What Is Tube Amp Biasing? How It Works, Why You Should Care

The what, how, when, where, and why behind bias

Ever since rock was born, tube amps of all shapes and sizes have
been used in the creation of those glorious, guttural guitar sounds we all know
and love. Yes sir, from AC/DC to ZZ Top; Alice in Chains to Led Zeppelin; Jimi
Hendrix to Van Halen; Gary Moore to Metallica; and from Sabbath to Slayer to
Slash — tube amps have been omnipresent. And the same is true of electric blues
guitar tone, too — from Stevie Ray Vaughan to Joe Bonamassa.

Along with these awesome tube amps — be they stacks or combos —
comes a bunch of oft-used terms such as “single-ended,” “push/pull,” “sag,” “harmonic
distortion,” and (of course) the dreaded “bias” word. I say “dreaded” because,
like some of the other terms just mentioned, bias is frequently not fully
understood by those who throw it around willy-nilly. I’m sure you’ve probably
met a self-proclaimed “tube amp expert” blowhard or two that spouts phrases
like “sounds to me like your amp’s bias needs tweaking” yet skillfully avoids
any and all questions about “what is a bias and how does it work?”

I’ve watched YouTube videos, perused online forums, and read a great many articles, blogs, and books about bias over the years. While there’s some truly great information out there from folk way smarter than me, there’s also a bunch of stuff that merely compounds the confusion instead of clarifying it. Yep, to quote the author of the excellent book Inside Tube Amps, Dan Torres: “Bias is one of the most mysterious and confusing subjects involved with tube amps.”

A few of the books and articles referenced when penning this piece.

The purpose of this short article is to demystify the confusing
and often intimidating tube-amp term that is “bias” — in guitarist-friendly
language, of course! First though, an important word from our sponsor
concerning safety.

Sometimes a picture does indeed paint a thousand words.

SERIOUS WARNING: At the risk of sounding melodramatic, all tube amps have one thing in common — under the hood, they all contain voltages that CAN KILL YOU! And this is true even when the amp is switched off and even when not plugged into the mains. Large capacitors like those used in amplifiers store a hefty charge, and all it takes to unleash that is to inadvertently touch the wrong part of the amp circuit. So, we strongly recommend that BIASING SHOULD ONLY BE DONE BY AN AUTHORIZED TECH. Doing this will not only protect you but also protect your amp’s warranty and well-being.

NOTE: There are plenty of “experts” on websites, forums, and YouTube that will “teach” you how to re-bias. Unless you’re a trained electrician (and be advised that being able to change a light bulb and owning some tools does NOT qualify you as such!), you shouldn’t be messing around inside a tube amp. Please, take the smart route and see a qualified tech. It could quite literally save your life.

Darn, that’s a heavy disclaimer but a very necessary one — I’ve seen a self-proclaimed “expert” literally get thrown several feet by a Marshall JCM800 he was “fixing” when his neck chain discharged a capacitor. Thankfully, the genius survived... but he fixed nothing!

“Trust me, I know what I’m doi—!”

What Exactly Is Biasing in
Electronics Anyway?

According to “the Googles” (© Henry Rollins), bias is, and I
quote:

“In electronics, bias usually refers to a fixed DC (direct current) voltage or current applied to a terminal of an electric component such as a diode, transistor, or vacuum tube in a circuit in which AC (alternating current) signals are also present in order to establish proper operating conditions for the component.”

Wow! I dunno about you, but just trying to read that massively
overrun, 52-word sentence gave me a blinding headache.

Another slightly less verbose definition of the term I also found
is this one:

“Bias is direct current (DC) deliberately made to flow, or DC voltage applied, between two points for the purpose of controlling a circuit.”

More concise? Yep. Clearer? Hmmm. If I’ve interpreted the above
two explanations correctly, then, in the case of an amp, bias is a voltage
applied to a tube that controls it and makes sure it is working properly, too.

That’s still way too vague for me, I’m afraid. “Works properly”
and “control” are more than a tad cloudy. I need to know more — a lot more! I
did more searching and found this definition:

“Vacuum tube bias is a DC voltage supplied to the tube that controls the flow of electrons within the tube when no guitar signal is present.”

That’s definitely a more guitarist-friendly explanation, but it
still doesn’t paint the full picture. I wanted not only to understand what bias
is but also other questions like “when bias needs to be adjusted,” “where
exactly bias is applied,” “how bias works,” and the “why it’s important”
aspects of tube bias, too.

NOTE: To aid us in the clarity we seek, I’ll refer to the voltage we’ve been discussing as the “bias voltage” from here on in.

At this juncture, I should point out that preamp tubes don’t need
to have their bias tweaked, thanks to the self-biasing circuits they’re
literally plugged into (read on, all will be revealed later). However, the same
is not true of the power-tube bias
circuits used in the majority* of tube amps — thus, they’re the ones that need
TLC when it comes to biasing. So, whenever someone checks or adjusts the bias
of your amp, they’re measuring and then, if necessary, tweaking the bias
voltage of the power tubes — not their smaller brethren in the preamp.

NOTE: There are some tube amps with self-biasing circuits for the power tubes, too. This will also be discussed shortly.

A tribe of 12AX7 preamp tubes... no finicky bias tweaking required.

Now that we’ve clarified that, let’s focus on bias circuits that
do not “self-bias” and find out more about the mysterious subject by digging a
little deeper. To do so, we need to have a rudimentary grasp of exactly how
tubes work, so here we go.

Basic Tube Talk

In very simplistic terms, a tube is an electrical device that, at the very least, consists of the following four active components — all situated within a glass-enclosed vacuum:

Heater (a.k.a. filament)CathodeGrid (a.k.a. control grid)Plate (a.k.a. anode)

When a tube is working, the heater warms up the cathode (which is negatively
charged) so it emits electrons. The resulting and ever-growing swarm of
electrons then starts flowing through the vacuum to the positively charged
plate they’re attracted to (remember folks: in electronics, opposites attract
whereas like charges repel). As you can see in the above diagram, in between
the cathode and the plate sits something called a grid. Its purpose? To control
the flow of electrons so that the tube works properly within your amp. Without
the grid, the flow of electrons to the plate would be continuous and totally
out of control — just like water running out of a tap that’s been left open.
That’s why the grid is sometimes referred to as the “control grid.”

So, the grid has effectively got two jobs to do, and both involve
controlling electron flow within the tube:

To control the flow of electrons whenever
you’re not playing.

To ensure the tube amplifies your guitar’s
signal powerfully and efficiently whenever you are playing.

Furthermore, how well the first job gets done has a direct impact
on the second. This knowledge about the grid segues us nicely into discussing
bias again, as we now know that both the grid and bias voltage are involved in
controlling the flow of electrons whenever you’re not playing. Yep, it looks
like we may have just found the “where.”

How Does the Tube’s Grid Control
Electron Flow When You’re Not Playing?

Once the tube’s heater causes the cathode to start producing an
ever-growing swarm of electrons, the only thing between them and the plate
they’re attracted to is the grid. The grid has very little surface area though,
so it’s hardly an effective physical barrier to stop the electron army heading
its way. And that’s where the bias
voltage we were talking about earlier comes into play. If the grid is given
a negative charge, then it will inhibit the flow of electrons to the plate by
literally repelling them to retreat back to the cathode whence they came. So,
the more negative the grid is, the fewer electrons will be able to flow past
it. Make sense?

The opposite is also obviously true, and, to this end, there are
two extremes:

If the grid is made too negative, then it will
stop the flow of electrons completely, as it will repel all comers! This is
sometimes logically referred to as “cutoff.”

If it isn’t negative enough, then the flow
will increase and could well get totally out of control. When the electron flow
(a.k.a. current) actually exceeds the power rating of the output tube being
used (e.g., an EL34’s rating is 25 watts; an EL84’s is 12 watts), it’s
sometimes called “red plating.” Can you say “burnout?!”

The desired biasing goal sits somewhere in between: The grid needs
to control electron flow within the tube when you’re not playing without
halting it altogether. The bias-voltage setting needed to achieve this varies
depending on the sensitivity of the tube.

GEEKY TECH NOTE: Because the bias voltage is applied to the tube’s grid, this type of biasing is often logically referred to as “grid bias.” It also has another name that we’ll discuss later.

Why Biasing Is Important

Aspen Pittman’s Excellent Car-Engine Analogy

When talking to the late, great tube guru Aspen Pittman of Groove
Tubes (who authored the authoritative The Tube Amp Book) about the
much-misunderstood topic of biasing the power tubes in an amp, he likened it to
the idle setting on a car’s engine. That’s such a good analogy that I’m going
to rightfully give him the credit and then play his words forward. Here’s a
summary of what this much-missed man taught me:

The idle setting of a car’s engine is a pretty simple concept that definitely relates to biasing a tube amp. When a car’s idle is set correctly, the engine doesn’t stall or rev up wildly when you’re not moving. The correct “tick over” setting ensures that the engine is running smoothly when you’re stationary and is ready to accelerate efficiently whenever you start pressing the pedal to the metal! And the same exact thing applies to the bias setting of your beloved tube amp — it controls how much current is running through your tubes when you’re not playing so that it’s ready to rock whenever you are.

The late, great, Titan of Tubes, Aspen Pittman

Bias not only affects power-tube performance, but it can also impact the sound and the lifespan of said tubes. As already mentioned, if the bias voltage isn’t negative enough, then the tubes will run hot. This is sometimes referred to as “biased hot,” “red plating,” or “running hot,” and it not only significantly shortens tube life but can also result in a lack of output power and punch. On the other side of the coin, if the bias voltage is too negative (a.k.a. “biased cold” or “running cold”) the amp will sound dull and lifeless, and some unwanted and unpleasant-sounding crossover distortion could well occur — and, much as we guitarists love distortion, this type sucks!

So, just like the idle on a car, the correct bias-voltage setting on your amp is imperative for performance, tone, and tube life. Thanks for playing your wisdom and way with words forward, Aspen, my friend.

Gratuitous “amps and cars” image — just because we could!

GEEKY TECH NOTE: On occasions, I’ve seen bias referred to as “the tube’s operation level.” Now it makes sense.

Is Re-biasing Really Necessary?

At this point, you might well be thinking: “My amp came from the
factory with 6L6 power tubes, so, surely, the bias voltage is already set for
them? Why would it ever need changing, unless I decide to try a different type
of power tube — like EL34s, for example?”

Based on what we’ve discussed thus far, this is a pretty
reasonable and logical question/assumption. Why doesn’t the manufacturer just
set the bias and then leave it there?!? If only it were that simple.

Power tubes are kinda like humans in a way, meaning that each and
every one is going to be slightly different. Even if they’re a matched* set
from the same brand, they could well have different characteristics than their
predecessors and, as a result, will probably require a different bias voltage.
As always, due to the fact that lethal voltages lurk within the guts of your
amp, any and all internal work — including biasing — should only be done by a
certified amp technician.

NOTE: Replacing power tubes with a matched set is highly recommended, as it’s pretty darned important that the new tubes are as similar as possible to each other for obvious sonic reasons.

Matched Power Tubes: The “Pair of Gloves” Analogy



When
discussing the understandable confusion that arises when an amp owner is told
that, even though their replacement power tubes are “matched” (and sometimes
supplied by the same brand as their amp), their amp will still need a bias
check, the following analogy definitely helps clear some of the fog. “Think of
a matched pair of power tubes as a pair of gloves and your amp as your hands.
Just because both gloves are exactly the same size, type, color, and brand (i.e.,
matched), it doesn’t automatically mean they’ll fit your hands!” Makes sense to
me.

The classic Marshall 50-watt “Plexi” (model 1987) is powered by a pair of grid-biased EL34 tubes. Even though Marshall offers matched replacement sets, the company recommends the amp be re-biased whenever the output tubes are replaced.

GEEKY TECH NOTE: While a change of a mere few volts might not seem like a big deal, in the case of bias, due to the way a tube works, it is. Why? Because a small change in grid voltage causes a large change in the amount of current flowing, or not flowing, to the plate. And those few volts either way can make the difference between your power tubes running hot, cold, or “just right” when you’re not playing. Yep, bias voltage is important!Also, at this point, it’s worth mentioning that, while some will talk about a specific bias-voltage value as being the “correct” one for a certain amp, that’s not strictly true. There’s invariably an upper and lower range for each case, and any point between the two is fair game. It’s all down to personal taste — be that of the amp’s designer or of the tech working on the amp.

All that said, the highly respected Mesa/Boogie company is a notable exception here. In fact, the company prides itself on not having bias adjusters on its grid-bias amps. The reason? The company has a sophisticated, computer-based tube-testing system it’s nicknamed “Robotube,” which rejects all tubes that don’t meet the strict, seven-parameter testing procedure. So, providing you use Mesa/Boogie-branded replacements for the power tubes in your Mesa amp, no pesky bias tech work is needed.

As long as you use a set of Mesa/Boogie-branded replacement power tubes with your grid-biased Mesa/Boogie amp, no bias checking or tweaking is necessary... nice!

What Mesa offers is somewhat confusingly referred to
as “non-adjustable fixed bias.” Read on, as all is about to be revealed behind
this prime example of tautology
— the fancy, literary word for redundantly saying the same thing twice.

Three Types of Tube Bias

Here’s a trio of ways power tubes can be biased — and don’t worry,
we’re not going to get even close to technical!

Adjustable Fixed Bias

Yep, I know, “adjustable fixed bias” is an oxymoron of “jumbo
shrimp” proportions! As silly as it sounds though, the name actually does make sense because the bias voltage
supplied to the grid is, indeed, fixed until you decide to — wait for it —
adjust it. This is the grid bias we’ve been zoning in on, and the vast majority
of tube guitar amps boast this type of bias.

How you adjust the fixed-bias voltage supplied to the control grid
often varies from amp to amp and from brand to brand. Some circuits have
internal trim pots, while others may have adjustment controls on the rear
panel. Whatever — once again, at the risk of sounding like a broken record
(remember those?) — biasing should only
be performed by a qualified technician using the correct tools.

AMP GEEK NOTE: Some pundits refer to this type of bias as “adjustable grid bias,” too. Makes perfect sense, as the grid is involved here.

Non-adjustable Fixed Bias

Just like the name states, these amps have a factory-set bias
voltage that is non-adjustable... duh! The only way you can change the bias
is to actually modify said circuit. These types of amps are, by far, the least
common of our trio for obvious reasons. As already
mentioned though, Mesa/Boogie walk this way, and, providing you use their
replacement tubes, all is well — and there are no hidden guitar-tech fees
either, unless you’d rather they did the tube change for you.

Cathode Bias

This type of bias circuit is a little different than the one we’ve
just been discussing. In order to explain it both clearly and succinctly, I’m
going to pull a quote directly from Gerald Weber’s wonderful tome, Tube Amp
Talk for the Guitarist and Tech.

“There are two common ways of biasing the output tubes — either fixed-bias or cathode-bias. In a fixed bias amp, a constant negative voltage is injected to the grids of the output tubes while the cathode is grounded to the chassis. In a cathode-biased amp, the grids are grounded through a grid return resistor and a resistor is placed between the cathodes and ground. This creates a slight positive charge on the cathodes. Basically, the tubes can’t tell the difference between having the grid negative with respect to the cathode or the cathode positive with respect to the grids, so either way can be used to bias an amp, however they do sound much different.”

What a great explanation. Of all the ones I researched, this was,
by far, the most “guitarist-friendly,” IMHO. Heck, even I could understand it
without too much brain ache.

Cathode bias is often referred to as “self-biasing,” as the design
ensures that the tubes — wait for it — bias themselves. Thus, no biasing is
required when replacing tubes — but using a matched set of replacement output
tubes is, once again, highly recommended for obvious tonal reasons.

Amps with cathode-biased power-tube circuits are lower-output ones — 30 watts or less. EL84s are a very common tube in such designs, 6V6s are, too.

The classic Vox AC30 combo is a great example of a cathode-biased amp.

NOTE: Sweetwater’s guitar-amp
technician and 22-year veteran Greg Bowers adds: It would be helpful to point
out an adjustable fixed-bias amplifier will need adjustment by a trained
technician every time the tubes are replaced. Discovering that this is
sometimes a yearly proposition, some owners regret owning an amplifier that
needs regular “pit stops” (following the car analogy).

Alternatively,
non-adjustable fixed-bias and cathode-biased amplifiers do not need adjustment,
and users can replace all their tubes without a trip to the service
shop. Some players have expressed to me that knowing this would have affected
their decision on which amplifier to purchase.

Interestingly enough, certain folk refer to cathode bias as
“variable bias” because the voltage automatically moves both up and down to
ensure the bias is always correct — kinda like the thermostat on your home’s
heating system. As Sweetwater’s very own technical and tonal guru, Mitch
Gallagher, mentions in his excellent book, Guitar Tone: Pursuing the
Ultimate Guitar Sound, this is because “the tube itself supplies the bias voltage, based on the voltage
flowing from the cathode to the plates. In a sense, the tube is continually
biasing itself.”

As Mr. Weber states, fixed bias and cathode bias sound different
and also feel different, too. Describing both sound and feel is, of course, a
very personal thing, but here are a few common terms used to describe the
differences.

Fixed bias: Firmer, punchier, crisper, and tighter, with a good level of clean headroom.Cathode bias: Smooth and compressed. The power section overdrives quicker and has more “sag” (a spongier, looser response), especially in the low end. Less clean headroom.

AMP GEEK NOTE: One of the reasons for the aforementioned differences could well be the fact that cathode-biased amps are often biased on the hot side. Fixed-bias amps, on the other hand, are usually biased a little cooler.

This all said, a cathode-biased amp can still be designed to be tighter and more aggressive sounding. Friedman’s brutal yet musical PT20 is a 20-watt, EL84-powered, cathode-biased monster with tight lows and a punchy midrange.

The brute that is the Friedman PT20 head. A cathode-biased amp that delivers a tight, punchy sound.

AMP GEEK NOTE: I should also point out that, in the case of a handful of Mesa/Boogie amps that feature a switchable low-wattage option, the bias sometimes switches from non-adjustable fixed to cathode.

Now You Know

Hopefully, this article has shed some light on the once-darkened domain that is tube-amp biasing. It’s all about making sure those power tubes do what they do best when you’re rocking out — and a big part of achieving that is by ensuring that they’re “idling” correctly when you’re not! All hail Aspen Pittman’s excellent car-engine analogy. Rest in peace, good sir. Rest in peace.

What’s Next?

Sweetwater not only sells tube amps of all shape and sizes made by the world’s leading manufacturers, but we also sell replacement tubes, too. We have a huge team of highly trained Sales Engineers, amp technicians, and tech support folk who will be only too happy to assist you with all your tube-amp and bias questions and concerns. So, give us a call for any of your tube-amp needs at (800) 222-4700... anytime!
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