles—even incorporating elements of the Irish folk music they had immersed themselves in while living abroad. In 2006, the duo released Rodrigo y Gabriela, a mix of original compositions and covers of classic songs by early influences Led Zeppelin and Metallica. The album was a commercial success, reaching the top of the Irish album charts and earning them a nomination for the Mercury Prize, awarded for the best album released in the United Kingdom by a British or Irish act. In 2008, they released 11:11, which featured 11 original compositions—each dedicated to a different musician who had influenced their music. In January 2020, Mettavolution, their fifth album, won Best Contemporary Instrumental Album at the Grammy Awards, cementing Rodrigo y Gabriela’s status as one of the most innovative and exciting guitar duos in the world.
“Gab has seven piezos inside her guitar, and everything is very tight. And I have five piezos.”—Rodrigo Sánchez
Gabriela Quintero’s Gear
Lead guitar provides the flash, but Gabriela Quintero’s right hand is what keeps the party jumping, with a driving, uncommon approach drawn more from traditional Irish music than flamenco.
Photo by Jim Bennett
Guitars
- Yamaha NCX5 Signature Model
Effects
- Boss FV-500L Volume Pedal
- Boss OC-3 Super Octave
- Boss TU-3S Chromatic Tuner
- Dunlop Cry Baby Standard Wah
- Dunlop DVP4 Volume (X) Mini Pedal
- Lehle P-Split III Box
Strings
- D’Addario Pro-Arté EJ45 Normal Tension
Self-produced by Rodrigo y Gabriela at their studio in the resort city of Ixtapa, Mexico, In Between Thoughts… A New World reasserts their seemingly innate ability for cultivating a musical repertoire that captures the zeitgeist. And while it may have begun without intention, that doesn’t mean In Between Thoughts lacks direction. Like its predecessors, there’s a familiar and explosive display of virtuosic guitar craft, including all of the hallmarks one would expect from Rodrigo y Gabriela. The powerful, percussive playing of Quintero and the deft melodicism of Sánchez remain the duo’s calling cards. But new, unexpected sonic elements abound as well, including the reverb-drenched slide guitar on “Egoland,” the energetic percussion on “Descending to Nowhere,” the kinetic electronic beats on “The Ride of the Mind,” the passionately chanted vocals of “Broken Rage,” and the dreamy mystique of the robotic vocal effects embedded within “Finding Myself Leads Me to You.”
In fall 2020, while recovering from Covid, Sánchez stumbled upon an online video on nondualism—the notion that there is a “single, infinite, and indivisible reality, whose nature is pure consciousness, from which all objects and selves derive their apparently independent existence,” as defined by author/teacher Rupert Spira. “Advaita Vedanta, or nonduality, is often called the direct path—accepting what is,” explains Sánchez. “We’re not saying that everything in this structure of the body/mind we live in is right. It is just what is, and we cannot really argue with that.”
“The beauty about music is that it’s always expanding.”—Gabriela Quintero
During the early stages of the pandemic, Rodrigo y Gabriela did what many other artists did: They turned to social media, posting short anecdotal performances from their studio. But when they finally got bored of that, they started to write music based on the concept of nondualism without really thinking it would become their new album. “It was just a project,” emphasizes Sánchez. “We were just here in the studio doing things that we would never dofor Rod and Gab. I started to work with electronics, I left my acoustic guitar [at home] and just took my electric guitars [into the studio]. We started writing the music at the same time as we were writing a story based on this philosophy that we were so much attracted to. If we had known that it was going to become the Rod and Gab album, we probably would’ve limited ourselves in terms of not using electronics, or not using too much electric guitar. But we didn’t really think that way. That’s how the album came about.”
Their new album began as a pandemic songwriting and recording project, and took shape almost by accident as they accumulated tracks and tunes.
As for Quintero, she took a slightly more pragmatic approach to the endeavor, particularly regarding nondualism. “I think me and Rod, we share a lot of things that we like, and we feel attracted to, but we process differently,” she explains. “That’s where the nondualism becomes dual [laughs]. I discovered these teachings through a book called The Power of Now [byEckhart Tolle]. To me, that book was incredibly insightful and practical, and such a ‘no rules’ type of thing. I tried to meditate but there was too much discipline with some of the spiritual teachings. I remember when Rod was into Buddhism, and he was meditating a lot of hours a day and learning some mantras that were very strict. And for me, it was too much of a discipline. When I discovered The Power of Now, it was like, ‘Oh great, you don’t have to basically do anything [laughs].’ And then, when the pandemic came in and Rod discovered these videos about nondualism, the way he presented them to me sounded super confusing and too much like nihilism. So, we were constantly having friendly debates here in the studio. And I was going, ‘This is too crazy.’ It felt to me that it was denying this existence. But then we discovered these are the same teachings as The Power of Now, but in different words, in a different way. Then we stopped the debates.”
Quintero, very late into their writing and recording process, asked Sánchez if they were, in fact, writing their next record. “And then she asked, ‘When are we going to record it?’” says Sánchez. “We’d been recording [what we were writing] from day one with quality, and so I went back to the studio that afternoon and I checked all the recordings and all the levels, and we had produced the album already. We had the record.”
“We love flamenco. My best friend in that scene, Vicente Amigo, is one of the best. But no, we never play flamenco.”—Rodrigo Sánchez
As for how they record, Sánchez says it happens all sorts of ways—sometimes tracking together, sometimes individually. Sánchez says the acoustic guitars get picked up by German-made Schoeps MK 4 mics, recommended to him by his close friend, Spanish guitar maestro Vicente Amigo. They also adopted some of what he calls his “old-school metal techniques” for recording. “Knowing that we were going to have orchestra and electronics and all that, I used room mics for Gabs—and instead of just copying her track, I have her record two guitars exactly the same,” he explains, noting he did not use the copy/paste shortcut many musicians use nowadays. “She would do one guitar rhythm and then she would double that to make it sound bigger. Overdubbing the same rhythms and the same parts actually give her much more presence on top of the electronics. And she’s so good at it.”
Due in large part to Quintero’s right-hand technique, which Sánchez recorded so well on In Between Thoughts, “heavy metal flamenco” is a label often applied to the duo. “Ah, the ‘F’ word,” laughs Sánchez. “We love flamenco. My best friend in that scene, Vicente Amigo, is one of the best. But no, we never play flamenco. I understand some people are confused because of Gab’s rasgueado[gesture to invoke her right-hand technique], but actually she’s not doing the flamenco technique at all. She learned most of these techniques from an Irish bodhrán player, Robbie Harris.”
Rodrigo Sánchez’s Gear
Rodrigo Sánchez wears his musical roots on his chest,
in a t-shirt proclaiming his fan status for the Bay Area metal band Testament.
Photo by Dan Locke/Frank White Photo Agency
Guitars
- Yamaha NTX5 Signature Model
- Fender Jaguar
Amps
- Fractal Audio Axe-Fx II XL+
- Marshall JCM900 4100 Hi Gain Dual Reverb
Effects
- Boss DD-3 Digital Delay
- Boss FV-500L Volume Pedal
- Boss OC-3 Super Octave
- Ibanez WH10 V3 Wah Pedal
- Lehle P-Split III DI Box
- MXR M133 Micro Amp
- MXR M234 Analog Chorus
- One Control Minimal Series AB Box
- TC Electronic Ditto X2 Looper
- Truetone 1 Spot Pro CS7 Power Supply
- TWA WR-03 Wah Rocker
Strings & Picks
- D’Addario EXL115 (.011–.049
- D’Addario Pro-Arté EJ46 Hard Tension
- Jim Dunlop Jazz III Black Stiffo
The bodhrán is a frame drum used in traditional Irish music that Quintero learned about when they moved to Ireland. “At the time, I was trying to imagine how flamenco players played their rhythms,” she explains. “I couldn’t figure it out, because back then there was not YouTube—there was nothing. Nowadays, you can go and say, ‘How to play rasgueado flamenco, how to play rhumba,’ and you’ll find something, but not back then. And I always got it wrong. And then I discovered the bodhrán.”
In the old days, the bodhrán was played with hands, not with a stick, as is often seen presently, and she says the Irish kept telling her she actually exhibited the movements of a bodhrán player, but on guitar. “They encouraged me to do certain rhythms. So, just watching them, it was easy to emulate a lot of the movements—it just came organically. The beauty about music is that it’s always expanding.”
“If I came back to a solo bit or something, there was not that beat—people were not jumping anymore, and it was like, ‘Ah, we’re losing the audience,’ so I tried to become more the drummer of the band.”—Gabriela Quintero
After weaning his guitar craft on West Coast thrash metal bands Testament, Megadeth, and Slayer, and New Yorkers Anthrax, Sánchez’s nylon-string style was originally grounded in a lot of the palm-muting he carried over from that style of electric playing. “First of all, I had to translate my palm muting [from electric to nylon string],” he explains. “Then, I used a little bit more of Al Di Meola’s technique, but he was playing steel-strings, right? So, I was like, ‘Okay, how can I translate this into nylon?’ And then I started to listen to Strunz & Farah, and they are incredible. I listened to the way they played, especially Jorge Strunz, who is so clean and so fast. And I started to learn some of his licks here and there, so I was in that zone already.”
They want a whole lotta folk! Rodrigo y Gabriela get down on the Newport Folk Festival’s Harbor Stage in 2014.
Photo by Tim Bugbee/Tinnitus Photography
It’s worth noting that the nylon-string guitars Rodrigo y Gabriela play live are the result of years of practical research and application in collaboration with Yamaha and are not models or designs your average nylon-string player would use, nor are they commercially available. “It’s not like any nylon-string guitar can just go and play in the middle of a festival of 40,000 people,” explains Sánchez. Originally, they were using guitars made by Irish luthier Frank Tate, which they still use at home and in the studio. But the guitars they now use live were specially designed over a 14-year period by Yamaha’s Japan-based Custom Shop for arena-concert environments. “These guitars have a very special system for us to sound the way they sound live,” he says. “Gab has seven piezos inside her guitar, and everything is very tight. And I have five piezos, which is really important for those kinds of shows.”
Within the duo, both players are very melodic and very rhythmical, but Quintero did gravitate to doing more of the beats, simply out of necessity, once they started playing bigger shows. “At the beginning when we used to play together, we swapped all the time—solos, arpeggios, and all of this,” she explains. “Eventually, when we started playing rock festivals, because I was the one who played the chords and the beat, if I came back to a solo bit or something, there was not that beat—people were not jumping anymore, and it was like, ‘Ah, we’re losing the audience,’ so I tried to become more the drummer of the band.”
Jumping from a metal band in Mexico City to an acoustic guitar duo busking the streets of Ireland seems quite serendipitous and grounded in the kind of ideology they eventually discovered via nondualism. Circling back to Quintero’s The Power of Now-influenced, pragmatic approach, she jokes that the decision was really quite simple. “Eventually, we were so internationally non-famous and miserable, we decided we’re going to quit the band,” she chuckles. “But we’re not going to quit music. We wanted to travel the world. So, our new goal was to travel and play guitar.”
YouTube It
While this live performance doesn’t capture the duo’s current blend of acoustic and electric sounds, it does afford a close-up look at their playing technique. In particular, check out Gabriela’s right-hand approach, which is based on the traditional Irish instrument called the bodhrán.
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.
Fig. 2
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.