Many people are not familiar with the difference between class A and class A/B circuits, even tough it has a huge effect on an amplifier’s tone. The basic answer is this: These two classes of amplification illustrate two different types of output tube (or power amp tube, as some people like to call them) wiring. Read more
The basic technology behind the modern vacuum tube has been around since 1906. Yet few musicians seem to have even an elementary knowledge of this rather low-tech device. In reality, the various functions and characteristics of tubes are easily understood. What’s more, once armed with this knowledge, you’ll be better equipped to troubleshoot amplifier problems and to improve your guitar tone. Read more
There seems to be an awful lot of interest these days in tube amps that put out lower amounts of power, especially in more boutique, cork-sniffing circles. Just about every amp company has some kind of lunchbox-sized head or ankle-biter combo that puts out five to fifteen watts of power, it appears, and many players are taking the bait and paying top bucks for these little guys. Devotees of these things claim that “five watts is plenty to gig with” and they are more than able to hang with a drummer. Personally, I have a hard time with that statement. Read more
There are a few central debates that continually rage in this thing of ours, topics that never seem to get exhausted during online forum or real time arguing: Gibson vs. Fender, Fender vs. Marshall, Ginger vs. Mary Ann, Bud vs. Miller, and, most importantly, tube amps vs. solid state amps. These are the things guitarists think about. Now, I can’t help you decide to play a Les Paul over a Strat or who to spend Read more
An electric guitar’s sound is deeply influenced by its pickups. For a musician who has little or no knowledge of electronics the matter can often seem to be incredibly complicated. When it comes to electronics in general, pickups are quite easy to wrap your brain around. This article will explain the connection between the electronics and sound. Read more
Guitar amps are, no matter how you cut it, black boxes. You may think you have control over them, but when it gets down to it, you can’t really see inside the black box, and even if you could, there aren’t any moving parts. Electricity is largely non-mechanical. Or in other words, magic. And when the magic stops, most people think all they can do is resort to prayer. Or an amp technician.
When your amp isn’t performing up to snuff, there’s still a lot you can do without having an EE degree, or even knowing how to operate test equipment. Here are some holistic approaches—and solutions—you can try yourself, as I did when my tube amp went on the fritz. Warning: Some of the following procedures involve messing around with the components of the amp, so be careful. Electricity can kill you. Proceed at your own risk. Read more
What is a Potentiometer?
Potentiometers, or “pots” for short, are used for volume and tone control in electric guitars. They allow us to alter the electrical resistance in a circuit at the turn of a knob.
It’s useful to know the fundamental relationship between voltage, current and resistance known as Ohm’s Law when understanding how electric guitar circuits work. The guitar pickups provide the voltage and current source, while the potentiometers provide the resistance. From Ohm’s Law we can see how increasing resistance decreases the flow of current through a circuit, while decreasing the resistance increases the current flow. If two circuit paths are provided from a common voltage source, more current will flow through the path of least resistance.
We can visualize the operation of a potentiometer from the drawing above. Imagine a resistive track connected from terminal 1 to 3 of the pot. Terminal 2 is connected to a wiper that sweeps along the resistive track when the potentiometer shaft is rotated from 0° to 300°. This changes the resistance from terminals 1 to 2 and 2 to 3 simultaneously, while the resistance from terminal 1 to 3 remains the same. As the resistance from terminal 1 to 2 increases, the resistance from terminal 2 to 3 decreases, and vice-versa.
Tone Control: Variable Resistors & Tone Capacitors
Tone pots are connected using only terminals 1 and 2 for use as a variable resistor whose resistance increases with a clockwise shaft rotation. The tone pot works in conjunction with the tone capacitor (“cap”) to serve as an adjustable high frequency drain for the signal produced by the pickups. The tone pot’s resistance is the same for all signal frequencies; however, the capacitor has AC impedance which varies depending on both the signal frequency and the value of capacitance as shown in the equation below. High frequencies see less impedance from the same capacitor than low frequencies. The table below shows impedance calculations for three of the most common tone cap values at a low frequency (100 Hz) and a high frequency (5 kHz).
When the tone pot is set to its maximum resistance (e.g. 250k?), all of the frequencies (low and high) have a relatively high path of resistance to ground. As we reduce the resistance of the tone pot to 0?, the impedance of the capacitor has more of an impact and we gradually lose more high frequencies to ground through the tone circuit. If we use a higher value capacitor, we lose more high frequencies and get a darker, fatter sound than if we use a lower value.
Volume Control: Variable Voltage Dividers
Volume pots are connected using all three terminals in a way that provides a variable voltage divider for the signal from the pickups. The voltage produced by the pickups (input voltage) is connected between the volume pot terminals 1 and 3, while the guitar’s output jack (output voltage) is connected between terminals 1 and 2. From the voltage divider equation below we can see that if R1 is 0? and R2 is 250k?, then the output voltage will be equal to the input voltage (full volume). If R1 is 250k? and R2 is 0?, then the output voltage will be zero (no sound).
The taper of a potentiometer indicates how the output to input voltage ratio will change with respect to the shaft rotation. The two taper curves below are examples of the two most common guitar pot tapers as they would be seen on a manufacturer’s data sheet. The rotational travel refers to turning the potentiometer shaft clockwise from 0° to 300° as in the previous visual representation drawing.
How do you know when to use an audio or linear taper pot?
It’s really a matter of personal taste when it comes to volume control. Notice how the rate of change is much more dramatic on the audio taper pot when traveling back from 100% to 50% rotation. This means that the same amount of rotation would give you a more intense volume swell effect with an audio taper than with a linear taper. Using a linear taper volume pot would give you a more gradual change in volume which might feel like you have more fine control with which to ease back the volume level.
For tone control, it’s basically standard practice to use an audio taper. The effect of the tone circuit is not very noticeable until the resistance gets pretty low and you can get there quicker with an audio taper.
How do you know what value of potentiometer to use?
The actual value of the pot itself does not affect the input to output voltage ratio, but it does alter the peak frequency of the pickup. If you want a brighter sound from your pickups, use a pot with a larger total resistance. If you want a darker sound, use a smaller total resistance. In general, 250K pots are used with single-coil pickups and 500K pots are used with humbucking pickups.
Potentiometers are used in all types of electronic products so it’s a good idea to look for potentiometers specifically designed to be used in electric guitars. If you do a lot of volume swells, you’ll want to make sure the rotational torque of the shaft feels good to you and most pots designed specifically for guitar will have taken this into account. When you start looking for guitar specific pots, you’ll also find specialty pots like push-pull pots, no-load pots and blend pots which are all great for getting creative and customizing your guitar once you understand how basic electric guitar circuits work.
Kurt Prange (BSEE) is the Sales Engineer for Amplified Parts (www.amplifiedparts.com) in Tempe, Arizona, United States. Kurt began playing guitar at the age of nine in Kalamazoo, Michigan. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.
I have a Pignose Hog 20, and every time I jam with other players, the drummers drown me out. I’m 15 years old and don’t have a lot of money. I’m wondering what size amp I should by so that I can be heard above the drums without the amp distorting. Also, how are amplifier wattage ratings determined?
Let’s tackle your last question first Johnny, since this will help us answer your question about what size amp you should buy. Amplifier watt ratings are determined by a calculation called “root mean square,” or RMS. It simple terms, RMS is a measurement of the average, or “effective,” voltage your amp can produce continuously under normal playing conditions without clipping (what happens when you crank up your amp and it starts to distort). Some manufactures give their amps a power rating that’s closer to the peak output—that is, the highest wattage that the amp is capable of producing. This isn’t a very accurate way to measure wattage, because amps don’t operate (or shouldn’t be operated) at peak levels for an extended period of time. If you were to operate an amp at its peak output, I assure you, not only would your tone suffer but the amp itself, including its speaker, would burn out very quickly.
Let’s look at an example. If you have a 50-watt amp and it starts to distort, or “clip” as the output approaches 35-watts, is it a 50-watt amp or a 35-watt amp? To put it another way, if you want an amp that can produce 50-watts of continuous power, don’t buy an amp that has a peak power of 50-watts, because the amp will start to distort well before it reaches the 50-watt mark. Read more
The bias of guitar amp tubes is the amount of electricity that each tube uses in relation to the other tubes in the amplifier. The sweetest overall tone and longevity of guitar amp tubes is attained when all of the tubes in the amplifier are drawing the same amount of current or electricity. Read more
Yellow Jackets tube converters allow EL84 power tubes to be used in place of the most common guitar amp power tubes including 6L6, EL34, 6V6, 7027, 6550 and 7591. Most Yellow Jackets provide a substantial output power reduction and a “self-bias” Class A configuration for the EL84 so that no bias adjustment is required. Yellow Jackets are like getting a whole new amp. Read more