When Good Amps Go Bad
There are two main categories of failures in most electronic equipment: the sudden asymptomatic failure, and the gradual symptomatic failure. We will examine both of these categories below.
In this case, everything is working normally, then suddenly stops. This can mean no output, a burning smell sometimes accompanied by smoke, a blown fuse, loud noises (hums, buzzes, crackles), etc. Usually a component has opened or shorted, or an electrical connection has come undone. If these symptoms are accompanied by the h/t or a/c mains fuse blowing, chances are an output tube has developed an internal short, causing a large current surge from the power supply through the output transformer. Just changing the output tubes may solve the problem, but it may not, and in doing so you may also ruin the new set of tubes and risk destruction of the output transformer. There is a pretty good chance that when the tube shorted, it burned out the screen grid resistor, or the cause of the tube failure was not actually a short, but an interruption of its bias supply (this is generally preceded by a hum which gets progressively louder, the tube's plate structure glowing bright red and then the fuse (hopefully) blowing). Other sudden failures can be due to a specific component's premature death, generally a preamp or power tube, a plate load resistor opening up, a capacitor shorting, or a transformer winding opening or shorting.
A fuse is a link between circuit points that will melt and open up the circuit if more current flows between those points than the circuit was designed to safely withstand. 99% of the time, if a fuse blows there was a good reason for it. Fuses are rated in amps (the unit of current measurement), and, in addition to that current rating, also have a voltage rating and other parameters such as time constant or delay. A simplified explanation breaks fuse types into two categories; fast blowing and slow blowing. A fast blow fuse will usually open up on an instantaneous current peak that exceeds its current rating. A slow blow fuse will withstand this overcurrent for a specific time, but open up if the higher current continues beyond the fuse's time constant. There are reasons why designers will chose one type rather than the other. In most audio equipment having a linear power supply, there are large value capacitors whose characteristics may result in a large current inrush while they are first charged during the power-on sequence. This short term current draw may be higher than during the normal operation of the device, so a time delay or slow blow fuse is sometimes used in the ac input or mains fuse position. This also allows for other brief overcurrent situations such as when the standby switch is turned on (more large value capacitors may be charged at this time), but protects the equipment (and particularly the mains transformer) from potentially damaging overcurrent conditions.
Fast blowing fuses are typically used in situations where protection against instantaneous current peaks is required to prevent damage. An example of this is the H/T or B+ fuse on Marshall and other amplifiers. This protects primarily the output transformer from overcurrent, usually the result of an output tube short.
There may be fuses in some other circuits in the equipment, particularly in other parts of the power supply, or, for example, in solid state channel switching circuits. The important thing to remember is that the designer of the equipment used a specific value of fuse for a good reason; you must replace any blown fuse only with the exact type specified! If you replace a fuse with the same value and it too blows, that means there is a serious fault in the circuit, and the fuse is doing its job -- protecting expensive components from damage. Installing an over-rated fuse is not going to get you through the gig or session, it's going to cost you money in repairs, possibly destroy your equipment, and create a fire or electrocution hazard as well.
These are sometimes very difficult to detect, as they occur over long periods of time (perhaps years), during which the performance of the amp gradually degrades to the point where it becomes unusable. A loss of gain at the frequency extremes, more hum than you have noticed in the past, strange noises that may come and go, low power output, tone that's just not "right" -- these are signs that the amp is really in need of some maintenance. Nothing lasts forever and it's truly amazing how many 30 year old tube amps are still working every day, but what their owners may not realize is that parts of the amp's circuits have seriously deteriorated. A great advantage of the relatively simple circuits and generally high quality construction used in many tube amps is the fact that they almost never die! The individual components do however, sometimes at a very slow pace.
Component Failure Mechanisms: causes and solutions
It's surprising how many amps come in for service sounding like they are on their last legs, with the client wondering if it's even worth fixing, only to find that a thorough cleaning of the pots, switches, jacks and tube sockets cures the problem completely, without requiring any component replacement. Many of these items are mechanical tension or friction connections that provide electrical contact between points in the circuit. If they don't reliably "make," it can indeed seem that the amp is near death, and can be symptomatically similar to component failure.
Corrosion is the result of moisture causing a chemical reaction in the metals of wire, solder connections and the chassis. An amp that is stored in a damp basement or garage, for example, or one that had liquid spilled into it or was rained upon, may have its connections deteriorating due to this corrosion or rusting. As the protective plating on jacks and tube socket contacts wears off, they are more susceptible to this corrosion and should be replaced. A common point of failure is a mechanical connection to the chassis (as in some jacks and pots) which provides the signal ground path rather than a soldered wire connection. If these points become loose or corroded, the ground now has a much higher impedance path, or an intermittent situation develops, causing loss of signal and increase of noise.
The internal wiring of an amp is also a potential failure point. The older cloth insulated wire used in some equipment can degrade due to mildew (if it has been dampened by humidity or liquids), hardening and cracking, and burning. Some of the older synthetic "plastic" insulations can also harden and crack over time, usually as a result of prolonged exposure to high temperatures. In either case, if the insulation is damaged, shorts of an intermittent or permanent nature can result, and can sometimes be quite difficult to diagnose.
This is one of the areas in which modern materials technology has made available superior components that weren't available years ago. Insulators made of PVC or PTFE (Teflon®) are largely impervious to heat, moisture, mildew and even most solvents; and wiring using these insulators may well last forever with no degradation.
Connections or points of permanent contact in circuits, whether they be soldered, crimped or mechanical, are possible points of failure. If a solder connection was properly made at the point of manufacture, it should be very reliable for many years. If it was poorly done though, by bad preparation, incorrect heating, or improper amounts of solder, it can be the source of problems down the road. More common failures are due to mechanical connections as described above, or poorly implemented crimp connections.
Tubes do wear out over time. In addition to the sudden failures such as shorts mentioned above, as the tube ages, several mechanisms are at work. Most common are mechanical problems, in which the internal geometry is altered as the tube undergoes numerous heating/cooling cycles and is subject to physical shock. (TIP -- to prevent this, don't move your amp while the tubes are hot). These changes can manifest themselves by way of increased microphonics, hum, and other noises, or loss of gain and even shorts. Occasionally the heater or filament opens up, completely shutting off the tube. Another common failure is caused by the chemical coating on the cathode (which when heated produces the free electrons attracted by the plate) being exhausted or stripped (TIP.........this stripping can be minimized by always allowing the tubes to completely warm up for about 2 minutes before turning the standby switch on), and causes a substantial loss of gain and an increase in noise.
The other main components that don't age gracefully are capacitors. A capacitor is two conductors separated by an insulator which is called a dielectric. Many older capacitors typically found in the power supplies of tube amps are the "wet electrolyte" type. The dielectric material is a paste having some moisture content, which over time dries out, reducing the functionality of the device. This can result in increased hum, loss of low frequency response, low gain, and sometime a phenomenon known as motorboating, in which the amp makes a continued popping or putting sound. One of the functions of capacitors in tube audio circuits is as ac coupling or dc blocking between stages. A common failure mode in this application occurs when such a capacitor "leaks" dc, which upsets the operating point of the next stage. When this failure occurs and a capacitor feeds a pot, the presence of dc will cause the pot to scratch when adjusted, and the pot will not respond to cleaning and subsequently be damaged.
Usually pretty stable and reliable, resistors can be the cause of several problems. One of these is a crackling noise which results from moisture being trapped inside older carbon composition resistors (usually in the plate load position of the circuit). Replacement with a carbon or metal film type will cure this. Other resistor failures can be related to high temperature, over-voltage, or over-current, though most amps are designed with the components overrated for these 3 factors.
All in all, guitar amp transformers are robust and very reliable. In the hundreds of amps we have repaired, only a few transformers were the cause of any problems. The typical transformer fault occurs when a winding opens up, usually due to an over-current, over-voltage or overheating condition; less common is a shorted winding resulting from the same "over" conditions. Both are usually "indirect" failures, caused by something else (a tube, capacitor, wiring) failing and generating the initial fault. There are two repair methods for defective transformers (once the underlying cause has been isolated and repaired); replacement or rewinding. Replacement is an option when an identical or very close substitution is available, but this may not be possible or desirable. Rewinding, though sometimes more costly than replacement, guarantees retaining the exact electrical specifications and physical dimensions of the original. It is also the best option when you wish to retain originality.
Though not generally resulting in total failure, component values can shift over time, as a function of age, temperature, frequency of use, and environmental conditions, as well as their initial manufacture quality. This can be a double edged sword - perhaps some very sought-after older amps actually sound they way they do as a result of an almost chaotically variable change in component values over time. On the other hand, as these unpredictable changes occur, tonal degradations can indeed take place, and the value changes may result in non-optimum or unstable operating conditions within the circuits.