
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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Though Lou Reed seemed to only use a handful of chords, his shrewd right-hand strumming created limitless rhythmic variations.
Beginner to Intermediate
Beginner to Intermediate
- Focus on syncopation.
- Add muted strums to enhance your rhythm playing.
- Learn how to modify complex rhythmic patterns into smaller, more comprehensible figures.
Though usually more lauded for his lyrics than his guitar playing, Lou Reed had a distinctive style of strumming that can make replication a challenge and seemingly impossible for beginners to comprehend. With a combination of syncopations and muted strums, Reed’s rhythm guitar—from his time with the Velvet Underground to his solo career—has a groove and feel that all guitar players can learn from.
We’ll start with a relatively simple strummed hook that shows Reed’s roots. Ex. 1 is a variation on “There She Goes Again” from The Velvet Underground and Nico, which was recorded in 1966 and released in 1967. This strum was clearly inspired by Marvin Gaye’s “Hitch Hike,” as it duplicates Gaye’s intro almost exactly, albeit in a different key. (It is worth mentioning that “Hitch Hike” also inspired the Beatles “You Can’t Do That.”)
Ex. 1
Ex. 2 is the samething but performed with cowboy chords instead of barres.
Ex. 2
With Ex. 3, a la “Sister Ray” from 1967’s White Light/White Heat, we get into Reed’s more syncopated strums. Note that after the first beat, the emphasis of each strum is on an upbeat, including the changes from G to F, then from F to C, followed at the end by a typical Lou Reed-ism of strumming while in mid-change (though in all fairness, many guitarists do this). Thus, the last upbeat notes, which imply a G chord, are in fact a byproduct created by the switch, not a specific chord.
Ex. 3
Ex. 4 is the same strum but capoed at the 5th fret, using cowboy chords instead of barres.
Ex. 4
Our final Velvet reference, Ex. 5, comes from “Sweet Jane” (Loaded, 1970), which is a song I loathe teaching to students as it is deceptively tricky—it was the inspiration for this lesson—and can make one feel demoralized. True, the song has only four chords, but the syncopated switching and muted strums can cause problems for many players, and not just beginners. Take your time to get the nuances of the switches and mutes.
Ex. 5
Better yet, start with Ex. 6, which is a simplified version of Ex. 5, using cowboy chords instead of barre chords, as well as a more straightforward strum. True, it does not conform completely to the original, but it is a worthwhile starting point. Once this is mastered, try playing Ex. 5 with the open-position chords before moving onto the barres.
Ex. 6
Solo Years
Ex. 7 is a take on “Walk on the Wild Side” from 1972’s Transformer, arguably the most famous Lou Reed song. Here we’ve graduated from eighth-notes to 16th-notes in the right hand. Once again, the muted strums play a vital role.
Ex. 7
Like the previous two examples, you might want to start with the simpler Ex. 8, although the only change is that the mutes have been omitted. This example demonstrates how essential the mutes are, as the feel changes completely when you leave them out.
Ex. 8
Ex. 9 also comes from Transformer and is a variation on“Vicious.” Additional syncopations and muted strums, both different from “…Wild Side,” complicate what should be carefree.
Ex. 9
Ex. 10 is, again, a simplified alternative.
Ex. 10
Examples 11 and 12 demonstrate two options for fretting chords, using Reed’s “Leave Me Alone” from Street Hassle (1978) as a vehicle.
Ex. 11
Ex. 12
Our last example, Ex. 13, is based on Reed’s late-’80s “comeback” hit, “Dirty Boulevard” from New York. Here we not only get syncopations, but a rhythmic variation in the chorus—same chords, different rhythms. Pay attention to that quick, almost-ghosted B note on the “and” of 1 in measure two. It almost seems like a mistake but it adds so much.
Ex. 13
The Lou Reed Legacy
When most fans think of Lou Reed, the first thing that comes to mind is his pioneering subversive, transgressive, and poetic lyrics. But, clearly, that is not all there is to his work. In truth, Lou Reed is an incomparable musician whose guitar playing should not be underestimated. So, take this lesson as a starting point and search out more of his idiosyncratic strums. You will be well rewarded.
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Nashville luthier and guitar tech Dave Johnson shows us the baker's dozen of tools he thinks any guitar picker requires to be a guitar fixer.
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Paul Reed Smith cradles one of his company’s Charcoal Phoenix limited-edition guitars in front of a PRS Sonzera amp.
The storied guitar builder reflects on his dozen months sharing his experience, knowledge, and perspective with PG’s readers.
Over the past year, I’ve written a series of articles in Premier Guitar going over some of my, and our industry’s, views of guitar making. You can find all of them all online (and for those reading this online, the articles are collected here). What I am going to attempt to do for this final piece is boil down each of these articles to a “sort of” conclusion. I have enjoyed the process of writing and editing each one. For the most part, it’s been a lot of fun. So:
Tonewood Doesn’t Matter. Wood Does (August 2024)
We don’t use the word tonewood in conversation at PRS. It doesn’t capture a lot of my experience of which combinations of wood make good instruments. First and foremost, we are looking for qualities of wood over species, and we make sure we dry and treat each one appropriately.
What Makes a Guitar Worth the Price? (September 2024)
Very often, the price of a guitar does reflect how good an instrument is—and very often the price of the guitar has nothing to do with how good an instrument is.
What You Can and Can’t See When Buying a Guitar (October 2024)
This one’s a pet peeve. When making a purchase, you can clearly see the color of a guitar and how beautiful it is, but you have no idea if the neck is going to warp over time because of many factors, including wood drying and truss rod installment design. Trust your experience.
Does Where a Guitar Is Made Really Matter? (November 2024)
There are really good, really smart, really talented people all over the world. While I won’t deny it’s helped our career here at PRS being in the United States, I do not believe it is a truly defining factor of our quality. A good example would be the PRS SE Series.
The Complicated Beauty of Electric Guitar Pickups (December 2024)
We’ve taken so many good stabs at humbucking design, and I’m getting incredibly good feedback on our new McCarty IIIs. But making pickups is a complicated art. There are a lot of ingredients in the recipe.
“I hope these articles have had some positive impact on you as a group.”
In Guitar Making, It’s the Details that Matter (January 2025)
Well, that title says it. What’s interesting for me is that all guitar makers believe that different types of details matter more than others. At PRS, we have our own beliefs.
When Building Guitars—Or Pursuing Anything—Go Down All the Rabbit Holes (February 2025)
This one’s about learning. It’s a process I really enjoy. Whether your passion is guitar making, guitar playing, or something else entirely, there is always more to keep discovering.
Paul Reed Smith on Buying Gear (March 2025)
Very often at a clinic, the people who don’t play guitar have a less calcified view of the sound of the instruments being demonstrated than those players who have already developed strong ideas. On the other hand, one time a very experienced player was at one of my clinics and heard something he’d been looking for for a long time and did not expect it out of the guitar he got it from. Trust your experience—not your predetermined views.
Learning from the Mistakes of Guitar Building’s Past (April 2025)
I think my beginnings as a repairman gave me a lot of insight on this one. Even still, just the process of defining a past guitar-making mistake is its own art.
Paul Reed Smith on Where Amp History—and Tone—Begins (May 2025)
This one I love. It’s a greatly exaggerated version of mods you could do to a TS808 Tube Screamer, which has a history on the web. Amplifiers are “modulated power supplies” and can have so much impact on your tone.
The Lifelong Rhythm of Learning (June 2025)
In some periods of time, there’s a lot to be learned, and in some periods of time, you can barely hold what you had before. It’s different for everyone, and for me, it’s been up and down. No matter where you are in the process, there’s always good work to do.
I hope these articles have had some positive impact on you as a group. I know some things resonate with some people and not others, and the opposite as well. I haven’t gotten a lot of feedback from these articles except that there’s not an overwhelming chorus saying “what an idiot,” which I assume means there’s been some meat on the bone of some of the views shared. It’s been a joy. Thanks for listening.