There are laws of physics involved in electricity.  You have heard of ohms, amperes (amps) and volts (voltage). There are very specific mathematical relationships between these things. A practical example might be that a piece of equipment is rated by the manufacturer for a 2 ampere fuse.  You are on a gig, the fuse blows, and you install a 3 ampere fuse because that is all that you had.  The fault that occurred, perhaps a vacuum tube shorts, ...whatever, ...which might normally be inexpensive to repair, just replace the tube, replace the fuse, and rarely also some resistor or diode fails when the faulty vacuum tube shorts;... which is still a relatively inexpensive repair job. ....BUT:

7.  Bits of advice/hints to save you some serious money:
I do a lot of repairs and restorations on older electronics.   Many items have been stored in garages and not used in a long time, and just how you go about 'firing it up' can be important to your wallet.  There are other hints I will add to in other regards, in this item 7, now and then.
a.  Please do not store your electronics item in your garage if you can avoid it, if you also have a gasoline or diesel powered car kept in the garage.  The exhaust, especially upon initial start-up of the engine, contains chemicals that will eventually get into the various switches, contacts, etc., in your electronics, and cause a type of (difficult to remove and deal with) tarnishing...and thus INTERMITTENTS & STATIC in use of controls and switches, ETC.
b.  DO NOT use WD40 in your controls and switches.  Over time, WD40 tends to have a bad effect on the internal parts of most switches.
c. For AC powered items that have been stored for more than a FEW months, DO NOT plug in and turn on such items without FIRST raising the voltage from near zero, slowly, over some hours, to the normal 115 (or 117 or 120) volts.  There are parts such as electrolytic capacitors that do not appreciate having a sudden blast of voltage on them.  These electrolytic capacitors can short-circuit, and the larger ones are $$$....and may be difficult to find, if at all.  My shop uses a Variac (or similar) to enable the proper powering-up, for long-stored items.  I do it over many hours, usually overnight, raising the voltage a bit at a time.  UNfortunately, most home-theatre receivers, and certain others, can not be easily dealt with to reform capacitors, because the section that allows use of remote controls needs to be bypassed first, and figuring that out is sometimes QUITE time-consuming.
d.  If you intend to remove vacuum tubes yourself, for whatever purpose, be gentle, do not crack/break the locating pin on many power or rectifier tube bases.  It's not a catastrophe, but you must re-install the tube very carefully to avoid wrong tube pins in the sockets positioning.
e.  There is a lot to know about vacuum tubes, in trying to test them, change bias settings, installing substitutes, etc.  Our advice is freely given.
f.   Many musician's amplifiers have 1, 2, or 4 or 6 output tubes.  Some of these amplifiers have hi-low power switches, many or most do not.  Most of the ones without such switches, and even some with the switches, can have some tubes removed (which ones is important), in order to reduce maximum power available, and change the sound to an smoother or harsher overload mode.  ASK.
g. 

8.   Capacitors...what is recapping REALLY about?  What do you REALLY need to know?  Hype versus truth, ...etc.

There is a lot of talk about replacing capacitors, which, when done in many positions at the same time in an amplifier or other electronics items is often called 'recapping'.  The term recapping tends to be tossed-around particularly when the equipment is quite old.  

Many capacitors used in transistorized (solid state) equipment are operating at quite modest voltages, and are not under any serious strained operation.  These capacitors tend to be very long-lived, unless the over-pressure seal fails; or, they are exposed to heat (external or due to nearby warm items).  Very old style round type, with 1 wire out each end, 'paper, wax, etc.' capacitors tend to fail from age, quite differently than electrolytic type capacitors.  These type capacitors are usually rated at less than one microfarad.  With aging, they tend to leak electrically, something similar to acting as if a high ohms value resistor is wired across them.  These types of capacitors are also used in low-power circuits to couple signals from a high DC voltage to a lower DC voltage; as well as being extensively used in tone control circuitry.    They can upset the circuitry, including, especially, reducing the necessary and designed-for negative bias on an output tube.  The result can be anything from poor sound or output to extreme overheating and multiple parts failures....including vacuum tubes.   This type of capacitor failure can usually be easily tested-for.    These types of capacitors, usually smallish and inexpensive, should be replaced whenever the slightest problem is found with them, and if antique type, better quality higher rated modern capacitors should be used.      

Ceramic disc and rectangular mica capacitors tend to last forever.

Electrolytic capacitors are generally types that have two chemically treated, and often DAMP, and always use long and super-thin strips of aluminum in them, wound together, & so made that the two strips are insulated between them.  These are still made in vast quantities, in less than 1 microfarad value to many thousands of microfarad values, and can be rated from 10 volts to 600 volts.  Typically they are rated at 85 or 105 degrees C rating, although some were rated at 55 degrees C.     IT IS THESE type of capacitors that should be 'reformed' if they have had no power on them for months, let alone many years!

There is another type of electrolytic capacitor, from the truly-old days...these had an acidic-liquid-containing type of electrolytic capacitors may be rated at perhaps 2 to 10 microfarads, often at 100 to 500 volts.  These were used in the power supplies of antique radios.  These tend to fail after many years by leaking or chemical changes, but, if the liquid is intact (shake and listen), they MIGHT be reclaimable, using the same reforming technique as the non-slosh types previously described. These 'wet' electrolytics were of small microfarad values, because the necessary methods to obtain high values in a relatively small space were unknown at the time.  To compensate for the low value, the circuitry would incorporate what is called a 'choke', which was a coil, of many hundreds of turns of thin wire, wrapped around an iron piece of some sort....and a popular design was to incorporate this coil as the magnet needed for the loudspeaker.   Super powerful magnets without needing electricity were unknown at that time, so a low value capacitor and a iron core choke (Inductor) was very popular.  MOST ALL pre-sixties (approximate guess) radios, etc., used the lower value capacitor and the wound-coil method.

Modern (sixties onwards, approx.) electrolytic capacitors of the dry type (damp parts internally) ...with 105 degree (C.) ratings ...are generally better (longer lived) than those rated at 85 degrees (or less).  Large capacitors should generally be tested after 10 or 20 years, for capacitance values, sometimes for more advanced testing information.     Very large electrolytic capacitors are in wide use in powerful amplifiers, etc.   Various sized ones are used to eliminate hum in power supplies; circuits requiring extended low-bass response; circuits requiring a capacitor to couple to loudspeakers; tone control circuits of certain types; circuits to increase instantaneous power output for music peaks; and, many other purposes.     Modern electrolytic capacitors can be rated at very low mfd (microfarad) values and low voltages (down to 10, sometimes 3 volts), and up to exceptionally high values, including tens of thousands of microfarads.  If also rated at very high voltages, these capacitors tend to be quite large and quite expensive to replace.

Occasionally an electrolytic capacitor of any size and values will start to electrically leak (physical contents leaking is only modestly rare) and then will age quickly, or, will upset circuit operation.    These types of capacitors are generally rated at 85 or 105 degrees CENTIGRADE, but, the higher the temperature they are exposed-to, the faster the aging.  Some are rated as low as 55 degrees C.  Vacuum tube equipment can get quite hot, depending on design. This is particularly so when power tubes are factory mounted upside-down, and if there is a lack of holes, etc., for enough ventilation.  Using equipment very close to a wall is not a good idea.

If the capacitors get warm or hot...as noted, they age more quickly.  I always inspect electrolytic capacitors for physical leakage; and, have specialized equipment that is not very available commercially, to test capacitors for estimated longevity, measure electrical leakage even if minor; and, test for other more-hard-to-measure characteristics, which affect life of the capacitor which directly affects the life of tubes the capacitors may be associated with, which is especially important for power tubes if capacitor electrical leakage is upsetting tube BIAS.   This is quite commonly seen!  Coupling capacitors are used to pass the AC (sound) signal to the output tubes, & even with modern capacitors, have a tendency to act as if they have a resistor across them, as they get quite old.   This happens very much less with capacitors using special 'films' as the internal insulator, such as Mylar.    Because this is such a serious and costly problem to the owner who has to purchase power tubes too often, or even more serious things are a-happening, we ALWAYS use a special meter to see if the BIAS is being affected, if at all, and if so, by how much, due to capacitor aging.    A moderate to larger failure of one or two of the output power tube 'driving' stage capacitors will often cause the output tubes to rather quickly fail.  Along the way, you may or may not notice increasing distortion out of your speakers.  Coupling/driving capacitors are very cheap, compared to new tubes. 

If the capacity value DEcreases, which is quite common with aged electrolytic capacitors (and, somewhat with non-electrolytic), the effect varies, depending on the circuit and amount of decrease in capacitance.  If located in the power supply, constant 60 or 120 Hertz (cycles-per-second) HUM will is likely to happen, usually not, or minimally, affected by a volume or gain control.   If in certain circuits, the BASS response may seriously degrade.  Seldom is an electrolytic capacitor minor failure going to affect the treble response...but it CAN happen.

Old style 'paper' or 'wax' capacitors (all have been used in vacuum tube equipment AND transistor equipment) have a rather serious tendency to fail by their 'equivalent series resistance' ....decreasing to the point that the circuit will fail to work properly.  These aged capacitors will act like there is a resistor wired across the capacitor's two leads.  We almost never replace these types of capacitors with the original design types, unless so requested. 

If capacitors have a lot of electrical leakage, or, a short-circuit, all sorts of ills will come about.  It can be minor, such as a change in tone, or major, such as causing damage to vacuum tubes, transistors, diodes, speakers (yes!, can happen), etc.  Fuses may blow, and if the short is irregular, the effects be irregular.

Electrolytic capacitors can fail suddenly and disastrously.  If the pressure inside rises fast enough, the capacitor, which may or may not have a type of rubber plug with a safety valve hold in it, can EXPLODE, scattering slightly damp, but very acidic bits of aluminum foil, and in some cases damp insulating 'paper' bits, all over your equipment.  The end result will be damage from light to nasty nasty nasty.

Capacitors can sometimes be tested with only one wire (of 2) disconnected.  Some multiple connections types will require full removal for proper testing.  In quite rare instances, a capacitor may need to be tested on equipment that can change its temperature, measure very high resistance values, measure ESR, and other things.

WE DO NOT recommend wholesale 'recapping', UNLESS TRULY NEEDED.   We understand that some folks have drunk the Kool-Aid, and want their equipment 'recapped'.  We will do it.  It will cost you $$$ for labor...and $ to $$$ for parts....yes, the cost can be considerable.  Many of the largest capacitors are quite costly, and often very difficult to find brand new (or, may be totally not available). NOTE that capacitors have special ratings beyond temperature, capacitance, voltage, and polarity identification;...and, replacement of some capacitors needs to have these special ratings carefully considered.    

Willy-nilly replacing capacitors (including what is called Recapping) can cost you a lot of money, YET, improve little to nothing, ...    Yes, this is true.
On the other hand, a good technician will test, evaluate, etc....and, advise you appropriately.    BE SURE YOUR TECHNICIAN REALLY UNDERSTANDS CAPACITORS, AND ADVISES YOU APPROPRIATELY.   DO NOT WILLY-NILLY REPLACE CAPACITORS.  KNOWING WHICH ONES TO REPLACE, IF ANY, IS THE BEST WAYS TO GO.  KNOWING WHAT TYPES WILL INSTALL AND OPERATE PROPERLY AND LAST A LONG TIME IS IMPORTANT WHEN CAPACITORS ARE REPLACED.
NOTE that while many famous names in the electrolytic capacitor business have been out of the capacitor manufacturing business for a very long time, in some instances the original manufacturing equipment has been purchased by a new company, and reproductions may be quite excellent!   We see this quite often when restoring old radios.  We have had a very good experience with these capacitors.

9.