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 Fendley, 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 ...

All About Tubes

Today I’d like to talk about tubes. Of all the questions I receive, issues about tubes and tube amps ranks right near the top of the list. You know, guitar players of today and perhaps fifty years ago are linked by some common threads … certainly the classic Gibson Les Paul, the vintage Fender Stratocaster, and of course – tube guitar amps. So, let’s talk about the heart of the tube amp – those little “fire bottles” we call tubes, and the British call “valves”. We’ll format this article along the lines of “Frequently Asked Questions”.

Why do guitar players prefer tube amps over solid state (transistorized) amps?
One word – tone. Tube amps work differently than transistor amps. When tubes “break up”, or begin to go into distortion, the clipping is compressed, gradual, rounded, warm and soft. In technical terms, it is rich in both even and odd harmonics. Transistor amps are clean and distortion-free until they reach the limit of their design. At that point, they clip the tops and bottoms of the signal sharply, symmetrically, and harshly. The resultant signal contains mostly odd harmonics, and generally sounds hollow, cold and fuzzy. Now, for about the past fifteen years, you can purchase digital modeling amplifiers introduced by Line 6 and copied by many others. These amps are solid state, and yet emulate very accurately the tonal quality of many vintage tube amps. Do these modeling amps blow apart the theory that solid state amps can’t sound as good as tube amps? Well, yes and no. These amps derive their sound from digital technology and sampling of the actual sounds produced by the tube amps they model. It’s kind of like “capturing” or “recording” that sweet sound of a Fender Deluxe or a Marshall JTM45, and being able to play back that captured sound, but with you at the controls (that is, your guitar strings). So, yes, they sound as good as the tube amps, but digital wizardry is their secret – not a new type of transistor circuit that behaves exactly like a tube amp. That circuit design still eludes engineers, in most guitarists’ opinions.

How does a tube work?
A tube is an electronic device that has at least four active elements: cathode, heater, grid and plate. These four elements are sealed in a glass enclosure with the air removed. A low voltage is applied to the terminals of the filament, producing a dim orange glow and moderate heat. Once the cathode reaches a minimum temperature, it begins to emit electrons which flow toward the positively charged plate. The grid is physically between the cathode and the plate, and the application of voltage to the grid (either bias voltage or signal voltage) controls the flow of electrons from cathode to plate. If the grid were not present, the flow of electrons would be uncontrolled, much like water running from a faucet opened all the way.

So, the grid acts as a valve to control the electron flow, which explains the origin of the British term “valve”. A small voltage signal applied to the grid produces a nearly identical large current change in the electron flow from cathode to plate. In this manner, the tube amplifies the input signal, transforming a tiny signal originating in your guitar pickup to a larger signal appearing on the plate of the tube. Cascading several stages of tube amplification builds up the signal to a level strong enough to drive a speaker and fill a room.

Why are output tubes sold in matched sets, and is it important to use matched sets of tubes?
Tubes are still primarily made by hand today, and there are naturally slight variations which occur between each tube that is produced. These variations are primarily minute differences in the spacing between the elements inside the tube, and in how perfect a vacuum is achieved inside the glass. (A perfect vacuum – no air molecules whatsoever – is not realistically obtainable, but the ideal tube is as void of oxygen and nitrogen as possible).

These variations cause tubes to behave somewhat differently. Since it is not economically practical to tighten quality control in manufacturing to the point where nearly all tubes would be essentially identical in behavior, the next best thing is to test and measure tubes after they are built, and group them according to their specific test results. Most amplifiers’ outputs are designed as what is known as Class AB; that is, an even number of output tubes are used, and the tubes each contribute as a team to producing the final output signal to the speaker via the output transformer. If you put tires on your car, you want them to be all the same type and size to provide the best ride. One tire larger than the others would produce a poor ride and eventually damage your suspension. Likewise, mismatched tubes produce a poor quality sound, result in shorter tube life, and can damage other components such as the output transformer.

What is bias, and why is it important?
Bias is a fixed control voltage applied to the grid of the output tube to balance the amount of idle current flowing from cathode to plate. Because tubes differ in characteristics (see the discussion above about tube matching), every tube requires just a slightly different bias voltage in order to achieve the optimum idle current flow. This is very much akin to the idle speed setting on your car’s engine. When properly set, you car idles smoothly and performs properly. If the idle speed is too low, your car coughs and gasps at low speeds, and frequently dies at stop lights. If set too high, your engine races, you waste fuel, and you have to apply the brakes harder to keep the car from moving. Likewise, if the bias is set so that too little current flows in the output tubes at idle, the amp sounds cold, brittle, and hollow. Lower volume settings on the amp sound fuzzy and lifeless, and background hum may be noticeable. If the bias allows too much idle current to flow, the amp runs very hot, the tubes’ plates may actually glow red-hot, and the overdriven sound of the amplifier sounds harsh. In addition, the tubes’ life is severely shortened due to the overheating. In an amplifier with adjustable bias, it is very important to have the bias adjusted properly each time you change the output tubes. Proper bias improves the sound of the amplifier and optimizes the life of your output tubes.

What about brands of tubes? I see so many different brands available. And what are NOS tubes?
First, let’s take a look at a little history. Tubes used to be made in many countries around the world. In the late 1970’s, demand for tubes began to shrink as fewer and fewer tube-type televisions and radios were still in operation. By the early1980’s, tube manufacturing in the United States had ceased. The last tube plant in America was only about 100 miles from Louisville on Old Hartford Road in Owensboro, Kentucky – the General Electric tube factory. Tubes being sold today which are NOS (new “old stock”) were made in these now-defunct factories some decades ago. NOS tubes are understandably costly, since they are a shrinking commodity. However, let the buyer beware. Not all NOS tubes are “new” – some (especially preamp tubes like 12AX7’s) could be actually used tubes which have been cleaned up and put back in their original boxes. And, there are actually counterfeit NOS tubes out there – new Russian or Chinese tubes which have been rebranded and placed in old boxes or even counterfeit boxes. So, if you are purist and want new vintage tubes and are willing to pay the price, choose carefully the seller from whom you buy.

Guitar amp tubes manufactured today come primarily from Russia, China, and the Slovak Republic. In Russia, the Reflector Xpo-Pul factory in Saratov is owned by Mike Mathews of Electro-Harmonix fame, and produces Sovtek, Electro-Harmonix and the “new” Svetlana brand tubes. The original Svetlana factory in St. Petersburg produced the SED Winged-C tubes. In my opinion, all of these tubes are of decent quality and reasonably reliable. A special nod for exceptional tone quality goes to the SED Winged-C brand, but take note that Winged-C tubes cost about twice as much as Sovtek, Svetlana and Electro-Harmonix.

Audio tubes in China come from the Shuguang factory, and are marketed under the Shuguang name, as well as the Ruby name. Chinese tubes are generally less expensive than Russian tubes, and their preamp tubes are especially warm sounding. However, quality control can be a problem, although it is improving. You may find that some of their preamp tubes are a bit microphonic, or their output tubes tend to flame out early. If you have a desire to try Chinese tubes, I would stick with the 12AX7 preamp tubes, and buy your output tubes and rectifiers from another country.

Years ago, Tesla had several factories in Czechoslovakia. An individual named Jan Jurco bought the Telsa factory equipment at a liquidation sale a few years ago, and opened a new factory in the Slovak Republic. The new tubes carry his initials followed by the name from which the new factory claims its heritage: JJ Tesla. JJ Tesla tubes tend to have high gain and high output, making them interesting choices when trying to “soup up” an old amp. However, this high gain also makes them candidates for hum, noise and microphonics in some amp designs. My experience with JJ tubes has been excellent.  In my opinion, if you need new tubes, go with JJ brand.

There are many “private” brands of tubes made for manufacturers. Many amp manufacturers have their own line of tubes with their brand name – commonly found are Marshall, Mesa Boogie, Peavey, and Fender. All of these tubes are made by one of the above factories. Some of the tubes are custom-made by these factories to the amplifier manufacturers’ specifications, such as the Mesa Boogie STR series. Others are just standard off-the-shelf tubes which are rebranded (most Fender tubes are standard Sovteks).

Other brands such as Mullard, Ruby, Tungsol, Genalex, etc, are made by one or more of the above factories. Some of these are made to different specifications than the other brands produced by the same factory.

Finally, there are the boutique tube marketers: Groove Tubes and ARS Tubes. These companies have tubes made for them by the Russian, Chinese, or Slovak factories with their brands, and then they retest the tubes and provide extensive matching and classification information on each package. A lot of Groove Tubes are Sovtek made, and a lot of ARS tubes are JJ.   In theory, you are getting the higher level of quality control which may be missing at the factory, and Groove Tubes or ARS tubes should be of the highest quality, premium matched, and most reliable. For that, you are paying about twice the price of the original tube before they tested and categorized it. However, my experience over the years has not been impressive with Groove Tubes – rather than seeing a higher degree of reliability, I’ve seen a higher rate of failure, which I can’t explain. ARS tubes seem to be better than Groove Tubes in terms of reliability.

How often should I change tubes?
Preamp tubes last a long time, and generally they don’t need to be changed frequently unless you have a failure. Rectifier tubes in amps which use them (such as the Mesa Boogie Dual and Triple Rectifier and the vintage Fender amps) also tend to last a long time. Output tubes are a different story. You probably need new output tubes (and the all-important bias adjustment) when your amp starts sounding weak, makes funny noises, lacks punch, or loses high or lows. In any of these cases, though, the tubes may not be the culprit. A professional can diagnose the problem and make the proper repair.

From a preventative maintenance perspective, you should change your output tubes before they create the problems mentioned above, just as you should change tires on your car before they are bald. For an average player, that translates to about once every year or two. If you are a heavy player (several nights a week), six months would be the recommended interval.

I hope this has shed some light on the mystery of tubes and tube amps. Until next time, remember that no trees were destroyed in the creation of this document. However, some electrons may have been inconvenienced!

Frank the Tech Guy