Shimmilou

Consider this before changing amplifier tubes

 Safety first! Always unplug the amp from the electrical outlet before servicing. Any instrument amplifier can hold a dangerous electric charge even when turned off and unplugged for a long time, so take the proper precautions to prevent a shock. If you don't know the proper precautions, please learn them first. We shouldn't assume that someone knows about safety issues when discussing any subject (guilty myself)

 It is important to consider some facts about amplifier tubes, and amplifiers, and set aside the hype and anecdotal evidence, before changing tubes. Well, it's only important if you like things to be the best that they can be. Tubes can be expensive, and so can amps and repairs. If you like to keep your expenses to a minimum and your sound to a maximum (quality), a little understanding about tubes and amps can go a long way. The key to understanding the tubes and other components inside a guitar amplifier is to learn a bit about electronics. No need to understand it all, just some very basic info can very helpful.

 Electronics is mathematics, and math is a science, no art involved at all. One great advantage to math is that opinions do not affect scientific fact, the truth is still mathematically correct regardless of opinion, and math is much more consistent than opinion. To say that something sounds good or bad is an opinion, and opinions often change. And sound is even perceived differently depending on the barometric pressure, or the humidity in the room, or the size of the room, or the location of the amp and many other factors. Electricity however, doesn't change its behavior based on someone's "feeling" about it. Electricity follows laws, such as Ohm's Law E = I x R (E is voltage, I is current, R is resistance). It is the same, consistently, time after time. Electrically, or mathematically speaking, two plus two ALWAYS equals four. As an example of using Ohm's Law, let's say that we measured the voltage (E) across a bias resistor for a tube, and we know the resistor value (R). We can insert our known values into Ohm's Law and solve for (I). If the specs for the tube bias current match the value we measured then the bias is correct. And every time the results will be the same, even on Monday. The only inconsistent things involved here are the values of components such as tubes, capacitors, resistors that are used to make the amp.

 Manufacturers of electronic components do not have secret ingredients that they use. The components that are of better quality, use better quality parts, it is that simple. The same can be said of guitar amplifiers, quality in equals quality out, and modern mass-produced amps are not the same as the old hand-made amps, it's just the nature of the business. It is really more a matter of the inconsistency of the parts and sometimes the quality. Whatever the reason, the differences do exist. Different brands of the same type of tube may have quite different specs, thus resulting in different sound. It is the same for capacitors, resistors, etc., they are not all created equal. For example, if you look at the Fender Groove Tube 12AX7 preamp tubes, you will find that they are relabeled Sovtek tubes. Sovtek makes great tubes, and I personally like to stick with my Fender Groove tubes, but if you look closely you'll see that some are 12AX7WA and some are 12AX7WC, and some are, or used to be, labeled Fender 12AX7A. The ending letter designation on the tube indicates a difference in the gain of that tube, and a C has a lot more gain than an A and will change the sound of the amp when interchanged, yet if you order a Fender tube you might get an A or a C because they are both labeled the Same, GT12AX7A. Tubes from different manufacturers can have very different responses even though they are all 12AX7A's. With sometimes three or more preamp tube stages in an amp (V1, V2, V3...) the gain of each stage makes quite a difference in how the amp sounds. Check the gain specs of the existing tube and decide if you want to replace it with a tube that has more gain, less gain or the same gain.

 Power tubes can also vary in their specs from tube to tube even from the same manufacturer. Look in the Fender Hot rod series of amps and you will see that there are "White Label", "Red Label" and "Blue Label" power tubes, all are 6L6 but they each have different specs and Fender recommends replacing "White Labels" only with other "White Labels", possibly because that would make it more likely that no bias adjustment will be needed. Power tubes, or output tubes, unlike preamp tubes, have to be set up with the proper bias for best operation and longest life. So if you hear a big difference in sound when you install new output tubes in your amp, it may be because of the new tube having different specs, not because of some "magic" in the brand name. And the different sound almost certainly means that the new output tubes need to have the bias checked with a meter and adjusted if necessary. Plus or minus 10 percent is generally accepted as "within spec". Contrary to what some people say, the proper bias on tubes will not ruin the sound of the amp. It will allow the best operation of the tubes and much longer life AND better sound. The whole point of using tubes is because of their fantastic ability to be overdriven and produce wonderful tone. Maybe that is a key word, over "driven". You may sometimes want to overdrive the tubes for that great break up that we all love, but do it with the signal, not by torturing the output tubes. A cheaper tube that is properly set up can last much longer than an expensive tube that isn't properly set up.

 The rest of the signal path in an amp is very important to the sound out of the amp also, certainly as important as the output tubes and preamp tubes. If the amps signal path components are not great, then the signal through the amp will not be great either, no matter how great the tubes are. Sometimes the component selection isn't the best that it could be in some newer amps, and with a little use they can get worse. Capacitors are often the weakest link in the signal path (and in the power supply), and they should be checked and replaced if necessary long before you start changing tubes. If an amp doesn't sound great, and without troubleshooting some tech suggests that tubes are easy to change, try "CC Rider" brand tubes, then they are installed and the amp sounds "better", it would be easy to wrongly conclude that; "Wow! These ‘CC Rider' tubes are the best and they solved my problem".  That is an example of poor troubleshooting, and quite possibly more work on the amp later, as well as the perpetuation of false information, which spreads more quickly than facts. And you will have spent more time and more money than you would had you changed the much cheaper capacitors to begin with. Sometimes you pay a tech for what he knows not how much he does.

 A good tech may be necessary to have if you are uneducated in electronics. Simply changing tubes and turning an adjustment until you "hear" the correct sound is ridiculous. It is also not a good idea to just change the tubes without checking the bias. Don't get me wrong, I love experimenting and pushing things to the breaking point as much as the next guy, but I certainly don't recommend it for long life. So, what about techs? Many people might call themselves a tech, but what does that mean? The term "tech" at least implies tech-nical knowledge. Techs and musicians are a dime-a-dozen. So too are good techs and good musicians. Great techs and great musicians are more rare. The most rare are those who are both artistic and scientific. A great musician can be purely artistic without knowing any of the science of sound or of music (i.e. SRV).  A great tech however, must know the science of the electronics or he will spend a lot of time chasing his tail and giving horrible advice. Artistic abilities are irrelevant when it comes to science, just because a guy is a fantastic guitar player doesn't mean he knows squat about the electronics in amps. That's not to say that great musicians cannot be great techs, because they can, it's just very rare. Maybe it is a left-brain right-brain thing? I encourage anyone to educate him/herself about their amp as much as they can, so they won't fall for some outlandish nonsense and spend more than they need to. A tube amp is one of the more simple electronic devices made, it is fairly easy to learn how they work.

  During the life of an amp, the values of the electronic components in it can change, mainly due to the constant heating and cooling of the components from the amp being turned on and off over the years. The changes in component values, in combination with the type of component used, are the main reasons that some older amps have that unique "flavor", but the changes can also result in amp failures. Capacitors are often overlooked components in the amp and some of them have a plus-or-minus twenty percent tolerance! The effect of the changing component values adds up over time. And if lower quality caps are used to begin with they can wear out even faster. The tubes on the other hand are probably the sturdiest of the afore mentioned components, so try to be sure about the condition of the rest of the amp components before changing tubes.  And be aware of the differing specs for tubes. Quite frankly, most tubes just plain fail dramatically, there isn't really much about a tube that changes over time if it is biased properly. A handful of good quality capacitors are much cheaper than even one tube. My Hot Rod Deluxe has been running the same factory installed tubes for over ten years and counting. My 60's Silvertone still has US made tubes in it and it sounds great (No US tube production for many years). Don't limit yourself to only getting two or three years or less use from a tube, they can last a lifetime. If you use your amp a few times a day, leave it on until you are done for the day to reduce the number of heating and cooling cycles and it will last much longer. If you want to put in "better" tubes then I say go for it if you have the money, but why not do it correctly and definitely at least consider that there are other parts in the amp that might need attention besides the tubes?

Capacitors in an instrument amplifier

 Capacitors, hereafter referred to as "caps", are found in abundance inside of an instrument amplifier in various shapes and sizes. Typically their appearance will be cylindrical or tube shaped, like small vials or cans with a wire at each end (Axial), or with both wires at one end (Radial). They can also be shaped like a small bubble, or a disc or other shapes such as the "Orange Drop" cap. Generally the material used to make it determines the cap's shape. In an amplifier caps are used as "filters" in the signal path, power supply and also for the tubes. Any cap, no matter the size, can hold an electric charge that could be dangerous, so do not touch them until they are properly discharged to avoid a shock. I will just cover some of the basics here; too much info can induce headaches. ;)

 A basic cap is simply two very thin pieces of metal, metal film, or aluminum foil, separated by an insulator material. Imagine two strips of aluminum foil, each one-inch wide and one-foot long, stacked on top of each other. In between the two metal strips is a thin piece of insulating paper, slightly wider than the metal strips, with another thin piece of insulating paper on top of the metal strips. Starting at one end, the strips are rolled up tightly like a cigarette and continued to the end until it is a one-inch long tube shape, maybe a half-inch in diameter. A lead is attached to each metal strip and it is encased in a cover to make the tube-shaped type of cap. Some caps are two small discs of metal with an insulator in between. The area of the strips or discs, the distance between them and the material used to make them, determine the value or capacitance measured in Farads, typically micro Farads (uF) or pico Farads (pF). You might see numbers, indicating voltage and capacitance ratings labeled on the caps like 600V and 47 uF.

 In simple terms, caps are generally referred to as filters, as they allow only the desired frequencies of a signal to "pass" through them, much the same as a water filter only allows the water to pass through it, filtering out the unwanted material. The tone of the guitar signal through the amp can be adjusted by filtering certain frequencies using specific values of caps, and resistors, called a tone control. Caps also filter certain unwanted frequencies away from the DC power supply, by allowing the higher frequencies to pass to ground. The frequencies in a signal are measured in cycles-per-second referred to as Hertz (Hz). The value of the frequency at which the signal is passed is called the "cut-off" frequency, and is used to choose the value of cap needed, 60 Hz for example. The cut-off frequency depends on the value of the cap. If it is desirable to filter out the 60 Hz and lower frequencies from a signal, select a cap that will only pass frequencies above 60 Hz. In a power supply, to filter out the unwanted 60 Hz and above frequencies, simply choose a value of cap that will pass frequencies higher than 60 Hz to ground. Sometime you might see specs on a stereo speaker that say; "Frequency response is 20 Hz to 20,000 Hz". That "range" or "band" of frequencies covers human hearing and then some. The frequency of a signal can be determined by dividing the number 1 by the amount of time it takes to complete one cycle of the AC signal. One complete cycle of the AC signal is called a "wavelength" and is measured in seconds. If the wavelength is 0.010 seconds (10 milliseconds), then 1 divided by 0.010 equals 100, or 100 Hz. This is important only if you need to calculate the value of capacitor needed for a circuit. It used to be common to use an oscilloscope to measure the wavelength, and then calculate the frequency. Now there are inexpensive voltmeters that also measure frequency. The frequency of the electricity from the wall outlet in the US is delivered at 60 Hz. The 60 Hz "hum" or "buzz" is what you hear in some amps as unwanted noise. If it wasn't for filtering, that 60 Hz hum would be a lot louder. And that hum can be reduced even more with more filtering, but at a higher cost to manufacture the amp.

 A cap will not pass very low frequencies or a DC signal (Zero Hertz); it will only pass a higher frequency AC signal.  It is technically incorrect to say that a cap "blocks" lower frequencies, but rather it allows higher frequencies to pass, hence the term "High Pass Filter". Using a specific value of cap, coupled with a specific value of resistor, allows control of a specific range or band of frequencies. You can see an example of this in an amp schematic, a typical tone stack in which each control, bass, mid and treble, consists of a variable resistor (potentiometer, the knob you turn) and a cap.  This Resistor/Capacitor circuit is called an RC network, or time-constant and the specific frequencies that it will control are referred to as the "Bandwidth", or a narrow band, or range, of frequencies. The more narrow the band, the more narrow and precise the range of frequencies controlled. An amp with only one tone control would have a wide band of frequencies to control and it is usually more desirable to have three narrow bands for more precise control of each set of frequencies, bass, mid and treble. These three tone controls together might be called an "EQ" which stands for equalizer. The more bands that there are on the EQ, the more precise control of the sound you have. Home stereos and rack systems for guitars might have an EQ attached that has twelve or more bands. This type of EQ is exactly like the tone control in an amp, but with more RC networks, or bands, for more control.

 In the power supply, caps are mainly used to filter the "noise" to prevent unwanted signal from being induced into the signal path. The power supply converts the AC voltage from the electrical outlet on the wall, into a high DC voltage called B+, to power the tubes, and provides smaller voltages for other circuits in the amp, such as relays, lights, etc. This conversion to DC can leave a small amount of unwanted AC in the supply. Caps will filter out the unwanted AC that is introduced into the DC power supply, as AC in the supply causes noise or hum. The cap actually passes the AC to ground, or in some cases acts like a buffer to "smooth" the unwanted AC signal in the supply to a minimum or negligible value.

 In the signal path, caps are used for mainly two purposes. One is to prevent DC from getting into the AC signal path, called a coupling cap. And second, to control the specific frequencies in the AC signal path, like bass mid and treble, tone control. Using different values of cap and resistor combinations will give different frequency responses for the particular tone control. The caps are usually a fixed, or set value and the resistor is varied to affect the tone. Varying the tone control resistor (potentiometer) will vary the level of the frequencies that are passed through the RC circuit. That way you can select the level of the band of frequencies that are controlled by each tone control. The tubes use caps for filtering also, the same way that caps are used in the rest of the amp. If the tubes caps are not good quality, the tube life and performance can be degraded.

 Any two pieces of metal that are close to each other will have some measure of capacitance. This effect in an amp is called "stray capacitance", and is sometimes figured in when designing an amp. Sometimes this effect can be unwanted, or unforeseen and can cause a redesign when the prototype is tested. Sometimes, simply relocating components and wires can reduce the stray capacitance. The chassis size, component locations and even shielding can add to the stray capacitance effect.

 Caps can be the weakest link in an amp circuit if high quality components aren't used. The tone in the signal path will suffer with low quality caps, and the power supply will be less efficient when using low quality caps. Typical capacitors have a tolerance of +/- 20% of their value, which is a pretty big acceptable limit and doesn't allow for very much precision in a circuit where they are used. Over time the values of caps can and do change. A high quality cap can have much more precise tolerance, like +/- 5% or better and tends to hold it's rated value longer. Excessive heat can also cause the caps to go bad or to be out of tolerance. So if you couple low quality caps with excessive heat, failure of the caps or other components, like the tubes, is a likely result. Many low quality caps may still work, but can become so far out of tolerance, or so leaky that the sound of the amp and the performance of the tubes is adversely affected. Many modern amps use the lower quality caps to save money.... go figure. ;) Caps are very important to the tone and efficiency of an amp, and shouldn't be overlooked as a possible cause of sound degradation. I would rather replace a few inexpensive caps early, than to replace the more expensive tubes later. The old amps out there are still going after all of these years, partly due to using higher quality caps and other components than are used today. I hope that this was simple enough for everyone to be able to learn a little bit about caps, and I hope that I covered enough info about them, but my fingers say; "enough for now".