Frank the Tech Guy

It’s the beginning of 2009, and best wishes for the New Year! A good new year’s resolution for musicians would be to become more technically knowledgeable in the upcoming year. I’m here to help with that goal, so let’s get started. Today we’re going to discuss cables and connectors used in electronic music and in pro audio.

A good place to start is the type of cable used in various applications. There are three major categories of cables used in the music business: balanced microphone cables, unbalanced shielded instrument cables, and speaker cables. One of the biggest problem areas I see in my work is when the wrong cable is used for an application. Now, this usually isn’t a problem with microphone cables, since their XLR connectors are a dead giveaway as to their purpose, and because the only products which utilize those connectors are microphones, direct boxes, mixers, etc. If the plug fits, you’re in business. It’s hard to make a mistake with microphone cables.

On the other hand, it can be very easy to make a mistake and confuse unbalanced shielded instrument cables with certain types of speaker cables. If you utilize the wrong cable for an application in this area, the quality of your audio will suffer and it may even cost you money for the repair of damaged equipment. So, here’s a quick tutorial on the finer points of speaker cables and instrument cables.

First, let’s talk about unbalanced shielded instrument cables. These are commonly referred to as “guitar cords”, because that is certainly one of their primary applications. In addition to connecting guitars to amps, they are also used to connect keyboards, synthesizers, electronic percussion, etc to their respective amplifiers. Shorter lengths are used to interconnect floor effects units (“stomp boxes”) and rack-mounted audio effects processors. They are characterized by their ¼” phone plugs on each end, and shielded cable is used as the audio highway. Shielded cable means that one conductor is concentric, or wrapped, around the other conductor. The problem is that many speaker cables also utilize 1/4” phone plugs on each end – the difference is that speaker cables use unshielded cable. Unfortunately, it may be hard to distinguish between the two, and therefore easy to use the incorrect cable for an application.

What happens if you should use a speaker cable to connect an instrument to an amplifier? In a word – hum, and lots of it. Cables which connect to the inputs of amplifiers (or interconnect devices which then in turn connect to the inputs of amplifiers, such as effects units) need to be shielded cables. Generally, no damage occurs should you inadvertently use a speaker cable in place of an instrument cable (unless you have the volume on the amp set so high that the hum is overpowering and damages your speakers before you can power down the system).

But what happens if you use a shielded instrument cable to connect a speaker to a power amplifier or powered mixer? This is especially bad, because there are no immediate telltale clues that you’ve done anything wrong. The speakers will work – but, unknown to you, there will be considerable signal loss in that instrument cable. Instrument cables use conductors that are quite thin, because they are designed to carry only a very small signal, and don’t really need to be very large. When used as speaker cables, you could easily be losing 10% to 30% of the output power of the amplifier in that instrument cable. Where does the power go? It is dissipated in the form of heat within the cable itself. Heating the cable actually increases the resistance of the cable, which in turn increases the signal loss – a vicious cycle. You will be setting your mixer and amplifier volume “by ear” to get the right sound level in the room, and so you will be turning up the level even louder, forcing more power through the cable. Now you have another vicious cycle on your hands. You may not even realize that you are running your equipment nearly wide open to achieve a decent volume in the room. Your PA system itself is running very warm, cranking out the watts that you are demanding that it deliver. Sooner or later, something has to give. The cable may melt down internally, causing a short circuit on the output of the amplifier. Or, the power amp or powered mixer itself may fail from its own overheating. Either way, your gig may be suddenly interrupted by the loss of audio, and you’re going to be out some big bucks for the repair.

If your PA system uses 1/4” – 1/4″ speaker cables, be absolutely certain that the cables you have for that application are indeed speaker cables, and make sure you do not intermingle them with instrument cables. I need to clarify one point on this issue. If you have powered speakers (speaker cabinets with a built-in power amplifier, meaning that the speaker cabinet must be connected to 120V AC in order to operate), then you should use instrument cables to run the output of your mixer to the powered speaker. This is a low-level signal, and the powered speaker requires shielded cable to its input. A better choice is an XLR-XLR microphone cable, if the powered speaker features an XLR input, and if your mixing board has a balanced XLR main output.

Today, many newer systems have eliminated the use of 1/4″ - 1/4″ speaker cables in favor of other types of connections. This is a definite improvement, as it eliminates the possibility of error in grabbing the wrong cable. Plus, 1/4” connectors were never designed for the amount of current needed by today’s high wattage systems. Here are some other types of connectors found on speaker cables today:

Speakon is a type (and brand) of multi-pin connector developed by Neutrik which is commonly found on speakers and amplifiers with high wattage ratings. Speakon connectors offer a very reliable connection, can handle extremely high power, are very durable, and are relatively low cost compared to other connectors.

A banana plug is an electrical connector designed to join audio wires such as speaker wires to the binding posts on the back of many power amplifiers or to special jacks called, of course, banana jacks. A common configuration of banana plugs is to have two of them molded together and spaced 3/4″ apart, which is also the spacing of the binding post receptacles on the back of power amps. Technically this assembly is referred to as a “double-banana” plug. An advantage of banana plugs is the relatively large metal surface area used for the connection, which virtually eliminates signal loss at the connection point. Disadvantages include the ease in which the plug can accidentally be reversed (turned over), causing speakers to be out-of-phase with each other. Also, the banana plug can be easily pulled out of the banana jack should you accidentally yank or trip on the cable on stage.

Bare wire is the most basic (and least recommended) way to connect audio. It consists of dual-conductor cable, stripped of insulation at the ends to expose the individual conductors. These can be inserted into binding posts or hooked around screw terminals that are then tightened down. Besides the general “messiness” of bare wire, you must always make sure to match the “hot” and “ground” conductors of the cable on the outputs of the amp and the inputs of the speaker to prevent phasing problems. We don’t recommend this type of connection other than in an emergency.

Some systems require a combination of these connectors, so a hybrid cable is needed. For example, your power amp may have banana jacks only, and your speakers have Speakon connectors – in this case, you’ll need speaker cables with banana plugs at one end and Speakon connectors on the other end. The good news is that with these type of cables, you’ll never inadvertently connect an instrument cable in place of a speaker cable – the connectors won’t fit.

Finally, what is the best type connector for speaker cables? I would enthusiastically recommend Speakon connectors at both ends, if compatible with your equipment. Speakon connectors have good contact area for minimal loss, are twist-lock by design so they can’t be accidentally disconnected, and are durable. Definitely a good choice for the new year!

Frank the Tech Guy

It’s my guess that there are few musicians who haven’t been faced from time to time with the unfortunate reality of a speaker that cries for attention. Perhaps it’s a blown speaker that succumbed to too much power, which means it is now totally silent due to an open voice coil, or it may be badly distorted due to a burnt coil that is rubbing the magnet or perhaps locked in place. Or, it could be a speaker damaged by accident – dropped, torn, rain-damaged or punched through. Or, there is that vintage Jensen in your prized blackface Fender Deluxe whose age has caught up with it, and the paper cone is slowly turning to dust. Whatever the situation, the speaker is in serious need of attention, and your attention is drawn to the choice you need to make: Recone or replace?

Speaker reconing is a repair technique which was popular back in the 60’s and 70’s, but then seemed to decline for a period of time. More recently, however, it is again a viable option and in many cases is worth your consideration.

First, let’s talk about the reconing process and what it involves. In a nutshell, speaker reconing involves replacing all of the moving parts of a loudspeaker - these are the parts that typically fail over time with use (or in some cases, abuse). In the diagram below, the cone, surround, dust cap, spider and voice coil are replaced in a speaker reconing project. The speaker basket, pole piece, top plate and the magnet are normally not damaged and therefore are not replaced.

In most cases it is best to consider reconing your old speakers. They are the speakers that the cabinets were designed for. Someone spent a great deal of time and money designing the cabinets. A great deal of math was involved in building the cabinets with a certain amount of air space. The manufacture “tuned” the cabinets by building the cabinets specifically for the old speakers. If you grab other speakers and throw them in the cabinets, they will not sound the same as your old speakers. Reconing is the best method to reclaim the sound of your original speakers when they were new.

What about the cost? For most name-brand speakers used in good quality musical instrument or pro audio cabinets, the cost of reconing is anywhere from 30% to 50% less than the cost of replacing the speakers with new speakers of exact or similar specifications. Now, there are exceptions to this, and cases where reconing could cost about the same as replacement – those situations are with lower wattage speakers, small voice coils and small magnet speakers. In such situations, replacing the speaker would be the cost-effective solution. However … if we are talking about a vintage amplifier with a low wattage speaker (think 1960’s Fender Princeton or Champ), it would be economically wise to recone the speaker rather than replace it. The incentive here is to maintain the vintage value of the amplifier. Replacing the original Fender-branded Jensen or Oxford speaker in a Fender amp with a current-day Eminence replacement could drop the resale value of the amplifier by $150-$200. Having the original speaker professionally reconed maintains the value of the amp.

There are professional speaker technicians across the country who will take the time and care to recone your speaker and restore the original sound. I personally have reconed a lot of speakers over the years and take personal satisfaction in listening to the sound quality of the speaker when audio is first applied after the reconing process is complete. Reconing is truly an art, and the reconing artists take pride in our work.

If replacement of the speaker is the chosen option, try to get the original speaker if still available. Many OEM speakers of more recent vintage were made by Eminence, and the Eminence factory offers a service to their dealers in which they will make a recommendation as to the closest Eminence replacement speaker based on the make and model of your unit. If your old speaker is Celestion, many of their vintage speakers are still being made (such as the classic G12M “greenbacks”). Jensen is back as a brand name, and they are making many speakers which emulate their original vintage lines. (It may be worth noting that this is not the same as the original Jensen company – the new Jensen manufacturing facility is in Italy). Another good guitar amp speaker company with Kentucky roots is Warehouse Speakers, located in Paducah. Warehouse specializes in making “British” type speakers which emulate the Celestion sound – at very reasonable prices.

Welcome, all! This new blog is designed to address some of the many questions I receive regularly from local 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’ve been repairing electronic musical gear in the Louisville area since the 1970’s. I’ve worked directly for four different music stores during that time, and have done contract work behind the scenes for several other local stores as well. So, if you’re a Louisville musician, there’s a good chance I’ve worked on some of your equipment in the past.

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, the Slovak Republic, and Serbia. 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 produces the new SED Winged-C tubes. In my opinion, all of these tubes are of good quality and quite 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 not been all that good – I’ve seen a higher-than-expected failure rate in preamp tubes. At this point, JJ Tesla is not high on my list. I recognize that many players love their JJ’s, and that’s fine – if they’ve had good success, they are certainly entitled to their opinion.

Tubes branded “EI” were made by Electronska Industries in Serbia. The company is currently undergoing restructuring by the Serbian government, and apparently is not currently producing tubes. If you see EI tubes on the market, they were made before 2006.

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 Tesla. 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, but many ARS tubes are JJ Tesla, and as mentioned earlier, I’m not a big fan of JJ’s.

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 per year. 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 electronics may have been inconvenienced!

Frank the Tech Guy