The power rating of a guitar speaker is an indication of how much power it can handle without being damaged thermally or mechanically. It is not an indication of how loud the speaker will sound in comparison to other speakers. Let’s examine the basics of how speakers work, how the power rating is determined and look at things from the perspective of the guitar amplifier so that we can choose guitar speakers that will last a lifetime.

How an Electrodynamic Loudspeaker Works

Guitar speakers are a type of loudspeaker known as electrodynamic or “moving coil” loudspeakers. The magnetic circuit (composed of front plate, back plate, pole piece and magnet) and voice-coil make up the motor of a guitar speaker. An alternating electrical current flowing through the voice-coil generates an alternating magnetic field perpendicular to flow of current through the coil. The magnetic circuit creates a strongly focused magnetic field in the air gap between the front plate and the pole piece on which the voice-coil is centered. The voice-coil is pushed and pulled through the air gap based on the interaction between these two magnetic fields. Since the speaker cone is connected to the voice-coil, it now has a mechanical force with which to push air particles and make sound.

Thermal Damage

Speakers are transducers. They convert electrical energy provided by an amplifier into acoustical energy. They are actually very inefficient transducers because most of the electrical energy is converted into heat instead of sound. The reference efficiency (ratio of acoustic power out to electrical power in) for most guitar speakers is around 2% to 6%, which means that 98% to 94% of the electrical energy is dissipated in the form of heat.

From the aspect of power dissipation, a guitar speaker can be modeled as a resistor. Most guitar amp enthusiasts are familiar with the equation for electrical power and how it can be used to determine the power dissipated across a resistor. Resistors can be thought of as transducers that intentionally convert electrical energy to heat in order to create a voltage drop. Resistors have a power rating that indicates how much power they can dissipate before being damaged and this rating is analogous to the speaker power rating.

The voice-coil is the electrical interface of the speaker and is given a “nominal impedance” specification (e.g. 4, 8 or 16 Ω) which can be used to approximate power dissipation when connected to an amplifier. (The actual impedance of the voice-coil varies with frequency). Just as a resistor will eventually burn up if its power rating is not high enough, the speaker’s voice-coil will burn up if it is overpowered by the amplifier. One of the most common symptoms of an overpowered speaker is a burned voice-coil, which usually measures as an open circuit on an ohm meter. No sound can be produced by a speaker with an open voice-coil. An overheated voice-coil former may also become warped and begin to rub against the pole piece causing the speaker to buzz loudly.

Mechanical Damage

Loudspeakers can be damaged mechanically by over-excursion of the voice-coil and cone. This is more common with old speakers that have worn suspensions and adhesives, but may also occur at extreme low frequencies outside of the speaker’s useable frequency range. When over-excursion occurs, the voice-coil can become misaligned or bottom out. The cone and suspension (surround and spider) can also become stretched or torn.

How the Speaker Power Rating is Determined

Many speaker manufacturers rate their speakers based on industry standards similar to IEC 60268-5 or AES2-1984. These standards specify a pink noise test signal with a crest factor of two (i.e. 6 dB) which is meant to simulate the transient character of music having an average value, as well as frequent instantaneous voltage spikes that swing up to twice the average value. Pink noise is a particular type of random noise with equal energy per octave and actually sounds like a space shuttle launch. The test signal is applied to the test speaker for a few hours, allowing for a reasonable way to test the speaker’s real world thermal and mechanical capabilities. After testing, the speaker must be in working order, without permanent alteration of its technical features. The power rating is calculated using the RMS value of the applied voltage and the minimum value of electrical impedance within the working range of the speaker.

Guitar Amplifier Power Output Ratings

The power output rating of a guitar amp is mostly a ballpark figure for what it can put out. Amp specifications commonly list power output in a form similar to the following:   Power Output: 50W into 8Ω at 5% THD

This type of power output rating is obtained by using a sine wave from a signal generator (usually 1 kHz) as the input signal. The 5% THD (total harmonic distortion) figure means that the sine wave was able to generate 50W of power output with relatively low distortion (near the threshold of clipping or overdrive). THD measurements were one of the first conventions used to objectively compare the fidelity of audio amplifiers. 

Guitar amps are unconventional audio amplifiers. While most audio amplifiers are designed to keep distortion as low as possible, guitar amplification has evolved to where overdrive distortion is usually a requirement. For example, the Marshall® JCM800 2203 is a 100W tube amp that has a highly regarded overdrive sound. The owner’s manual lists the power output as follows:

Typical power at clipping, measured at 1kHz, average distortion 4% 115 watts RMS into 4, 8, 16 ohms. Typical output power at 10% distortion 170 watts into 4 ohm

This example shows that for many guitar amplifiers, the power output rating (100W in this case) is not a maximum power output rating, but more of a ballpark clean power output specification.

RMS and Overdrive Distortion

RMS (root mean square) is a kind of average value that can be used to compare the power dissipation from different signals on equal terms. For example, a 20 VDC power supply dissipates the same amount of heat across an 8 ohm resistor as a sine wave with an RMS value of 20 VAC.

Guitar amp output ratings are usually based on a sine wave at low distortion, but if the volume is turned up further or a gain boosting effect is used, the sine wave becomes more overdriven and can approach the shape of a square wave. The RMS value of a square wave is equal to its amplitude, while the RMS value of a sine wave is equal to its amplitude divided by the square root of two.

Plugging the RMS values into the equation for power shows that a square wave dissipates twice as much power across the same load as a sine wave with the same amplitude.

This simplified overdrive distortion model illustrates how the 100 watt Marshall® amp which puts out 115 watts at 4% THD could put out an additional 50 watts at 10% THD.

Tube vs. Solid State Outputs

Many tube guitar amps use output transformers with secondary taps connected to an impedance switch allowing for the same power output when connected to 4, 8 or 16 ohm load impedances. Solid state amps do not use output transformers and do not have the same power output when connected to different load impedances.

For tube outputs, it’s important to match the load impedance to the amp’s output impedance. For solid state outputs, it’s important to use a load that is greater than or equal to the rated minimum load impedance and to know the amp’s power output at that load. For example, the Fender M-80 is a solid state amp rated for 69 W(RMS) at 5% THD into 8 ohms and 94 W(RMS) at 5% THD into 4 ohms (the minimum load impedance).

With solid-state amps, overdrive distortion generated by the power-amp is not generally considered musically pleasing, so most people will not exceed the amp’s low THD power rating. Tube power amps, on the other hand, are often played well beyond their low THD rating.

Amps with Multiple Speakers

When an amp uses multiple speakers the output power is divided between them. The nominal impedance of each speaker should be the same value so that power is distributed equally and so that the output impedance of the amplifier can be matched.

Choosing Guitar Speakers to Last a Lifetime

There is no standard method used by all amp manufacturers when selecting an appropriate speaker power rating. If you want to choose a speaker to last a lifetime, you’ll want to choose a speaker that can handle the maximum amount of preamp and power amp overdrive distortion that can possibly be put into it and safely avoid exceeding the speaker’s thermal limits. In the case of single speaker setups, this means choosing a speaker rated for at least twice the rated output power of the amp. For multiple speakers, choose twice the rated power that would be distributed to it.

You might decide to go with a lower power rating because you know that you’ll never be cranked at full power and love the sound of a lower power rated speaker. In the same way you may choose a speaker with a much higher power rating because of the way it sounds.

A Real World Example: Speakers for a Fender® ’65 Twin Reverb® Reissue

1) Determine the rated output power of the amp.

Amplifiers have two power ratings: power consumption and power output. The power consumption is always much higher than the power output. In this case the output power is 85 watts RMS and the power consumption is 260 watts.

2) Determine the output impedance for that output power rating.

In this case it is 4 Ω.

3) Determine the number of speakers.

In this case there are two 12” (8 ohm) speakers wired in parallel for an overall impedance of 4 ohms.

For this amp, speaker choices to last a lifetime should be rated for at least 85 watts each. There are a lot of speaker choices rated for 100 watts and this rating would be very safe. Actually, the stock speaker for this amp is the Jensen C12K and it is rated for 100 watts.

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Kurt Prange (BSEE) is the Sales Engineer for Amplified Parts in Tempe, AZ(amplifiedparts.com). Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.

Written by Kurt Prange

Kurt Prange (BSEE) is the Sales Engineer for Amplified Parts in Tempe, AZ (amplifiedparts.com). Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.

The Reverberation Effect

A listener standing some distance from a sound source will perceive sound that is actually a combination of direct sound and indirect sound that has been reflected from the boundaries of the listening area. The reflections are referred to as reverberation. Reverb can enhance the perceived sound from a source by adding depth, color and liveliness.

Reverb can be thought of as being composed of two parts:
1) Early reflections - shape the listener’s conception of room size
2) Cluttered reflections - convey the liveliness of a room

Imagine you are inside a large hall and you clap your hands once. The length of time required for the arrival of the very first reflections is called the delay time (usually on the order of tens of milliseconds, e.g. 33ms) and is related to the volume of the room (or distance of the reflective surfaces from the listener). The number and density of reflections increases rapidly with time and they become cluttered while simultaneously decreasing in level until they are no longer audible. The length of time required for a sound to decrease in level by 60 dB is called the decay time (usually on the order of a few seconds, e.g. 3 s) and is related to the acoustical properties of the reflective surfaces in the listening area. For example, poured concrete walls will reflect more (absorb less) acoustic energy than drywall.

Electro-Mechanical Reverberation Devices: The Reverb Tank

Historical use in Musical Instruments

Laurens Hammond of Illinois popularized the use of artificial reverberation devices through his church organs in the 1940’s and 1950’s. “The early (pre-B-3®) Hammond® organs were sold to churches on the principle that organ music is greatly enhanced by reverberation, but the minister’s speech in the church is hampered by reverberation. Therefore, churches were designed to be acoustically dead, and the Hammond® organ had to have its own artificial reverberation.” ”[Reverberation] made its debut in the Fender® line as a separate item, using a spring [unit] bought from Hammond®, [in] 1961. It was first incorporated in a Fender® amplifier with the Vibroverb® of 1963 and then spread widely throughout the amp line, just as vibrato/tremolo had before it.”

To read the whole article click here.

New Old Stock Supplies being tested for use in Mojave™ Ampworks LLC amplifier products.

Chances are, if you’re reading this article, you know what a Vacuum Tube is. The Vacuum Tube, most often referred to by guitar players, Hi-Fi buffs or recording engineers as just “Tubes” or the European definition, “Valves”, vacuum tubes have been a dominate device in the audio world. Sometimes an antiquated technology such as vacuum tube technology, can be a bit more expensive when compared to cheaper alternative electronic products such as transistors or ICs but even though there are some small learning curves to be had or a few pounds more to cary, all is forgiven in trade for the results that are yielded when sound is involved. Nothing makes sound like a good quality Thermonic audio device we all know as the vacuum tube.

New Old Stock G.E.C KT-66 and GE 6BQ5 vacuum tubes

So what is it that makes the vacuum tube so desirable to the music enthusiasts? The simple answer is the sonic qualities inherent in the vacuum tube sound. If your a guitar player, then your no stranger to the benefits of vacuum tube amplifiers. It’s been said the “Guitar” is the icon of Rock and Roll, then it stands to reason the tube guitar amplifier is the other half of Rock and Roll.

If you are interested in recording and making your sound harmonically rich, you are fully aware that not only are there tube microphones, tube Microphone preamps, tube compressors and tube equalizers but that these components go hand in hand with, Magnetic tape and transformers which are large contributors to that process in creating the unique distortions and harmonics needed to give texture and musicality to digital sound. Just as it is in video, a video recorded on a cell phone or camcorder has a sterile look to it, not really the vibe you would consider as professional, that would be “FILM”. In the world of movies, FILM is what give the movie industry that color and vibe as does vacuum tube sound in digital recording. The majority of the greatest recordings ever made were done with vacuum tubes. The vacuum tube is responsible for what is commonly referred to as a “warm” sound and in other descriptions “harmonically” rich. Vacuum tubes are responsible for that character and unique sound that is highly coveted.

Test Results for the N.O.S. G.E.C. KT-66 show it is in excellent condition and above factory spec.

No matter what your interest in tube audio, guitar amplifier, recording processing or fine hi-fi stereo, there is one common factor that needs to be in place, quality dependable vacuum tubes. there in lay the tricky part. What is the definition of a quality dependable vacuum tube? The definition will be the same for all users no matter the application if it be for audio.

• Strong Emissions
• Solid high transconductance (GM)
• Low noise
• Low gas emissions
• Little to no heater to cathode leakage current
• Little to no contamination of the Oxide cathode coating
• Quality construction

These are factors that are generally very elusive to the end user if he or she is not a electronic tech type who is well versed in vacuum tube technology. The end user are people who buy and use vacuum tubes for audio products like guitar amplifiers, microphone preamps, studio compressors or any other audio product will be at the mercy of those selling them the tubes.

New Old Stock G.E.C KT-66

It would be really great if there was a reliable and guaranteed way a person could know the vacuum tubes they are looking to purchase do indeed meet all the criteria stated above.  What is needed is a modern day product that can reliably test all of these things and provide a time and dated stamped printed test result, which would have the factory specs on it and the resulting measurements. That would be easy, you just look at the factory spec and look in the column next to it where the test result is. If the result is the same or better, you know the tube is a safe and good purchase. That would eliminate all the worry and take all the guess work out.

Maybe a printout that looks like this:

The computer time and date stamped test results which are able to be printed and packed with the tubes include the factory spec and test results side by side.

Mojave Ampworks Has two Amplitrex AT-1000 testers for volume testing and batch screening.

Even if you do not know the technical details and what they mean, you can see if said tube is good just by seeing that the numbers match. It’s peace of mind and it is critical when you are a professional who needs quality tubes that are guaranteed to be what your paying for.

Thankfully there is such a way. There is a modern day product, a computerized piece of test equipment that is a laboratory grade tube tester and offers test results with time and date stamped results that can be printed and packaged with the tube. This little marvel is called the the Amplitrex AT-1000. Designed and built by hand in Southern California the Amplitrex AT-1000 is one of the finest vacuum tube testers ever built. Creator and Amplitrex owner Chris Terraneau is the mastermind behind the AT-1000. Terraneau (pronounced |te(ə)r|uh|now|) started the production of the Amplitrex while still working as a design engineer working with Microprocessors and X-Ray equipment. His unique skills in both microprocessors, vacuum tubes, computer programming and electronics engineering yielded the dream come true for tube enthusiasts around the world.

The New Old Stock G.E.C. test results prove this is an excellent tube

First, you need to know what the Amplitrex is. The Amplitrex AT-1000 is the state of the art computerized tube tester and one of the only tube testers produced that is able to work independently and has custom interfaced software for PC operation. The Amplitrex AT-1000 is a current production tube tester and unlike tube testers built 35 to 55 years ago, the Amplitrex offers laboratory grade testing under microprocessor control and makes it possible to check a tube according to the original manufactures design specifications. Even more valuable is the ability to duplicate not just our guitar amplifiers circuit parameters but any guitar amp parameters. With the Amplitrex AT-1000, we can test our tubes under the same conditions for which they are going to be used within our Mojave tube amps. Real life guitar amp circuit parameters work with more then 150 volts which is what most old testers use. The Amplitrex can put up to 500 volts on the plate @ 160ma of plate current. a Marshall super lead runs the power tubes anywhere from 460 to 500 volts on the plate. With the Amplitrex, we can duplicate these parameters. This means the tube will run on the tester as it would in the amplifier.

The ability to heat the tube up at the same temperature means any mechanical defect that would be subject to change because of temperature will be weeded out on the spot. Many other important variables are more likely to be brought to bare under real operating conditions. This important tube performance data cannot be duplicated with older “Proportional” type tube testers that use lower test voltage because the typical voltage applied (around 150 volts) simply does not stress the tube or create the heat and current flow that will be seen inside the guitar amplifier on stage in front of hundreds if not thousands of people where a failure is out of the question. Even if your just at home, your not going to have to worry that you paid for tubes which might not be good.

Because of this difference, you could feasibly buy a set of expensive new old stock power output and preamp tubes which were tested on a 35 year old tester and not get the facts. You wind up wasting money because you get your new tubes and they crap out after they are put in a guitar amp because they run hotter & harder then the tester they were checked out on. With the Amplitrex AT-1000, your tubes are tested exactly the same as the intended circuit they are to be used in.

In short, by having your tubes tested with the Amplitrex AT-1000, you see the following benefits:

• Your tubes are sure to be dependable.
• Audio tubes are certain to sound good.
• Tubes graded on the Amplitrex are known good and are going to last longer.
• N.O.S Pre amp and Power tubes are worth what you paid for them
• An audio noise test is performed. Only low noise tubes are passed.
• The Amplitrex assures your tubes are not contaminated internally
• Peace of mind that your tubes are certain to work without question.
• You can get your tubes exactly matched so you keep spares that do not need to be re biased by a technician.
• Your tubes are going to make plenty of clean power
• your tubes will not have weird sounds
• Higher gain tubes help produce better harmonics and clipping for overdrive.

A New Old Stock G.E.C. KT-66 test results are displayed.

The Amplitrex AT-1000 is without question the best tube testing product available. Mojave Ampworks builds top quality amp and we spare no expense even down to the quality of our tubes and especially our tube testing which is fundamentally critical for your product performance and reliability.

The Amplitrex provides a dated and time stamped data sheet for all tested tubes. We are happy to provide you your dated time stamped test results upon request.

Make sure you ask your favorite tube supplier, amp builders or service tech for Amplitrex Graded tubes. Only Amplitrex Graded tubes will due.

NEWS RELEASE

THD Electronics, Ltd. announced June 18, 2010 it has sold its Yellow Jacket Tube Converter line to CE Manufacturing, LLC. The sale includes the Yellow Jacket trademark, the THD designs, and the know-how to allow for a seamless transfer of the line. CE Manufacturing will continue to produce the hand made, high quality, and well respected Yellow Jacket tube converters that musicians love.

They will be made in the U.S.A. using the same quality materials at CE’s headquarters in Tempe, Arizona. The Yellow Jackets will be distributed through CE Distribution, LLC, a sister company. Yellow Jackets are specialized adapters which permit the use of EL84/6BQ5 power tubes in place of 6V6-, 6L6-, and EL34-family tubes and 7591 types. Because the Yellow Jackets ignore the amplifier’s grid bias voltage, it is possible to take them in and out at will to adjust your power level and sound for the individual gig or session. Andy Marshall, President of THD said, “The sale of the Yellow Jackets will allow us to concentrate on our core products, especially our Hot Plates. CE was the natural choice to purchase the line because for years, they have been our largest Yellow Jacket customer.” Noreen Cravener, President of CE Manufacturing and CE Distribution remarked, ”We are pleased to have made this purchase and have enjoyed working with Andy and THD throughout the process. We assure our customers we will maintain THD’s high standards of quality and excellent customer service.”
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CE Manufacturing, LLC and CE Distribution, LLC, are subsidiaries of Campanella Enterprises, LP. CE Distribution also supplies vacuum tubes, parts, supplies and literature to the music and sound industries.

CE Manufacturing, LLC                  CE Distribution, LLC
6221 South Maple Ave                     6221 South Maple Ave
Tempe, AZ 85283                             Tempe, AZ 85283
www.yellowjacketstc.com www.cedist.com

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From tubes to tools, speakers to sockets, and books to bridges we’re confident that you’ll find everything you need to upgrade your rig.

AmplifiedParts also has a very informative Tech Corner. Here you can find helpful information such as the ‘12AX7 Comparison Chart’ which details the differences between current production 12AX7 tubes. You’ll also find the Jensen Speaker Tone Chart, Useful Conversion Factors, and much more coming soon!

All items shown are in stock with lightning fast shipping (98% of our order ship the same day.)

AmplifiedParts -  what you need to tweak, freak, and speak your tone!

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The JBL 4311 came in in the late ‘60s and were used throughout the 70s by almost all of the major studios. These monitors are still considered a high end industry standard product. Designed to have a flat low frequency response at a mind boggling 22hz just below the human hearing level. 4311’s also have a user adjustable midrange and tweeter frequency adjustment on the front face of the speaker, this helps you to smooth out your sound to get the right balance in your room. The 2213 woofers were coated with a material called Lansplas which is easily recognized by it’s white color, this coating dampened out mid and high frequency response to ensure an efficient low frequency transducer. Not only is the 2213 woofer an alnico magnet but built with a super heavy duty cast aluminum frame. The midrange model LE 5 is a huge alnico magnet equipped 5” driver. It too is a cast aluminum frame and housed in it’s own isolated compartment within the cabinet. To top it off the LE 25 tweeter is positioned in a cluster form so the woofer is a top on the speaker and the mid and tweeter are at the bottom which  should set them at ear level when placed vertical a very distinct configuration. The purpose was to make them a near field monitor. These are a large near field monitor compared to what you would find today but the sound is incredible. To top it off these monitors have a beautiful reddish walnut veneer that gives any studio a warm classic look.

For those who are after that “Holy Grail” of tone, there are ways to find it. Many fell most confident with vintage guitars and Guitar Amplifiers. That makes perfect sense in many cases because you are in fact using the exact same gear. Granted the gear today reflects the sound and retains its integrity all these years later. But what about those of us who can’t afford the rare and hard to find? We turn to a more modern solution. The building of the great amps in todays market is all around us. Brands that are becoming well known from stage view and Studio. One way to judge a good amp is by its sound. Even if they are overbuilt and look exactly like the amps of yesteryear, you need to be certain your tone is what comes first and hopefully all the benefits of the upgraded new amp come with it.

Enter the Mojave Plexi 45 Special Edition. Under the unique and privileged opportunity of amassing 26 very rare and excellent condition Marshall JTM45s, so many of the greatest early JTM45s all at once, Mojave Ampworks (Also operating the business known as The Plexi Palace or Vintageamps.com) had the opportunity to build a great and exact likeness of the Marshall JTM45 but do it in its earliest of versions.  The close study of these amps gave us a unique and privleged opportunity to truly uncover the exact characteristics that make those old amps sound like they do in such a wonderful way. The Plexi 45, a member of the Mojave Ampworks amp line was already in production. the Special Edition was created from that model.