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.
Words of wisdom from the legendary engineer, proprietor of Chicago’s Electrical Audio, World Series of Poker champion, and, in the band Shellac, the compass for brutal guitar aesthetics.
“All day every day, we’re grinding it out,” says engineer Steve Albini of his team at Electrical Audio, the Chicago studio he built and has run since 1997. “We’re constantly in session, constantly under fire.”
While it might be tempting to geek out and ask Albini about all the iconic albums that he’s recorded with the utmost finesse—and surely, there would be value in rapping about recording some of the biggest names in guitar music—that’s all been done.
What’s much more interesting is the work that goes on every single day at the studio. So, when he tells me, “My colleagues at Electrical Audio and I are constantly having to interrogate our methods and validate the things that we’re doing and come up with arguments for why we should do things this way or that,” that’s the stuff I want to know about. If you want to learn about how he recorded In Utero, go listen to Conan’s podcast. (Albini was a guest, along with Dave Grohl and Krist Novoselic, on the October 23, 2023 episode of Conan O’Brien Needs a Friend, and it’s a thorough discussion that is totally worth checking out.)
The fact is, Albini has recorded countless records. I’m sure he has a tally in his books somewhere, but it would be exceedingly difficult to know for sure how many albums he’s engineered. That’s because, as extensive as his credits are in various record-collecting resources, he’s also impressively accessible as a for-hire engineer. All levels of artists—from superstars to harder-to-track, mostly unknown road dogs—have carved out their time with him. Plus, he’s been at it since he started renting four-track demo packages on the weekends during his high school years in Missoula, Montana.
The body of engineering work that Albini has amassed is monumental not just in size, but also in musical scope, which extends further than from Sunn O))) to Magnolia Electric Company, Cheap Trick to Neurosis, or Low to The Thing. And with those artists, he often helps capture a landmark album, or at least the record that fans refer to as “the one they recorded with Albini.”
Aside from his day job, there’s also his guitar playing. Albini serves as the compass for cutting, brutal tone in the punk and underground rock scenes. Since his early days in the hard-hitting Big Black through his continuing work alongside bassist Bob Weston and drummer Todd Trainer in the band Shellac—whose soon-to-be six (no info yet, but a new one is confirmed to be on the way) full-length albums and some other recorded odds and ends are maybe the purest documents of his overall sonic aesthetic—he’s used a fairly concise rig of well-suited esoteric gear to shape his incisive, metallic, and esoterically personal guitar sound in the creation of angular riffage and gnarly feedback.
Albini’s sonic mastery seems to know no bounds: He’s probably the most-cited proponent of analog recording. His live-band-in-a-room sound is unparalleled. And his drum sounds are peerless. But, while it’s not as if he never talks about guitar, it’s rare to see him dive deep on his guitar-specific processes.
So, we called up Electrical Audio and had a chat about his methods on recording guitars and how they’ve evolved, his take on modern guitar culture, and the definitive details of his sound.
In addition to his gear collection, Albini is also a good-bandname-T-shirt collector. (If this isn’t proof enough, go look up a photo from when he won his first gold bracelet in the World Series of Poker.)
Photo by Daniel Bergeron
When you’re going to record a guitarist, what’s your process of deciding how you’re going to choose a microphone, and how you’re going to mic their amp or cabinet?
Steve Albini: You have a conversation—what kind of sound are they shooting for? Who are some guitarists whose sound would be appropriate for their music? That sort of thing. And sometimes that’ll give you a clue about how to get started.
If they like a thick, bass-y, chewy, distorted sound, you want to make sure you’re using mics that capture that low-frequency stuff with good definition and not get muddy or soft. If the kind of guitar sound they’re shooting for is very bright and very crisp and dry, you’ll want to make sure to avoid using mics that can have some resonance or bloom to them that will soften that sort of precision.
I think it’s a bad idea to have a standard method where when whatever guitarist walks in you stick an SM57 on it and call it good. A lot of people do that as a default just because it resolves the issue quickly, and they can get on with their day and do more fancy stuff. But I think it’s absolutely critical to pair the microphone with the actual sound that’s coming out of the amp.
After having a conversation with the guitar player, understanding what their aesthetic is, I have them set up their gear and just play a bit, to get a feel what their playing style is like. Are they using a lot of feedback and sustain or are they hopping on a bunch of different pedals all the time? Is the sound derived from their playing style or from particular layering of pedals? Getting intimate with the exact specifics of the guitar style and sound and aesthetic guides you on what microphones to use and physically where to put them.
The main thing is not to have a preconceived notion about what mics are good for guitar. I’ve used everything from vocal-caliber condenser microphones to quite limited electret microphones to high-quality ribbon microphones to pawnshop junk microphones—I’ve used absolutely everything you can imagine on a guitar amp, and that selection is always based on the aesthetic of the person playing and then the actual sound that’s coming out of the cabinet. In your mind, you might have an idealized notion of what a heavy guitar sounds like or what a clean guitar sounds like, but until you get down on all fours and listen to the sound coming out of the speakers, you don’t really know what you’re dealing with.
“When you listen to the speaker when the guitar player is playing, the sound that’s coming off—you should consider that the goal. What you’re trying to do is you’re trying to make that sound happen in people’s homes.”
How do you interpret what you’re hearing then?
Albini: When you’re down on all fours listening, you need to be forming a mental image of what that sound is like. Are there spikes and dips in the frequency response? Is there a lot of granular treble detail? Is it a really smooth sound? Does it have a sort of billowing quality, like a trombone-like fundamental, or is it really dry and raspy? Even using wine-tasting words like that, it helps you form an internal image of what that guitar is supposed to sound like when you hear it on playback, and from your experience with your mic collection, you’ll know what microphones are best suited to sounds like that, or you’ll know where to start anyway.
When you listen to the speaker when the guitar player is playing, the sound that’s coming off—you should consider that the goal. What you’re trying to do is you’re trying to make that sound happen in people’s homes.
Steve Albini's Gear
Hands on faders, Albini and his team at Electrical Audio are “constantly in session, constantly under fire.”
Photo by Kevin Tiongson
Guitars
- Travis Bean TB500
Amps
- Tapco/Intersound IVP Preamp
- Fender Bassman
- Custom homemade speaker enclosure based on Electro-Voice TL Series plans with 10" and 12" Celestion Greenbacks
Effects
- Interfax Harmonic Percolator
- MXR Smartgate
Strings and Picks
- Ice picks with the points cut off
- D’Addario XLs (.012–.016–.020w–.028–.038–.048)
Once you’ve chosen a mic, what’s next in the decision-making process?
Albini: One thing that I do that I think is probably distinctly different from what a lot of other engineers do, I tend to have whatever microphone I’m using on the guitar in the middle of the speaker cone, and I don’t generally use microphones pressed up close to the grille cloth right next to the speaker. I tend to use microphones at a working distance of between eight and 14 inches from the cabinet.
A lot of engineers made their bones as live engineers, where they’re trying to get isolation on stage, so they have the mics as close as possible to the speaker cabinet, and that practice translated into the studio. I experimented with that technique because I saw everybody else doing it, but I just never got good results with it. It always sounded slightly tweaked and muffled and weird. I found that when I put the microphone dead center on the speaker, then the sound hitting the microphone sounded more like what I heard when I was down on all fours listening to the speaker myself.
Working distance has a big effect on the sound quality. If the microphone is choked up tight on the speaker, you get a lot more low-frequency energy. You get a lot more muscular pumping low end from the proximity effect of the microphone, and, especially with ribbon microphones that are bi-directional and have a fairly exaggerated proximity effect, you can really use that to tune the response of the microphone. So, I say that I use a working distance of between eight and 14 inches. If I’m in the closer part of that range, six to eight inches from the speaker, there’s going to be a lot more of the sub low end emphasized in a bi-directional ribbon microphone, and that can be great to add weight and heaviness to a heavy guitar.
Then, if the microphone is backed off more like 12 to 14 inches, then you get much more of an overall picture of the sound of the cabinet, where it’s not emphasizing any particular region, for lack of a better word. It’s a flatter representation of the sound coming off the speaker. Being able to tune the behavior of the microphone by moving the microphone in and out just by a matter of inches can make a noticeable difference in the sound quality.
At this point in your career, do you know what mic to use as soon as you listen to someone’s playing?
Albini: It’s really rare for me to listen to a speaker, listen to somebody playing guitar, grab a microphone, put it up, and have it be right in the first instance. When that does happen sometimes, you feel like a fucking genius. That’s really satisfying. That means the first 30 years of your career weren’t wasted, but it doesn’t happen often.
Often, you have to move the microphone, or sometimes you have to swap the microphone out completely, like this microphone just can’t handle that much high end, it sounds too raspy, it’s just too midrange forward, it starts to sound nasal and different parts of the playing vocabulary can sound different as well. Sometimes, you’ll have a setup that sounds amazing when the guitarist is just playing rhythm stuff, but then when they go up the strip and start showing off, it can be too piercing or too woolly sounding, so it’s often a good idea to have a complement microphone.
“It’s really rare for me to listen to a speaker, listen to somebody playing guitar, grab a microphone, put it up, and have it be right in the first instance. When that does happen sometimes, you feel like a fucking genius.”
You’ll have a couple of microphones in the same position, one that is maybe a brighter, drier sound and one that’s maybe a fatter, darker sound. And that way you can either balance those microphones against each other for a composite sound or use them in stereo to synthesize a stereo image. Or when the lead kicks in, you can nudge the brighter microphone for a little bit more bite and attack.
I think having an ambient character available on the recording often helps with the sense of realism. If you’re just using a single guitar, for example, then having an ambient microphone that you can use to create a stereo image helps add to the sensation of hearing the sound in a room, even if it’s a very dry room. Having close mics on the amp and then also having a distant mic out in the room eight or 10 feet away gives you a little bit of air on that secondary mic, which you can then use to create a stereo image to help localize the guitar in the stereo image of the whole thing.
All of those little things, if you don’t have it set up so you have those kinds of options available, then you can’t make those choices down the road. I have been in sessions where some engineers have an array of microphones around a speaker cabinet. They’ll have eight or 10 microphones in a sort of swarm around a speaker cabinet. And that, to me, just speaks of really poor decision making. If you’re recording eight or 10 microphones at once and with the idea that you’ll sort it all out later, that just puts all your critical decisions off until the last minute and means that you’re going to make those decisions poorly. I think it’s much, much better to listen to it on the first playback and decide if you are on the right track or not. And if you’re not, just stop and fix it. Don’t just carry on with the plan to deal with it later, because when you get to later, you just have way too much shit to deal with.
Onstage with Shellac, Albini wields “Old Ironsides,” his Travis Bean TB500. Behind the guitarist lurks his customized amp head, which contains a Tapco/Intersound IVP Preamp and Fender Bassman, and his homemade speaker cabs.
Photo by Tim Bugbee
It’s like it creates option anxiety, and at that point, you’re just making the mix all that much longer.
Albini: Yeah, it’s not even the amount of effort that it takes. I don’t think it’s possible to make 10,000 critical decisions simultaneously and have them all be as valid as if you were to make those decisions one at a time as they came up with all your attention and full consideration. So, while you’re setting up the guitar, figure out which microphones you want to use and commit to them, and once you’ve committed to them, then that decision is made, and you can just get on with your day and you don’t ever need to re-litigate those decisions.
I also feel like over the course of working on a record, you get acclimated to the sound that you’re listening to, and then that becomes the basis on which you make other decisions. If you pull the plug on that by changing the sound around at the last minute, then all of those predicate decisions that were made based on that original place keeper have somewhat been invalidated. And I think that’s a dangerous thing as well.
In one of the videos on the Electrical Audio YouTube page, you talk about developing your practices through an iterative process of trial and error. Is experimentation still a part of your process?
Albini: I actively question myself and verify my preconceptions or challenge my preconceptions. One nice way to do that in a kind of a programmatic way is something I stole from Bob Weston, the bass player in the band Shellac that I’m in. He’s also a fine recording engineer and mastering engineer. I read an interview with him maybe 15 years ago where he said that on every session he does, he tries to do just one thing that he’s never done before. It might be the choice of microphone or positioning a microphone or a processing choice or a routing, just something, and that seemed brilliant to me. Just a very simple way to make sure that you’re always expanding your repertoire and always expanding your knowledge base. You don’t get set in your habits. And so, I stole that, and I do that to this day.
“While you’re setting up the guitar, figure out which microphones you want to use and commit to them, and once you’ve committed to them, then that decision is made, and you can just get on with your day and you don’t ever need to re-litigate those decisions.”
Also, microphones come in over the transom. There are microphones being designed and invented every day, and we get a chance to hear a lot of those either as trial or because people want our opinions on them. They’ll send them to us to put them in use for a while and play around with them. So, I get to play around with stuff that I’ve never heard before pretty regularly, and I like to try microphones I’ve never heard before.
This has proven enormously valuable over the course of the last 15 or 20 years. My routine behaviors have changed quite a bit as a result of these little, tiny experiments that I’ve done one at a time.
With Shellac bandmates Todd Trainer (drums) and Bob Weston (bass) in view, the most legendary Harmonic Percolator is at Steve’s feet, next to his MXR Smart Gate. If you’re wondering, Albini uses a waist strap for his guitar.
Photo by Jordi Vidal
I would imagine that, making as many records as you do, that’s like constant revision.
Albini: I promise you, the moment you get complacent about how you do things, someone will show up with a rig that’s freakish in a way you’ve never encountered before.
I did an album with the group Sunn O))). Their music is really slow-moving, impossibly heavy riffs, but the sound is really minimal. It’s just two guitars most of the time. In the studio, they added a few guests. One of is Hildur Guðnadóttir; she plays the cello.
There’s an instrument that was invented for her by a friend of hers called the halldorophone. It’s an electric cello that has built into it an amplifier and loudspeaker, so it’s a self-resonating, self-feeding-back, infinite-sustain cello. It’s a super bizarre thing, but she’s an expert. There’s one in the world and I’m staring at it and I have to figure out how to record it.
The fact that I am confronted with these new and different things all the time means that my vocabulary and my skillset and my facilities are constantly being tested and improved. And that’s one of the great joys, for me anyway, of doing what I do for a living, that I do get to do these freakish things once in a while.
You use a small pedal setup as a player, but you’re engaging with different kinds of players all the time. What do you think about modern pedal technology?
Albini: The stage that we are at now, where every player in every band has a pedalboard and have this sort of a curated collection of sounds that they come up with, I actually got a preview of that in the late ’80s. The first time I went to Japan, most guitarists that I worked with had a pedalboard with a half a dozen pedals on it, and that’s how they would craft their sound. They could bring that anywhere and plug it into any amp and they’d be happy.
Something very similar is happening now in the U.S. where a lot of people are doing demo recording at home through modeling amps or through interfaces, and rather than using an amplifier for its inherent qualities, they’re kind of defeating the amplifier by using pedals as the principal source of their sound. It’s a trend. I don’t really have an opinion about it.
“I promise you, the moment you get complacent about how you do things, someone will show up with a rig that’s freakish in a way you’ve never encountered before.”
There are some people who are more adept at it than others, but it’s absolutely the case that most players in most bands now have multiple pedals that they’re using, and the songs are arranged in a way where you use this combination for this part and this combination for this part. And nothing about it seems bad to me. It’s a little more cumbersome, especially when you’re in the studio and you’re trying to track down problems. But when you see somebody who’s really put some thought and attention into it and they’re really using the pedals in an expressive way….
I did a session with Reba Myers from Code Orange. She has this really expansive pedal setup where she’s got a main soundboard where the general tone for a given song comes from, and then she’s got a kind of an expression board, which is just all the crazy shit, and she’s constantly going back and forth. She’s an example of someone who’s put a lot of thought and attention into the specifics of the pedalboard and is using it as a creative tool. I’ve seen other people where it’s kind of pro forma—like, Kiss wore funny outfits on stage, and so for a while a lot of bands felt obliged to wear funny outfits on stage.
I know some old school guys are like, ‘Plug the guitar straight into the amp, and if you can’t get it done with that, you’re not a real musician,’ or whatever. That’s horse shit. That’s just boomer shit. I’m not into that at all.
How Steve Albini Gets His Guitar Sound
How did your personal guitar sound develop over the years?
Albini: When I was in Big Black, that band was predicated on the do-it-cheap, do-it-quick, take-no-prisoners approach. That was very much the cornerstone of the behavior in the punk rock scene. Don’t try to get it perfect, just get it. So, everything about that band was done sort of extemporaneously. I made the first Big Black record on my own in my apartment, so I needed an amplifier that I could use for either guitar or bass. I stumbled onto this bizarre preamp called the Tapco/Intersound IVP. It had a clean channel and a distorted channel. I didn’t find much use for the clean channel, but the distorted channel sounded great on either bass or guitar—or great toward my aesthetic at the moment, which was a pretty brutal one.
When Shellac started, I was looking for a fatter, fuller sound than the scrabble-scratchy sound I had with Big Black. I eventually gravitated toward the Fender Bassman as the perfect tube amp for me. But when I would play just the Bassman, I missed a little bit of the bite and the sizzle from the old transistor days. So, I ended up making a hybrid setup with the Tapco IVP preamp, typically recorded direct. And then on stage, I’ll have a monitor cabinet for it that has a horn in it, so it’s like a full-range speaker, and the Fender Bassman going into a fairly bass-y cabinet, typically a 4x12 when we’re on tour in Europe and we’re using backline.
The cabinets that Bob and I made for our amps—I have two Celestion greenbacks in that, a 10" and a 12"—are based on the TL series cabinet that are the published plans that Electro-Voice made available for using their speakers in an enclosure. If you just built a cabinet along those published plans, you would end up with exactly what Bob and I use for our speaker setups.
When you record yourself for a Shellac album, do you always use the same gear?
Albini: No, it has been different on literally every session. I often use the amp that I use on stage. Often, I do not. Often, I’ll use some other transistor amp and some other tube amp as the two complement signals. It’s essentially always two amplifiers, a transistor amp and a tube amp. The transistor amp is typically being recorded direct, and the tube amp is always recorded acoustically through a speaker cabinet with microphones and stuff.
But I have used an Orange OR80. There’s an amp that was made by a company called Sam Amp, and I believe there are very few of them in the world, but I ended up with one of them, and I’ve used the Sam Amp. I’ve used the Traynor YBA-3, Traynor YBA-1, a Marshall JTM-45. I’ve used a lot of different amplifiers for the studio recordings.
The Travis Bean that I use is such an indestructible sound. It’s weird that I’m so fussy about my amp because I’ve demonstrated myself that it kind of doesn’t matter what amp I play through, I can always get something that I like out of it.
We did a tour of Japan very early in the band’s tenure, right after we started. In Japan, it’s normal practice for the venue to have a backline. Every night it was a different, quite crappy by our standards, amplifier on stage. One night, it was a Roland Jazz Chorus. I used a Guyatone amplifier several times, and other Japanese brand names that I was unfamiliar with. Every night sounded fine. As specific as I am about what I like and don’t like, I have sort of taught myself that it’s not that important and that I can zero in on what I like and don’t like about even an imperfect setup.
For pedals, do you use anything other than the Harmonic Percolator, which you’re most known for using?
Albini: I’ve used a noise gate since I first started playing on stage. For many, many years it was just one of the original old-school MXR noise gates. They’ve all crapped out and been repaired and crapped out again many times. There’s an updated version of that MXR called the Smart Gate. I switched over to that. It’s set so that I can just touch the guitar and it opens up, but if I’m not actively playing it, it doesn’t open.
The output of the noise gate goes into the fuzz tone. And the fuzz tone has been a Harmonic Percolator [made by Interfax] since, I want to say, 1986. My friend Jay Tiller from Milwaukee worked at a head shop, record shop, and pawn shop combo in Milwaukee called Record Head. When I was there one time, he said, ‘We have this cool fuzz tone this guy made here. You should try it out.’ And I loved it. So, I bought one from him, and then over the years, I’ve bought a couple more when he stumbled across them at record swap or whatever, or at guitar fairs or whatever, he’s picked them up and I got ’em from him.
I’ve referred to the Percolator as a labor-saving device, because as soon as you hit the switch, the guitar just starts playing. I don’t even need to tell it what notes or anything. It just goes, and that’s my favorite thing about the Percolator, how it’s completely unhinged using it for feedback or whatever. It will choose little melodies that it wants to play, and it’ll just whistle them for you. But you kind of need to be physically moving. I’ve noticed that if you stand in one spot, it just squeals. But if you’re moving around, if the distance between you and your amplifier changes, then the fundamental frequency changes from the physical distance, and you get these really great psychedelic melodies that it creates.
Have you played any of the Percolator clones?
Albini: They all sound very slightly different, but they’re all basically the same. Chuck Collins made a complete, meticulous resurrection of the Harmonic Percolator [through his company, Theremaniacs] a few years ago—those are absolutely perfect. They respond exactly the same way. They sound the same. Almost all the others that I have seen—people send them to me because they feel like I should pass my hands over their Percolator or whatever, I’ve had maybe six or eight others—I can’t use any of ’em. They all behave differently somehow.
I think one of the perversions of my setup is that coming out of the noise gate, the signal into the Percolator is buffered, so it sounds different if you just plug your guitar straight into it, and I never do that.
How to use a Super Reverb, with a slew of pedals quietly looped in, for late-night practice.
Hello Amp Man readers. This month I’ve picked two interesting questions, and since the first references a Fender Super Reverb, I’m going to answer both based on that amp. So read on!
Question #1
Dear Amp Man,
I’ve been maintaining and modifying my meager amp collection for a few years and studying amp technology/topology. You know … for fun. (Who doesn’t have a folder of Fender schematics near the john?)
One aspect of modern amp design that I don’t understand is the headphone out—especially with respect to the load (the output not “seeing” the amp’s speaker). How does it work? Is it something that can be added to an amp? Would I want a wire connecting a Super Reverb and my ears?
Cheers,
Bill Fugate
Okay, let’s get started with question #1. As far as “would I want a wire connected between a Super Reverb and my ears?” Well, that’s for you to decide, but if you choose “yes,” here’s how to do it.
Fig. 1 is a simple circuit I came up with to allow you to play your amp through your headphones without waking the kids or disturbing your neighbors at 2 a.m. So you can understand what’s happening, I’ll go through it and describe the function of each component.
Fig. 1: This diagram illustrates the modifications required to add a headphone out to a Fender Super Reverb.
Jack 1 is the input from your amplifier’s speaker output, at left in Fig. 1. The resistance value should be as close to the output impedance of your amp as possible. For a Super Reverb, that would be 2 ohms. If it’s not possible to get the exact resistance, it’s okay to go up in value, but not down. This resistor will be replacing the speakers in your amp, which need to be disconnected, so it will be absorbing your amp’s full output power. I recommend that the resistor’s power-handling capability be at least double your amp’s output power. A Super is rated at about 40 to 45 watts, so I recommend at least a 100-watt resistor here. Also, if you’re using the large, gold anodized aluminum resistors, they get very hot and need to be mounted to properly dissipate the heat. If you’re building this inside a Bud box or something similar, mount the resistor to the box and be sure to install feet on the bottom of the box so that the heat doesn’t damage what the box is sitting on—like possibly your amplifier. It would also be a good idea to vent the box.
Next, the signal goes to the volume pot. A 1k-ohm, half-watt or higher, linear pot works fine here. A 22-ohm 1-watt resistor gets connected to the pot’s counter-clockwise arm to provide a little isolation from the amp chassis and add a bit of a signal drop.
Then the signal from the pot’s wiper gets connected to the headphone output jack through a 100-ohm resistor. For testing, I used a set of Tascam headphones with a 32-ohm impedance, which is pretty standard today, and the 100-ohm resistor worked fine and provided plenty of volume. If you need a bit more level, decrease the value of this resistor. Also, the output jack should be insulated from the chassis because we’re trying to maintain a bit of isolation from input to output. A typical “British-style” jack works fine here. Just make sure it’s stereo and don’t forget to connect the tip and ring connections together. The last component you see is a 5 µF 50V non-polarized cap. Since a resistive load on an amplifier causes it to react differently from a speaker load, amps tend to sound more “spiky” with a resistive load, so this capacitor helps smooth out some of the brittleness of the sound in the headphones.
There you have it—a way to play your amp silently.
Question #2
Dear Premier Guitar,
I’ve grown to appreciate your DIY pieces, and they’re well written to the targeted reader. I would like to get your take on the possibility of using the reverb “send and return” loop as an effects loop. It seems easy: An adapter cable changes the RCA plugs to 1/4". The cables go to your effects pedals and return, instead of the reverb can. The cool part is the reverb control would now mix the wet and dry signals. Will this work? Can you use “Y” cables and a switch to include the reverb as well? Are the impedances so far out to lunch that it’ll never work? Is that why I’ve never heard of anyone doing this?
Wysong Perabula
Question #2 asks about the possibility of using the amp’s reverb circuitry as an effects loop. In essence, it already is an effects loop, but it’s optimized for use with the reverb tank, which is far different from an effects pedal. We can, however, get it to work as a pedal loop.
Fig. 2: This schematic shows how a reverb circuit can be turned into an effects loop. Unfortunately, it’s at the price of the reverb—but there are pedals for that.
The biggest difference between a normal effects loop and a reverb circuit is basically in the “send” department. A typical Fender-style reverb tank has very low input impedance, and this requires a substantial level to drive it. This level is far too hot to feed into any effects pedal, so we must first tame the beast. Looking at Fig. 2, R1 serves as a load on the reverb drive transformer. Next R2 and R3 form a voltage divider to reduce the signal to an acceptable level for an effects device. Since the reverb drive circuit is actually a small amplifier output stage using its own little output transformer, removing the inductive load from the reverb tank and replacing it with the 100-ohm resistive load is similar to removing the speaker load from an amplifier and replacing it with a resistive load. Things get a little “spiky.” To compensate for this, we add the .001 µF capacitor—identified as C1—to smooth out the sound.
Now we have a signal that will be a better match for a pedal. Regarding the effects return, the effects can be connected directly to the reverb return (out) of the amplifier, either by 1/4" to RCA cable or, if you’d like to contain this all in a project box, simply connect the loop return jack directly to the reverb return jack, as most pedals should be able to comfortably drive its 220k input impedance. The loop can now be used for your time-based effects (delay, reverb, chorus, flange, phase, etc.). The reverb knob will mix in the amount of effect, so set your effects for 100 percent wet, if possible, to minimize any phase cancellation problems. The footswitch will now turn the effects on and off as well. As far as including the reverb tank back into the circuit, the output impedance is too low and would more than likely attenuate the output of the effects devices.
Well, there you have it. Enjoy the experimentation!
This oddball 1963 Fender head demanded serious examination.
Hello Ask Amp Man fans! Once again, just when I think I’ve seen about every iteration of vintage Fender amp, another interesting one crosses my bench. This month’s beauty is a blonde 1963 Showman head (Photo 1) that just came into the shop.
You ask, “What’s so special about that?” Granted, we’ve all seen blonde Fender Showman amps before—at least those of us who are fortunate enough to see lots of great vintage gear have—but this one is transitional. Not just because it’s a blonde cabinet with a blackface control panel, but because it has some very rare transitional output tubes as well.
7355 output tube.
If there’s one thing vintage Fender amps are known for, it’s their use of 6L6 (5881) and 6V6 output tubes—as opposed to their EL34 and EL84 counterparts utilized in amps produced across the pond. This was pretty much ubiquitous across the historic Fender product line and was responsible, in large part, for giving the company’s amps their signature “American” sound.
Hi-fi vibe. So why change for this amp? That question may never be answered, but my baseless possible theory is Fender may have wanted a more hi-fi sound from the amp—something with more fidelity—so they opted to use the 7355 output tube. Around the same time, Ampeg was producing amps utilizing the 7591 output tube, which is also a hi-fi-style tube. The Ampegs were great-sounding amps, so maybe Fender took a cue from the guys on the East Coast. I’ve also read there may have been availability issues in obtaining 6L6s at this time. Who knows?
Photo 2
Anyway, the problem here is the 7355 is rated at a maximum plate dissipation of 18 watts, while the typical 6L6/5881 is rated for a maximum of 30 watts. Unfortunately, tube data is not provided in any sort of standard format, so attempting to compare tube specs on an apples-to-apples basis is difficult at best, but tube data suggests output from a pair of 6L6 tubes can be in the 50-plus watt range, which is typical, and that the output from a pair of 7355s can be 40 watts, which is past the amp’s stated design maximum of 36 watts (18 watts each).
Photo 3
That said, maybe they were hoping to design an 80-watt amp with a better frequency response. Again, who knows? But a quick look at these tubes (Photo 2), which are no bigger than 6V6 tubes, screams “good luck!” On top of that, Fender was following up the blonde 1961 Showman (model 6G14) and 1962 Twin (model 6G8), which were 80 watts and had a quad of 5881s and a huge chunk of iron for an output transformer (#45268) (Photo 3). This four-output-tube amp is using undersized tubes and a redesigned output transformer that is half the size (#125A18A) (Photo 4). That doesn’t quite sound like a recipe for success to me.
Photo 4
The plot thickens. And there are even more questions. According to the tube chart, this is a Showman amp, model AA-763 (Photo 5), but not according to schematics. In fact, no schematic that I could find reflects the use of 7355 output tubes or the 125A18A output transformer. Furthermore, the name “Showman-Amp”—as it appears on the faceplate—has typically denoted a single-speaker cabinet, while the name “Dual-Showman” designated a two-speaker cabinet. Apparently not so here, because the owner claims his dual 15” cabinet is the stock, matching cabinet for this particular head.
WARNING:
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.This normally would be easy to discern, as the blackface Showman and Dual Showman amps this model quickly morphed into utilized specific output transformers depending on the speaker load. The Showman used the 125A30A for an 8-ohm load and the Dual Showman used the 125A29A for a 4-ohm load. There is, however, a bit of information gleaned from researching more transformer history that may fill in the blank here. There was a blonde Fender Twin-Amp manufactured for a very short time as well. It’s possibly even more rare and short-lived than this Showman on my workbench, and was built with 7591 output tubes and utilized the same transformers as this Showman. Since the specs of the 7591 and 7355 output tubes are relatively close and the Twin-Amp combo is, of course, a 2x12 with a total load of 4 ohms, one may assume the output impedance of the A18A transformer to be 4 ohms and that the 2x15 cabinet could indeed be its mate. Boy, that was a long way home!
Photo 5
Anyway, this also provided me with some useful service information regarding output tube replacement/substitution, because they did need to be replaced, since most OEM tubes by this time have been run ragged and are probably as microphonic as the day is long. The first inclination would be to replace the 7355s with a typical 6L6, but that’s probably not a good idea. Not only would the sockets need to be rewired, but primary impedance of the output transformer is not a great match and the filaments of the 6L6 draw more current and would put additional load on an old mains transformer that, theoretically, is already running on more wall voltage than it was originally designed for. The better solution would be the 7591s, as they are currently manufactured and available. The 7355s were obviously produced with unobtainium, since they’re virtually unavailable in the NOS market and no one has done a reproduction. The 7591s, in my opinion, are also a better sounding tube, so they make a great replacement for the 7355s.
Photo 6
There will, unfortunately, be a couple modifications necessary. (And, as always, unplug the amp and drain the power supply caps before servicing any amp. If you don’t know how, find someone that does!) First, the 470-ohm screen grid resistor and associated wiring needs to be disconnected from pin 4 of every socket (Photo 6). Then stand the resistor, still connected to pin 8, up in the air and reconnect the wires to the top of the resistor that were disconnected from that socket. Do this for all four sockets and resistors. Be sure they won’t come in contact with the inside of the cabinet once the head is reassembled. You may need to bend them down to be sure of this. Next, there may be a change in the bias supply voltage necessary, as the 7591s require less negative voltage. Unfortunately, you cannot measure the bias current using a standard octal socket bias-measuring device, but to measure the current, connect a 1-ohm resistor between pin 5 and the ground on the output tube socket, and measure across it. It’s a 1:1 ratio, so a reading of 30 mV is equal to 30 mA of current.
Well, there you have the story of a rare beast, although we may never know why it exists.
’Till next time…
A rare find from 1964 lands on the workbench and is up and running after a little ground control.
Hello Ask Amp Man followers. Greetings from Amp World!
An amp crossed my path that I think is interesting enough to warrant an installment of the column, so I’m once again going to forgo a reader question and focus on this somewhat rare brown box on my bench called a Gibson Titan. A friend who recently acquired the head and cabinet pair brought them to me. He said that he’d wanted a Titan set since seeing the trapezoidal head sitting atop its matching extension cabinet in a music store back in 1965, so this was a very welcome blast from his past.
First, a little history on these amps and then we’ll get into the servicing. According to the information I’ve found, Gibson manufactured Titans for five years. In the first two years, 1963 and 1964, they were produced with a brown vinyl covering, and in the last three years, 1965 through 1967, they were covered in black vinyl. Titans were also offered with multiple speaker configurations. The cabinet in the Titan I set came loaded with two 12” speakers. The Titan III had one 15” and two 10” speakers, and the Titan V had two 15” speakers.
All of these configurations used the same head containing 11 tubes, utilizing a quad of 6L6 output tubes, and it was rated at 65-watts output. The cabinet associated with this head says Titan III on its decorative metal panel, so it’s the one 15” and two 10” version. The interesting thing about this configuration is Gibson installed a crossover in the cabinet so the lows and highs would be split between the 15” and 10” speakers, respectively. That’s not something seen often in guitar world. As for the head and cab, they’re covered in the earlier brown vinyl and the date codes on the parts are split between late 1963 and early 1964, so that should firmly date this Titan as a 1964 model. Nice find indeed!
The owner said this amp was sitting for a while and he wanted it serviced and brought back to its glorious self. He also mentioned that one of the output tubes was either missing or looked bad, so he installed a new quad of output tubes and turned it on, but it didn’t sound right and was making noises. As some of you may know from reading my columns through the years, that was a bad thing to do, but it was also good information for me to have.
WARNING:
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.I, as well as many others in the tube-amp service industry, have always said, “If an amp has been sitting idle for years, it’s not a good idea to just plug it in and turn it on.” The electrolytic capacitors in the amp tend to dry out, and if there’s any hope or desire of keeping the amp original, as well as in service, the caps should be brought up slowly over time with a Variac to allow them to “reform.” Not doing this can compromise the performance of the caps or possibly leave you with a mess that’s a chore to clean up after one decides to vent or explode. Luckily neither of these things occurred, but I made the decision that the original caps should be replaced for this Titan to become a usable, reliable amp.
Photo 2 — The amp arrived in original condition except for the prior replacement of one output tube socket.
Let’s get into the servicing. As you can see in Photo 2, the amp is pretty darn original. The only real change to this point had been the replacement of one output tube socket. Things looked good for the most part, except for a couple small problems. The screen grid resistor on one of the output tube sockets had been fried to a crisp and needed to be replaced. This is obviously where the missing or bad output tube was located, but no worries. The amp was originally built using 1/2-watt screen resistors, but, since we had to replace one, I upgraded them all to much more reliable 5-watt versions. There was also one signal capacitor in the phase inverter section that had broken away from its connection on one side (Photo 3). Luckily there was enough lead remaining on the capacitor that I could attach an extension, properly re-attach it to the connection, and keep the part original.
Photo 3 — A signal capacitor in the phase inverter section had broken away from its connection on one side. There was enough lead remaining to attach an extension.
So, I repaired the signal cap and upgraded the screen grid resistors. Then it was on to the filter caps, and I’d like to impart one very important piece of information here that has come from decades of experience. A fact I’ve learned that may be one of the most important to consider not only in building amps but also in repairing them is that “ground” is not just ground.
Photo 4 — When installing new caps, be sure to use the same negative connection used with the original caps, even if replacing them with a different style cap.
What do I mean by this? Connecting a component that needs a ground connection to just any location on the chassis or to any electrical connection to ground does not mean the unit will function optimally. In many instances, this will cause a low-level hum that should not exist. Proper ground location is crucial for the best performance, and in most amplifiers, that has been optimized during the design and development process. For this reason, whenever I replace filter capacitors, I make sure to use the same negative connection used with the original caps, even if I’m replacing them with a different style cap, as I did for this install. I chose to use the original wires from the multi-caps and attach them to the replacement discrete caps, which I mounted in the same locations (Photo 4).
Photo 5 — The amp’s original two-prong power cord had a hole for a ground wire that was manually attached to a wall outlet’s center screw for grounding.
There you have it. A little piece of someone’s childhood music store experience is back up and running, hopefully to produce more memories. Until next time…