Here we discuss amp tech as well as common myths and misconceptions about transformer-related information as it pertains guitar amplification. We add to this list regularly so be sure to periodically stop by to see what's new. If you have a guitar amp transformer question, feel free to email us.




Industry Terms & Definitions (in progress):

The Phenomenon of Ear Fatigue

Did you know... that ear fatigue is related to poor transformer quality? If while playing (or listening to someone play) through an amp you find yourself tiring easily, or the experience seems unpleasant… you are experiencing the tell-tale signs of ear fatigue. It is nature's way of telling you that something's wrong. In nature all sounds are composed of layers of frequencies producing all sorts of harmonics and distortion. Ear fatigue is caused by information commingling with non-musical, unnatural noise that "beats up" your internal mechanisms. The mechanisms that make your hearing possible are working overtime trying to separate the desirable from the less desirable sounds. It's the output transformer's job to emphasize the even-order harmonics and make tones more musical, appealing, and less fatiguing. There's no short-cut to building a quality transformer. The result is less effort by your ear to receive this information.


The Paper vs. Nylon Bobbins Myth

Fiction: Transformers made to look vintage with paper tube bobbins (PTBs) somehow have better vintage tone than those using nylon.

Fact: PTBs were used in the early years of transformers only because they were the cheapest material available. Back then tone, consistency and long term reliability usually took a back seat to meeting a low budget. This still remains true today. If the issue of using PTBs for the sake of sounding more vintage was that simple then sporting a set of sideburns like Elvis should give you the ability to sing like him. Impersonators/imitators rarely scratch below the surface to find the heart and soul of what advances art and science.

Mercury's goal of providing you with the finest sounding transformers on the planet could never be achieved by peddling myth, voodoo, junk science or unqualified opinion. The bottom line is that under close scrutiny PTBs fell short of our tonal expectations, not to mention the valued opinions and advice we’ve received from the finest players, recording artists, technicians and engineers of the past fifty years.

Let's leave the question of paper or plastic to the box boy at your local grocery store. At Mercury we combine the best of what tradition has to offer with the best of today’s technology -- and of course plenty of hindsight to boot. If you love music as much as we do, then you won't settle for anything less than a set of Mercs in your amp -- the best in tone without compromise.


Transformer Rust -- Friend or Foe?

Question: I heard that rust is bad for my transformers. I’m being told that rust causes an increase in eddy current losses which will heat up and further damage my transformer. Rusty transformers will also hurt my tone, so I must remove the rust or replace the transformers to make my amp operate properly and extend its life.

Answer: Nothing could be further from the truth. While iron possesses the necessary magnetic properties, it is also electrically conductive. Eddy currents are circulating currents in the core induced by a magnetic field emitted by the energized primary winding of the transformer. These electric currents are undesirable. The core is acting much like a shorted secondary winding. They are power-robbing losses that convert your guitar playing (input power) into heat that beats up your transformer. This effect is certainly not helping your tone!

On the other hand, rust (iron oxide) is a nonconductor of electric current. The high resistivity of rust reduces eddy current losses. Since iron is also a conductor of electric current and the goal here is to minimize eddy currents, which is why the construction of the core is made up of a stack of thin plates of iron (laminations) which are insulated from each other by an oxide (rust-like) coating. This confines the eddy currents and prevents them from circulating between the laminations. The higher resistance created results in a reduction of power losses within the core.

What’s this mean in plain English? Back in the early years, manufacturers of laminations intentionally exposed their iron to moist air to encourage rust to naturally form on the surfaces as an insulator! Like a fine patina on an antique bronze the rust on your transformer could be viewed as badge of honor. Assuming that your transformer doesn't have other problems like coil damage from excessive moisture exposure (common with paper bobbins), you can actually expect some improvements in output transformer tone and lower losses in power transformers as rust develops.

If you are still bothered by the cosmetic aspect of rust, never scrape off the rust to the point of exposing bare metal because you will bring back those nasty eddy currents. Instead, brush off the loose particles and re-paint the area with varnish. If you want to send your transformer to Mercury, we can re-dip it in varnish and bake it for a small fee. Transformer evaluations are always no charge providing you cover the shipping costs to and from Mercury.


The Power Transformer -- Your missing link to better tone?

Many people, including more than a few amp techs, have the mistaken belief that when it comes to a power transformer, "volts-is-volts," and upgrading a PT won’t have much of an effect on a guitar amplifier's overall tone. Not so! My tone transmogrifying tweakers!

The audio circuitry of your amp is completely dependent on the quality of its power supply. The very first component in this food-chain-of-tone is the power transformer. It makes the necessary connection between your amp and the power company. How it behaves sets the standard for your amp's performance. The PT has a profound affect on your amp's sonic characteristics especially its ability to handle musical dynamics. What this means to you is crisper note attack, better note separation, and added sparkle in the upper mids and treble frequencies. And keep in mind that with a properly-designed power transformer you'll hear all-around higher definition with the amp's clean and dirty tones.

Do not assume that all amp manufacturers know this, or that they have provided your amp with the best possible transformers (unless they are already equipped with Mercurys!). Instead you might just be stuck with low-cost, short-cut transformers. Remember, there are dozens of ways of building cheap transformers to meet certain voltage values but they all fall short of giving your amp the best musical qualities—that's the magic of Mercury Magnetics!

Which turns you on more? A tranny built only to please a volt meter -- or one that maximizes the sonic potential of your amp? You no longer need to be a victim of the sonically-challenged. It's time to liberate your amp with the only guitar amp transformers actually designed for better sound—Mercury Magnetics.


Next-Generation Trannys: Vintage tone -- but more, more... MORE!

Did you know that Mercury™ has been developing and perfecting THE next-generation of guitar amp magnetics? When it comes to guitar tone our Axiom® series may be one of the most significant advancements since the mid-1960s.

Here's why. We now have the "rear-view-mirror" benefit of knowing what made certain amps sound good... and just as important -- what didn't. Mercury was the first, and still is the only company, to capture and retain the sound of the most desirable old school amps with our ToneClone® series trannys. But, along the way, we discovered that we could now do things the old school guys never dreamed of. The Axiom® series was born!

Our Axiom® series answers many of the "what if" questions that has plagued both players and amp designers for decades. For example: What if you could mix the characteristics of one amp’s outstanding magnetics with another -- say, get both Fender- and Marshall-tones from the same amp without the lifelessness of digital emulation? Or, what if there was no quality limits imposed on how transformers were made (you wouldn’t believe how dumbed-down most transformers are!)?

The Axiom® series isn't only the result of engineering expertise. We received profound contributions from many of the finest players who ever recorded—musicians who generously gave their insight and direction to this on-going project. Axiom® "high def" transformers take your tone, clean or distorted, to the next level. Axiom-ized amps give you more vivid harmonic overtones, added sparkle, more dynamic headroom, and much better note separation. Even if you’re into weapons of mass distortion, Axiom® transformers deliver a far wider spectrum of tonal variations and possibilities than any previous design.

So, whether you're upgrading an existing amp, or working on a new design, give our Axiom® line a test drive—you won't be disappointed.


Output Transformer Bobbins -- Is Paper the Real Gold Standard?

Source: Musician's Hotline Magazine. Tech Views article by R.G. Keen, Chief Engineer, Visual Sound.


Most of us agree that amps built with vacuum tubes have superior tone. Therefore, it’s the tubes that take all the credit for great tone... right?

It is easy to see where one might get that impression. So far, the tube remains the ultimate conductor of musical signals. But in reality the tube is completely dependent on the circuitry connected to it. Consider the perfect tube hooked up to a perfect circuit. The resulting amp would have little to no distortion. Signals would pass through exactly as they came in. Nothing added—nothing subtracted. Amplified, but cold and sterile. The ideal conditions for neutering your guitar tone.

As you can see, blindly seeking design symmetry can work against us when it comes to electric guitar tone. To better understand this concept let’s compare it to rock vocalist style. Imagine in your mind your favorite singers -- do they have perfect vocals? Far from it. There’s always at least some grit or grind to their singing styles. The use of unusual vocal techniques is what makes their voices interesting and memorable. Opera is not what we’re looking for here.

On the other hand, the presence of odd harmonics in music is generally annoying. Better-sounding amps work to minimize these odd harmonics. At Mercury we design and build transformers that enable your amp’s circuitry to emphasize even order harmonics between specific frequency ranges. Our methods intentionally showcase the virtues of electric guitar tone.

To get desirable tone from a tube it must be coaxed, bothered or even irritated in some way. This is where the tubes share interdependence with the transformers. Great transformer designs are the key to provoking tubes to behave in a manner that is conducive to getting desirable tone. Having the right transformers has long been one of the fundamental insider secrets behind great tube amp tone!

Remember, your amp’s transformers are the cornerstone of its tone. Whether you’re a player or a builder, using Mercury transformers is unquestionably your best investment in superior tone, reliability and amp value.


When the Smoke Gets in Your Eyes

Did you know that the transformers and chokes in your amps are aging? If your amp is 20 years or older you really need to know this. Over time, the materials that were used in the construction of these transformers have changed -- actually degraded, and become increasingly fragile. Before the "smoke gets in your eyes" hang on to your power cord and don't plug in just yet until you understand the following.

Basically, transformers can be broken down into three components. The first being "copper," which is what the wound coils inside and the lead wires are made of. The second is "steel" which is what the stack of metal plates in the core are made from. Third, and most important to overall life expectancy, is "insulation" whose function is to keep things alive and running properly by not letting the other components touch each other and/or conduct when they are not supposed to. A breakdown with any one of these components will eventually grenade your amp!

Copper by itself doesn't really age, but the thin, clear coating of insulation around it does in the form of microscopic spider web cracking. This will lead to high voltage break down in the winding. Lead wires are also susceptible when their vinyl or cloth jackets become old and brittle -- sometimes causing arcing through the cracks and splits in the insulation. Phantom clicks and pops heard through the speaker are also telltale symptoms of aging.

Steel can degrade over time and extended use, especially if contains traces carbon. Carbon is a contaminant and has no positive contribution towards tone. The older the amp is, the higher the possibility. When that happens, the core begins to be less conductive to magnetism (magnetism is the essence of how transformers work!). Symptoms of that occurring include higher transformer operating temperatures, general dulling out of tone with the absence of treble frequencies, fuzzy mids, and a noticeable softening of the bass notes. None of which has anything to do with rust. Remember, rust on your core is a helping friend. For more information on this check out the article "Transformer Rust—Friend or Foe?"

Insulation is where most of the aging problems lie. The older your amp is, the more likely its transformers were wound on paper tube bobbins -- with Kraft paper as its insulation (which is no different than the paper used to make shopping bags at your local grocery store). This material has a tendency to suck up moisture and retain it. Moisture alters the delicate balance of reactive values in the transformer throwing off your tone into less interesting domains. The more destructive side to this moisture absorption is that it harbors mold and bacteria which consumes the paper, further diminishing its insulating properties. Even if this material has avoided moisture altogether, it will eventually crumble into dust over time.

Are all vintage amps time bombs? Certainly not. But to be safe don't play Russian Roulette with your valuable, irreplaceable, and highly-prized collectible vintage amps. If your amp hasn't been fired up in years, don't just plug it in and flick that switch on. You risk seeing your investment go up in smoke.

If you are in possession of a Variac, a volt meter and a current meter then you have the necessary tools to properly and carefully ramp up the input voltage while monitoring the current draw. This will also help reform the filter caps as well as avoid the "smoke test" with your transformers. If you don't know what any of this means then we strongly recommend getting an amp tech to do it for you. If you don't know of one, call us and we’ll help you find one qualified to do the job. When the fuses in your amp keep blowing it is your amp's way of telling you something's wrong and to take it to a professional.

One of the most important services Mercury Magnetics has to offer is free to all customers. We will evaluate your transformers with the most thorough and advanced testing. We do this at no cost providing you cover the shipping costs to and from Mercury. Transformers with no fault found are sent back to be reinstalled in the amp with a least the knowledge and confidence that things were double-checked. Transformers that failed in testing are either rebuilt/restored or replaced with ToneClones®. Free estimates are always given in advance. Either way you leave satisfied with the confirmation that your original works fine or the problem can be fixed.


Take Your Transformer's Temperature, Part I

Caption: Photo shows measuring the temperature of a transformer's laminations with a NON-CONTACT thermometer. In this shot the transformer is outside of an amp and hooked up to a Variac. This type of handheld thermometer does not require direct contact with the transformer and can read temperatures from a safe distance.

  • Temperatures below 100°F are generally considered "cold" or "cool" in terms of the operating temperature of a transformer.

  • Temperatures between 100°F and 110°F are considered "warm."

  • Temperatures between 110°F and 120°F are considered "very warm."

  • Usually temperatures above 120°F are considered "hot."

  • Temperatures above 130°F may cause second or third-degree burns on adult skin.

Transformers designed with high-temperature insulation systems can run safely at temps up to 200°F. But remember, a hot-running transformer is an angry transformer.

Want to know the health of your amp’s transformers? Take their temperatures! A hot-running transformer is an angry transformer and it may very well be telling you its dying. It could be an older vintage unit that is showing signs of aging materials. Or, you may have a newer amp with a far-to-common undersized "runt" transformer. Either way heat means a waste of power and potentially shorter transformer lifespan -- not to mention strangulation of your tone. Transformers should normally run between warm to very warm temperatures. Hot is a bad sign -- especially in the case of output transformers (OTs). Make sure that other components are not the cause of excessive transformer heat. For example the tubes should be the only hot-running component in your amp. But keep in mind that if they're too close to a transformer they could artificially heat the unit, also leading to damage over time. Of course there's more to the story, but if you are in doubt then see your local amp tech or give us call. By the way, Mercury recommends using a non-contact thermometer to take your readings. If you do have a dying transformer it's time to think about a rebuild, replacement or upgrade. Remember, Mercury offers you not only the best-sounding and most reliable transformers, but more tonal choices than any other company!


Take Your Transformer's Temperature, Part II

Did you know that hot-running transformers not only drain your amp's reliability but can also cause major tonal issues?

The problem is so common that many players mistakenly assume that transformers should run hot. Not so. In fact, excessive heat is an all-to-common sign of under-powered "runt" transformers that are potential tone-killing accidents waiting to happen.

Heat is nothing more than a symptom of energy loss. And hot-running transformers certainly do nothing to contribute to your tonal quality. Energy loss means one thing—inefficiency. It's like driving a hot rod around with the parking brake on. Think of "runt" transformers like brakes on your amp, making your tone dark and muddy.

Here's why: Insulation aging is accelerated by higher temperatures. As insulation temperature approaches redline, resistance begins to decrease. A breakdown occurs when insulation becomes damaged by the unchecked rising heat. Transformers wear-out from the effects of usage, time and high temperatures.

Exposure to electrical spikes can also contribute to stress on the transformer's insulation system. If that wasn't enough, the coils of wire inside your transformer are often beaten up by vibration! These internal vibrations sometimes cause the magnet wires to rub against each other, wearing off the insulation and causing short circuits.

The life expectancy, and resulting tone, of a transformer is completely dependent upon its insulation system. The build-quality and consistency of the transformer design must be greater than the working conditions and stresses you subject it to (i.e. how hard and often you play). This explains yet another reason why transformers wound with paper tube bobbins are the most susceptible to thermal- and aging-related breakdowns.

Remember, a hot-running transformer is an angry transformer. It is angry because it is overstressed. This may not cause your amp to immediately fail, but eventually it will. The ideal transformer should run between cool to very warm. Excessive heat is nothing more than the amp’s circuit demanding more power than it’s transformer can give. The proof is when you correct the problem. Swapping the "runts" with higher-efficiency upgrade trannys gives you enhanced tone by way of increased headroom, higher loaded voltages, better response and often a very perceivable tonal depth improvement.

Getting you both superior transformer tone and reliability isn’t easy. At Mercury we tackle the problem by making uniquely musical transformers for specific guitar amp designs. Each transformer is made by hand, one-at-a-time, then individually hand-tuned for maximum tonal performance -- and coolest running temperatures. No other approach comes even close.

With Mercury transformers you’ll have one less thing to worry about in your quest for great and consistent guitar amp tone.


Looking for Tone in all the Wrong Places

Question: My amp guy told me that "core saturation" (overloading the transformers) was part of my amp's distortion -- and important to my tone. Is this true?

Answer: This is a popular misconception. You can irritate a tube to distortion -- but the rules are completely different for trannys. Here's why. Think of the musical signal that comes out of your electric guitar like the fuel you put in your car. Where does it go? It goes into the gas tank of course. The inductance of the output transformer is a lot like that gas tank. It holds the fuel and makes available as much as the engine demands. The engine's ability to generate horsepower and torque can be severely limited by restricting the flow of fuel.

The same goes for your amp. Deep and tight bass notes, and how large the perceived soundstage your amp can deliver to, requires a lot of inductance to do the job properly. Undersized transformers and circuits designed to overload or saturate transformers won't cut it. Inductance gets sucked out which converts your good tone into heat! So you not only get tonal loss but potentially shorter life expectancies from both your transformers and tubes!

You'll know your amp's trannys are being pushed beyond their operating capacities when you hear:

Make sure your amplifier is up to the task of delivering all the tone you demand with the proper output transformer installed. Don't feel like you’re stuck with a poor sounding amp. There are many choices available to you from Mercury that will fix the problem. Buy the best and transform your uninteresting amp into a force to reckon with.

Technical explanation: Core saturation is the condition where the core material (usually iron) is completely magnetized. Any further increase in input energy will not produce an appreciable increase in magnetic flux. When this happens, the inductance of the coil is greatly reduced because no counter EMF is produced when the flux is unable to increase.


Give me your dirty tone...
the way your nasty little amp makes you play...

So where does great tone come from if nearly everybody is using the same parts from a small circle of high production manufacturers? What gave vintage amps their great tone and why do some amps sound better than others? How come so many new amps don't have that inspiring tone anymore? And where do babies come from?

The answer to (most of) these questions, and the absolutely mission of Mercury, is in the TRANSFORMERS. We're not kidding when we call them the "heart and soul of your amp." Transformers establish the character and behavior of your amp's ability to generate tone, from clean, to dirty harmonic distortion. That's why Mercury goes through the trouble and expense to build and tune transformers the hard way: by hand, one-at-a-time, with old-world craftsmanship and the most tonally effective materials money can buy. The end result is all the guitar tone you want without a speck of cereal.

But what does the future hold for the electric guitar player and vacuum tube guitar amp owner? Is there truly something out there beyond the limits of vintage tone that isn't going to be solid-state? Yes, we think so. With just the right combination of know-how and passion, Mercury is now helping amp builders re-think guitar tone by re-inventing it. We call these new tone-generating machines Axioms. The Axiom line showcases new advancements in magnetics through transformer design. Designs that give amp builders better tools to work with and tone that we feel will better connect you to your guitar. All without following conventional vintage designs or recipes.

One fine example of Axiom-based advanced transformer technology is Mercury’s Epiphone Valve Jr. Kit*. What started out as a simple demonstration of Axiom transformers gave a $100 economy tube amp $1,000+ worth of tonal performance! It's hard to believe there isn't a vintage bone in their windings! We're not surprised that this best-selling kit is fast becoming the first choice of pro players and recording studios alike.

Imagine an affordable entry into the world of boutique amps that delivers top-shelf vintage American clean tones all the way to the overdriven distortion of the old British amps. Also imagine an amp that totally responds to every tone knob and pickup adjustment setting on your guitar. It's fun stuff!

Our little upgrade kit is a showcase of what the future has in store for harmonically-rich tone. We invite you to check out our Epiphone Valve Jr. Kit and hear for yourself just what everybody's talking about. Or ask your amp tech about other Axiom upgrades for your amps. Remember, in addition to our already amazing-sounding ToneClone and Radiospares lines we make Axioms for most popular amps.

*With special thanks to friend and amp guru, Alan Cyr (


Dealing with Ghost Notes:

-- Doug Roccaforte,

Many amplifiers are prone to "ghost notes." It helps to know what kind of amp we're talking about.

Certain capacitor brands, like F+T, LCR have high leakage current. Other capacitors for coupling have ESR problems, sometimes caused by the physical construction, and lead type. Pure copper leads reduce this, and tubular capacitors are less prone. This is because they do not have any flat sides for the signal to bounce off of.

Some amplifiers like Fender Black/Silverface models that have the ghost note issue can be reduced by cutting the phase inverter "output" coupling capacitor values in half.

Others generally are just suffering from old, or leaky filters. Vintage amplifiers like Vox AC30s have very low filtering, I believe this was not because of design, but more as an economical approach to save money. The same is true with many amplifiers designed in the 1950s. Increasing the filtering can cure the ghost note problem.

With Marshalls there are two ways to help cancel this out:

  1. Install a luf 600-1000V non-polar plastic capacitor across the last decoupling electrolytic in the line. This is the last filter, which filters the supply for the preamp. The capacitor will be wired to "by-pass" this filter, one lead to ground, the other to the capacitor lead.

  2. Increase the filtering. On 50 watt models, it seems to take another 50mf across the main B+, and another 50mf for the screen supply.

For 100 watt Marshalls expect to add another 100-150mf across the main B+, and at least another 100mf for the screen supply. Now, the amp will be stiffer on bass notes, if not acceptable, you can add anywhere from a 100-200 ohm 209 watt resistor in series with the output transformer center tap, this will give back a looser feel.


Clean Headroom:

Question: Can a changing an amp's OT create more clean headroom?

Answer: Changing out an OT with a tranny that has the same impedances as the one being replaced won’t deliver more power because volts-is-volts and watts-is-watts. Meaning: no extra power is delivered here for free.

You can force out more power from a tube by lowering the primary impedance of an OT. This usually does more harm than good, shortened tube life and harsher tone for starters. A player can get more out of his amp like extended clean tone headroom and a noticeable presence of a bigger soundstage effect by way of transformer solutions. By increasing an OT's “tank capacity” with more of the right kind of iron and copper resulting in more inductance to the primary winding. It’s this inductance that has a more profound effect on tone and how large it sounds that goofing around with starving tubes with lowered impedances.

OTs that are physically larger in size do sound larger even if they may have the same impedance as their smaller counterpart. The reason for this is the energy carrying the guitar tone information through the tranny is seeing less power-robbing resistances and going through its magnetic field transformation with faster responding iron helps the tranny to have a larger window for tone to flow through.

The same holds true for the PT. While the OT benefits the player with extended headroom and frequency range, the PT supplies more energy and the quicker delivery of it to the amps circuit resulting better bottom end and improved note dynamics in the form of note attack and separation.

This is also why amps that run hotter than a bitch kitty usually don’t have good tone as well.



Working inside a tube amplifier can be dangerous if you don't know the basic safety practices for this kind of work. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure yourself or get yourself killed! This is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one.



Victor Mason of Mojave Amp Works and PlexiPalace fame shown with his find the fine of the century!


Some interesting discoveries were made during the course of our research, development and playing a whole bunch of the original Marshall JTM-45 amps to help us achieve our benchmark ToneClone transformer series for this mighty amp.

Many thanks also to our player and collector friends for loaning us their amps and offering their insight for the cause. Many hours of tasty licks included. Incidentally, we do actually own one of the first coffin logo offset chassis amplifiers built by Mr. Marshall and crew.

There are more JTM-45 amps built using EL34 tubes than with the legendary KT66 tubes and there is nothing wrong with that. But a change of tube type also requires swapping out the output transformer to get the correct operating impedance and tone.

All of the original (limited run) handmade amps had 6L6, 5881 or KT66 tubes connected to a Radiospares "De Luxe" output transformers. The "De Luxe" model being the premium offering followed down by the "Heavy Duty" and "Hygrade" models. Primary impedance was selected at 6.6K.

The Bluesbreaker’s legendary tone was made by using KT66 tubes and Radiospares (RS) premium transformers. The transformers did most of the work here with their unique high primary inductance and low leakage inductance combined with padded (surplus) primary impedance as compared to Fenders of the time.

Note: With the exception of the transformers the JTM was a copy of the Bassman. An interesting discovery came to our attention here. It seems that most of the amps made during that era had the output leads connected in reverse phase (backwards). Was it by intent or a happy accident? We and a number of pro players seemed to prefer the reverse phase connection after many A/B tests. We had the impression that the speaker cabs sounded better, more articulate.

When the JTM-45 went into full production, the KT66 tubes were switched for EL34s and the Radiospares transformers were replaced with Drake transformers (judging by build quality and style). All to save BIG on costs since at the time Marshall was being hammered by a distribution deal that shot up retail prices for his amps right through the roof (a deal Jim Marshall later regretted). The EL34s were decent sounding tubes, but the new transformers (not so Bluesbreaker-ish) bore no resemblance to the original RS designs. These simplified transformers had a primary impedance of 3.4K with a much more simple (lower labor cost) design.

The bonus here, though, was in the easier to hook-up speaker connections for the player on the road, which also made it more difficult to connect speakers to the amp in reverse.

Another interesting piece of historical data is that when the EL-34 started earning the reputation as an affordable, reliable and great-sounding tube, the people at Radiospares added it to their compatible list of tubes for the RS "De-Luxe" output without changing the 6.6K impedance. And the EL34 still sounds great through the RS output transformer!!


Industry Terms & Definitions

Key electronic terms as they relate to your tube amp
and transformers, in a musical industry

5V rectifier winding: This winding is needed if your amplifier uses a vacuum tube rectifier in its power supply circuit. It is a separate, isolated winding which supplies energy to heat this tube to its operating temperature so it may perform its function of converting AC (alternating current) to DC (direct current) which is the form of energy your amplifier's circuitry requires.

6.3V winding: Technical definition. This winding is also known as a "Heater winding" or a "Filament winding." It is usually configured in one of two ways. First, with two wires or terminals, meaning with no center tap. Second, with 3 wires or terminals which includes a center tap. The choice depends on which method of hum reduction the particular amp's circuit is utilizing. Generally, the older vintage amps came with the two lead system and newer amps used the three lead system. The specific voltage of this winding is designed to match the voltage requirement and ultimately, the operating temperature range of the heater elements located within the vacuum tube. While this specific voltage is common to most "MI" (musical instrument/industry) amplifiers, there are other tubes which have different heater/filament voltage requirements for their correct operation. So, make sure your transformer specs are compatible with your choice of tubes.

Friendly definition. The quest for the best tone a vacuum tube amplifier can give you begins with the power transformer. The build quality and the precision of its specifications have a profound affect on the ability of the amp's compliment of vacuum tubes to run at their peak performance. Each individual winding of the power transformer supplies energy critical to the needs of the amplifier's circuit. Often times, and mostly by ignorance, the heater/filament winding and its purpose is dismissed as incidental. Manufacturers of lower cost transformers usually make it an opportunity to cut costs on labor and materials here. How sad! Especially when you consider the stability and consistency of the energy provided by this winding then becomes the temperature standard for the tube's performance. You may ask, “Why is all this energy/temperature thing being stressed as important to my amp's tone?” The answer is found in the vacuum tube itself. The tubes in your amp work by thermionic emission. This means that within the vacuum contained in this glass tube is a piece of metal which is heated to the point of incandescence giving the electrons enough energy to fly off its surface. These free electrons flow through empty space and are attracted to another element in that space which carries a positive charge. The movement of those electrons and their direction are influenced by the charge of other elements also contained that empty space. This means that current is being conducted through vacuum and not a solid conductor or "semi" conductor which you know as "solid state." So would you prefer your tone to flow via electrons through vacuum or stumble through solid and/or semi-conductors? (Hint: a vacuum is superior.) I can just see the bumper stickers now -- "In space, no one can mess with your tone." Remember, better transformers make tubes happier which improves your tone.

B+: Technical definition. It is a term used to identify a DC voltage supply with a positive polarity.

Friendly definition. Why use the term "B+" and not "V+"?? Long ago, those who ponder such things referred to this voltage as "Battery" voltage or a "Battery" supply. So how long ago was that? Well, the roots of modern electric engineering can be traced back to about 1819 when the relationship between electric currents and magnetic fields were first discovered. By far the biggest power demand placed on the power transformer of your amp is for the B+. Hence the B+ winding which pretty much determines the audio wattage rating of the amp. The quality of this winding will affect how much air your amp will blow through speakers when it is "dimed out" (putting the volume on "10"). Having said that, this is the part of the transformer where the low budget guys really cut corners on materials and labor. Their "good enough" approach works with the belief that the typical musician couldn't hear the difference and treats them like low rent customers. The character and tone of your amp along with its output power heavily depends on the B+ winding build quality.

bias: Bias or rather grid bias on a vacuum tube amplifier is a DC current influence to flow the current inside the tube in a specific direction. Bias voltage in your amp is usually negative in polarity. Think of it as a kind of governor or "rev-limiter" to the tube. Keeping the tube's operation in check and preventing its amplifying energy from running away and grenading. Biasing your tubes correctly will extend overall tube life and optimize them for their best possible tone.

bias tap: This is a single wire non-isolating tap from the B+ winding that supplies energy for the bias supply [negative voltage] of your amp. This configuration is commonly found in older vintage amps.

bias winding: This is an independent fully isolated winding dedicated to supply energy to the bias supply [negative voltage] of your amp. It is easily distinguished from the bias tap setup by having two leads/connections and measuring no continuity between it and the B+ winding or any other winding. This is the preferred choice of today's amp builders because of its inherent stability and independence from the potential energy fluctuations the B+ winding incurs during amplifier operation.

bobbin: A coil form which is made of insulating material such as plastic, Phenolic or paper where upon wire is wound to form a coil.

choke: A power-supply filter whose inductance is used to limit the flow of AC while allowing the flow of DC. Chokes naturally maintain a more constant flow of current. Output voltages do not rise as high as they would without this inductor in the circuit; neither does it drop as low when compared to a capacitor input filter circuit. Chokes hold the variation of output voltage (ripple) to a relatively small value.

coil: A number of turns of insulated wire wound around a form of insulating material which may or may not include an iron core. Some coils are rigid enough to be self-supporting and do not require a winding form and/or a core to secure it. A coil is also known as an inductor. Inductors possess the property of having considerable opposition to the flow of AC while allowing DC to pass through the flow.

conductor: Any material which offers little to no resistance to the flow of electrons. Examples:

GOOD conductors =

  • gold

  • silver

  • copper

  • human beings

  • aluminum

  • mercury

  • lead

  • etc.

BAD conductors =

  • paper

  • glass

  • plastic

  • rubber

  • wooden parrots

  • leather

  • etc.

core: The center around which a coil of wire intended for a choke or transformer is wound.

core saturation: The condition where the core material (usually iron) is completely magnetized. Any further increase in input energy will not produce an appreciable increase of magnetic flux. When this happens, the inductance of the coil is greatly reduced because no counter EMF is produced when the flux is unable to increase.

Myth: "Transformer saturation is a part of my amp’s distortion; it is an important part of my tone." -- Knucklehead Smith

Wrong! Don’t you believe it—nothing could be further from the reality! Audible symptoms of this type of saturation are the absence of bass and the fuzzing out of your mids along with dark and dull treble frequencies. Does that sound like something you find desirable? Core saturation also greatly increases running temperatures of the transformers and tubes. Your amp at this point is crying out in pain! Sorry, not the best working environment for your amp and for your tone. Think of core saturation as playing with a virtual noose around your neck (and your amp's tone). It's a bad thing.

counter EMF: As a change of current flow through a coil induces voltage, the counter EMF polarity opposes that change in current that produced it.

eddy currents: Circulating currents induced in magnetically conductive materials in the core cut by the varying magnetic flux of the coil. The resulting heat from these currents are unwanted and are considered as power loss.

EMF: Electromotive force.

FatStack (SuperStack): FatStack transformers are custom-designed Mercury Magnetics transformers that have extra iron. In an output transformer it extends the bass frequency response. In the power transformer it can minimize tone drift giving your amp more stability and headroom. Our SuperStack is similar but built higher to accommodate smaller mounting footprints.

flux: A shared term to identify electrical and/or magnetic lines of force in a given area.

flux density: The number of magnetic lines of force passing through an area being considered or in question.

lower flux density: Means higher available inductance which gives you more operating headroom. LFD creates a much more relaxed operating environment for the transformer – so it does the job easier. It’s important to note that most off-shore or otherwise low-quality transformers run hot because of high flux density issues. It costs more to build LFD transformers.

inductor: See coil definition.

insulation: A non-conducting material that contains, supports or separates conductors to prevent the flow of current between them and/or other objects touching or in close proximity to the conductor(s).

iron core: Iron is a transition metal widely used for power and audio output transformers and chokes because of its excellent magnetic properties. It is one of the chemical elements with the atomic number of 26. Atomic weight 55.847. Symbol Fe (ferrum).

hysteresis: A delayed response by an object to changes in the forces acting on it, especially magnetic forces.

hysteresis loss: This type of loss occurs when the magnetic material of the core doesn't have the ability to keep up or change its magnetism as quickly as the changes in current flow it is subjected to.

lead wire: A jacketed insulated copper wire which attaches to or from a component like a transformer or choke. Typical insulating materials used for this type of wire is vinyl, Teflon, fiberglass cloth sleeving. Sometimes "self leads" are used from a transformer or choke which is the extension or continuation of the magnet wire exiting the coil.

magnet wire: A thinly insulated copper wire of high purity that is available in various sizes exclusively for winding coils in transformers and other related magnetic devises/components.

output transformer (Mercury's circuit equivalent): (CLICK ON THE THUMBNAIL TO THE RIGHT.) The transformer is a reactive component -- its values change depending upon the information feeding it. In this case, your guitar tone. Note that transformers for tube-based electric guitar amps are designed to intentionally provoke the tubes into distortion -- the polar-opposite of demands of hi-fi. The best guitar tone comes from harnessing and manipulating the inherent flaws of the imperfect transformer. This is where art meets science in our pursuit of great guitar tone. Follow this link for an information sheet on this topic (includes schematic).

primary: The transformer's input winding which carries incoming energy/current and transfers it to one or more of the secondary windings.

ripple: An AC component in the output of a DC power-supply. Also known as the beverage of choice for the frustrated amplifier designers when they are in the process of "self medicating."

secondary: The transformer's output winding(s). The energy/current that flows from this winding(s) is due to the inductive coupling between it and the primary winding.

transformer (iron core): An electrical devise made up of two or more coils of wire commonly referred to as "windings" wound on a core of magnetic material. These inductively coupled coils transfer energy from one winding to another without a direct connection, i.e. one wire in contact with the other. The core increases the inductance of the winding so that fewer turns are needed to induce an amount of voltage while requiring relatively lesser amounts of current to do so (when compared to coils not employing a core). Since the coils are located within the continuous magnetic path of the core, most of the magnetic field produced by the current going to the input coil "primary winding" transfers itself to the output coil "secondary winding." What is keeping this transfer of energy from being 100% efficient are power losses from the resistance of the wire used to make the coil (I2R) and losses from the core caused by eddy currents and hysteresis. These power robbing losses can be minimized by construction practices such as laminating the core by use of stacking thin sheets of iron which are insulated from each other. Copper losses can be reduced by using more copper like increasing wire diameter. All in all, iron core transformers are usually more efficient than their transformerless switcher power supply counterparts. 95% vs. 80%. So much for high-tech!

Variac: An iron core toroid (doughnut shaped) transformer built with a sliding contact which moves along an uninsulated "bare or exposed" portion of the secondary turns to vary the output voltage while being connected to a steady state line input voltage.

Caution! This type of transformer is non-isolating. The devise or circuit you are connecting to it will be hard wired to the power company line. Always double check your ground connections and test for live chassis before touching anything. When in doubt, please consult with a qualified technician before energizing your circuit or gear. Remember you make a better conductor than a wooden parrot. See conductor definition above.


For more information, assistance, quotes or inquiries:

9167 Independence Ave. • Chatsworth, CA 91311 • FAX (818) 998-7835

(818) 998-7791

Map to Mercury Magnetics

CAUTION! The voltages in your amplifier can be dangerous. Transformers and chokes are not user serviceable parts. Installation of these components should always be performed by an experienced technician. The simple ability to use a soldering iron is not enough to qualify a "do it yourself person." Those who are inexperienced in working with electronic circuits should never attempt to service their amplifier. Household line currents can be deadly!! Transformers, chokes and large filter capacitors can store a dangerous charge for several days or more after the amplifier has been unplugged. Never touch the terminals of such devices without being certain of their charge status. Risk of shock and damage to equipment may result from mishandling and/or improper use of these components. Please use common sense and always think safety first. After all, tone is most enjoyed when you are alive to hear it.