Welcome, all! This new blog is designed to address some of the many questions I receive regularly from musicians about technical issues. We’ll cover a variety of topics, and I welcome comments and questions.

First, a quick introduction. I’m Frank, and I am the founder, owner and president of Studio Sound Electronics.   I have nearly 50 years' experience in amp repair.   Although I no longer repair amplifiers myself, I want to share what knowledge I have gained along the way.   As a bit of background, I earned a bachelor's degree in electrical engineering in 1970, did graduate course work following that, and  spent 35 years in the telecommunications industry as a senior engineer at BellSouth and AT&T.   During that time, I also began to repair guitar amplifiers for various music stores in the Louisville, Kentucky area.    I started Studio Sound Electronics in 1997 in order to provide an online source for technicians and do-it-yourselfers to find the parts they needed to repair their amps.   So, let's get started ...

So, Exactly What is Bias?

Tube bias refers to a voltage setting on the control grid of the output tubes which controls the amount of idle current of the output tubes (power tubes). Much like the idle in your car’s engine, a tube amplifier needs to have the output tubes biased properly for optimal performance. If the output tubes are under-biased (idle current is too HIGH), the audio signal will tend to distort earlier and the tube life will be shortened. This condition is sometimes referred to as biased “too hot”. If the output tubes are over-biased (idle current is too LOW), the audio signal will deteriorate more quickly and the amp will sound thin, cold and sterile. Therefore, this scenario is sometimes referred to as biased “too cold”. Typically, a hotter bias will give you louder, punchier and fuller sound; a colder bias will give you a cleaner, thinner sound.

What’s the perfect bias?
There is no such thing as the “perfect” bias setting that is acceptable for a particular tube type in a specific amp. There are vast differences in output transformers and tubes, varying quality between tube manufacturers and even significant variation between individual tubes of the same brand and type. Bias ranges are also subject to personal taste.

The (boring) intricate details
For those who love the intricate technical details, this paragraph will describe the physics of vacuum tube operation and how the bias voltage plays an important role. If you’re more content with a broad-brushed version of the bias overview, skip on down to the next paragraph. A vacuum tube amplifies by taking a small voltage on the grid element and delivering a much larger voltage on the plate element. Here’s how it works: A tube has a third element called the cathode which emits electrons when heated. The heat from the tube’s heater warms the cathode, giving these electronics sufficient energy to jump through the vacuum space of the tube and land on the plate. Why do they want to do this? Because electronics have a natural negative charge, and the plate of the tube is connected to a rather high positive voltage (several hundred volts). In the physical laws of electronics, opposites attract and the negative electronics are just dying to get to that positive plate. The grid element of the tube is located between the cathode and the plate, and acts as a control valve for how many electronics can successfully make the trip. The grid has a voltage applied that is somewhat more negative than the cathode (typically by about 40 or 50 volts). This is called the bias voltage. Since the grid is negative and like voltages repel, the grid tries to repel or push away the electron flow coming from the cathode. But you can think of the grid as like a sieve or screen (or a protected border with insufficient guards!) – some electrons do make it through, and they wind up making it to the plate. The more negative we adjust the voltage on the grid, the more the electrons are repelled, and fewer make the journey to the plate. The less negative we set the grid voltage, the larger the number of electrons that make it through. Without the grid, there would be no control on the flow of electrons from cathode to plate, and the tube would soon burn up due the excessive current flow and the resulting heat that would be generated. If we set the bias much too negative, no electrons can flow and the tube is “cut off”. If we set the bias insufficiently negative, the tube is in “runaway” and will soon burn up. Now, in addition to the constant DC bias voltage, the grid voltage is modulated slightly by the input signal applied – that’s the audio signal of your guitar reaching the output tubes. This slight variance in grid voltage causes the current to the plate to change in the exact same manner. The plate voltage is applied to the tube through the windings of the output transformer, and the resulting change in the current through the primary of the transformer creates an identical and rather sizable change in the secondary of the transformer – and that is connected directly to your speaker. Whew!

Why must bias be reset over and over again?
So, once you’ve had the bias set and adjusted on your amplifier and you have the perfect sound that you like, why do you need to ever have it adjusted again? Because, as I mentioned before, tubes will vary greatly in their specific characteristics. First, as you might imagine, it’s tough to build a tube in the first place. Much of the process of assembling the components of a tube is done by hand. The individual plate, grid, cathode and heater components are assembled on a jig using hand tools, and then wired to the pins in the base, before the assembly is enclosed in the glass envelope. Naturally, there are slight variations in spacing, size and positioning between the components from tube to tube – and these variations mean that the electrical characteristics will differ. After they are built, each tube is tested and graded according to its specific characteristics, and then sorted matched up with other tubes of similar performance. You can purchase a pair or quad of matched tubes, but there is no guarantee that this bundle of two tubes or four tubes will be exactly like the previous set that you purchased. So, you must “tweak” the bias in your amplifier to fine tune the bias setting so that the new tubes will again perform properly and sound the way you like.

What about fixed bias?
But, you say – what about a fixed bias amp, such as a Mesa Boogie design? How do they get by without requiring (or allowing) you to change the bias?

Permanent fixed bias is pretty simple, really. If you understand the adjustable fixed bias, then you’ll find that permanent fixed bias is a design which uses fixed resistors to set the negative bias voltage on the grid permanently, rather than offering an adjustable resistor (rheostat or potentiometer) to allow the bias to be adjusted. The advantage of this concept has been heralded for many years by Mesa Boogie, who proclaims that their amps have an automatic bias system that requires no adjustment, thereby making it easier and simpler for their customers to change tubes themselves. In itself, that is quite true – no adjustment needed, and anyone can change the tubes. The disadvantage is that it’s always necessary in a fixed bias design to set the bias on the “cold” side, to ensure that regardless of the characteristics of the tubes installed, the current through the tube won’t exceed the maximum limit. In other words, they have to err on the side of caution. This means that the output tubes response is pretty sterile – by setting the bias cold, the output tubes add very little coloration to the sound of the amplifier – no early breakup, no “bluesy” overdrive. But most Mesa Boogies are crunch machines, and blues isn’t their sound. The overdrive in a Mesa is handled completely in the front end of the amp, by stacking extra preamp stages. The bottom line is that permanent fixed bias is a low maintenance design, requiring no action by the owner. But (and this is just my personal opinion) – you almost might as well have a solid state output, because you’re getting very little in tonal enhancement from the output stage.

Cathode bias is a type of “automatic” bias. It not adjustable, but it’s handled differently than what we are calling fixed bias. Generally, cathode bias is deployed in smaller wattage amplifiers. It doesn’t require a negative voltage supply from the power supply. So … if you’re up on the technical aspects of using a negative voltage to repel and “slow down” the quantity electrons leaving the cathode headed for the plate, you might be wondering how this control can be accomplished without having a negative voltage available? Well, in electronics, voltages are relative to each other. If we can raise the cathode voltage to a moderate positive voltage (say perhaps 10 to 50 volts, depending on the tube), and we then keep the grid at or near 0 volts (ground potential), then the grid looks negative with respect to the cathode. The electrons on the cathode are still strongly attracted to the plate, because it is a few hundred volts more positive than the cathode. But the grid is still a partial barrier to the electron flow, because it is more negative than the moderate positive potential of the cathode. So, as far as the electrons are concerned, they can’t tell the difference – they behave the same as they do in a fixed bias or adjustable bias configuration where the cathode is at 0 volts, and the grid is at -10 to -50 volts DC.

So how do we achieve this positive voltage on the cathode? It turns out that it’s pretty simple. Whenever you force current flow through a resistor, a voltage develops across the resistor, and the polarity of the voltage depends on the direction of the current flow. If we put a resistor of typically a few hundred ohms between the cathode and ground, the current flow through the tube automatically “lifts” the cathode voltage above ground. The final voltage of the cathode depends on the value of the resistor, and can be calculated fairly accurately though the use of some formulas. By choosing the correct value of resistor, the cathode is set at a moderate positive voltage, and a sort of “automatic” bias is achieved.

Cathode bias has a couple of advantages over permanent fixed bias. It offers some compression, some even-order harmonics, and earlier break-up. It also is somewhat self-adjusting, depending on the specific characteristics of the tube installed. To a significant degree, it’s responsible for the warm bluesy tone quality of the old Fender Champs, the Vox AC15 and AC30, the old Silvertone amps, and pretty much any amplifier that uses 6V6 or EL84 output tubes.

How To Check Bias
We offer different bias testing devices in different price ranges on our bias page.

So, that’s pretty much our discussion of bias in tube amplifiers. Yep, it’s pretty technical, but it’s a necessary, integral part of how the amp works, and one of the important reasons that a particular amp sounds the way it does. Meanwhile, keep on rockin!