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1.  Speaker connections, if done wrongly, can DAMAGE your amplifier.  The most common cause is shorted wiring (transistor amplifiers) by using a shorted 1/4" speaker plug or touching/shorting by excessively removed insulation on speaker wiring located at the amplifier output.  Most amplifiers have 'SOME' protection circuitry, but I've never seen any amplifier that was totally foolproof, protecting against all problem usage.

Amplifiers with vacuum tube output stages are usually tolerant of abuse that might seriously damage transistorized amplifiers.  Conversely, transistorized output stages are quite tolerant of abuse that might kill vacuum tube output stages.  Vacuum tube amps CAN be damaged by OPEN speaker connections/wiring.  But, they are immune to damage from shorts on the speaker connections/wiring.  Transistor output stages can be damaged by shorts or near shorts (including too low speaker resistances/ohms), but generally are immune to opens.

Vacuum tube amplifiers usually have limited protections besides the sometimes ineffective quite modest, if even there, electronics circuitry protections.  SOME types of chassis-mounted 1/4" output jacks on vacuum tube output stage amplifiers have a built-in switch, so if no plug/wire/speaker is connected, the jack ...hopefully ...does the necessary safety-shorting.  Some vacuum tube amps do NOT have that sort of jack, and thus may be highly susceptible to damage from 'speaker circuit opens'. 

Some (especially musician's) vacuum tube amps may or may not include shorting style output 1/4" jacks, and some amps have diodes at the output stage that can help prevent very high voltages from being produced in the output transformer if the amplifier does not have a speaker connected and if there is no operating protective short in the output jack.  The diodes method may not be 100% effective.

Saying some of this differently, transistor output stages are often seriously damaged by SHORT circuits; or, from excessively low ohms loads, such as from too many speakers or wrong speakers.  Short circuits in the speaker wiring, or a bad speaker, or bad speaker wiring of some sort, can cause transistor currents to rise spectacularly, and the output transistors might be destroyed, usually doing lots of other damage.  Transistorized amplifiers usually have, and NEED, MUCH more sophisticated protection circuitry, compared to vacuum tube amps.  A lot of transistor protection circuitry is not all that protective.

If you wish to connect speakers in parallel, series, or have other requirements, please ASK US.

2.  Speaker Phasing...what is it?  Why is it important?  

A loudspeaker (except for some solid-state tweeters or certain other types) is, at its most simplified explanation, simply a coil of wire attached to the speaker cone, with that wound coil immersed in a strong magnetic field, via a narrow gap in a magnet.   You probably learned in school that there is a North and a South pole in a magnet, and you may remember other details about moving coils or moving magnets, etc.   For our purposes here, the important thing is that in most situations (excluding some complicated Public Address or other systems such as vertically driven dual sub-woofer cabinets, or cabinets with many speakers, that could otherwise interfere with each other due to echo or signal canceling distances, etc.) ....you WANT your sound system's movable cone type speakers to ALL move in, or out, at the SAME time.  If not so connected, the usual result is a THINNESS of bass response, as well as a reduced bass output volume.  Over-all, your speaker cabinet with two or more wrongly phased speakers will sound lousy, as the speaker outputs try to cancel each other, to some degree or other.  There are a few exceptions, for push-pull operation in the same cabinet, so beware!

At any one moment of time, where speakers are mono used in particular, but also as stereo connected if there is a mono signal, the following is the situation:
SOUND is air movement, so if a sound wave from one speaker is PUSH connected, and it reaches you approximately or exactly when sound from some other speaker is a PULL connected type, the sounds can cancel, considerably or partially.   While the effective volume decreases, the worst part is that, typically, the result is POOR and WEAK mid-range to bottom bass.  This is exactly the area that amplifiers need the most power...in the bass region....thus, the canceling can be severe.  This effect tends to 'gather' at some small distance from the speaker cabinet.   The over-all effect is much less at higher frequencies, and almost non-existent at near the high frequency limits of human hearing, because of how humans hear things.    But, the lower mid-range to bottom bass effects can be PRONOUNCED. 

It is possible for "Effects" units that are popular with musicians, to cause your sound to be exceptionally BAD, if the connections are wrong.

When a speaker is backwards (reverse) connected with regards to another speaker, we say they are "out of phase".
The effect can also be heard as the type of noise canceling I've mentioned.  You will always find lowered volume, 'thinness' of sound, etc. ...if the speakers are not too far distant from each other.

The electronics industry, speaker manufacturers, engineers, ETC., all 'agreed' many decades ago, that a STANDARD for speaker polarity was needed. The reason was so various manufacturer's speakers could be interchangeable for connections.

What was agreed upon was ...and I am making this simple for you ....if a low voltage battery was suddenly connected to a speaker, ....if the + terminal of the battery connects to the + terminal of the speaker (which is to be marked red if color marking is desired, otherwise it is to be marked with a + symbol, although many manufacturers do both) (for - terminal, it is black) .... then the speaker cone, at the instant of battery connection, will move OUTWARDS; that is, away from the magnet.

If the battery is connected backwards to properly-marked speaker terminals, then the cone moves INwards (towards/into the magnet). 

If one speaker moves the same for the same signal, as another speaker, they are said to be IN PHASE.  If not, they are said to be OUT OF PHASE.   Speaker phasing of separate speaker boxes, for musicians and home stereos, are designed to be IN PHASE.   For Public Address systems, it is more complicated (with many speakers in use, over distances, echoes, etc.); and there are also some types of speaker boxes that have speakers in front and rear; and they may purposely be anti-phased, usually to give increased in-phase (effectively) loudness or a thing called dampening.

The battery test is USUALLY the easiest, quickest, and most accurate method of identifying speaker terminals precisely.  I have seen speakers wrongly marked.  Of course, you have to be able to see the speaker cone. Note that some speaker makers have marked their speakers, or the speaker terminals for the speaker box, or both, BACKWARDS.  JBL did this, a very long time ago.  Thus, it is a really good idea to CHECK speaker phasing, using a small battery, such as a common 9 volt type which can't deliver too much current.  Those are adequate for small soft cone speakers.  I recommend a larger battery, to perform the check on powerful speakers, because they generally have much stiffer cones or cone suspensions, and movement requires substantial current flow to the coil.  A battery should not be connected for any length of time....ONE-HALF SECOND is plenty.   You can do multiple connections, one after the other, if you need to.    Have the speaker DISCONNECTED from the amplifier for these tests.   Used properly, the method ALSO can "help" with phasing of wiring in businesses, churches, and some homes.  For large areas, and P.A. use, we can advise of the more sophisticated tests and connections that are usually needed.

One other method of checking speaker phasing is common, and is often done with Stereo-HiFi equipment, where all the terminals and connections for the speakers and/or amplifier; or, visual access to the cone ....might not be easily gotten-to ...or seen.  This method is also used with such as home theatres, with their multiple speaker systems.  This method is done with any two speakers at the same time, and repeated in all combinations of left-right-front-rear, for a home theatre.  While professionals do this with special test equipment and tones, you do not have to.   You can simply tune an FM radio (connected to the amplifier that drives the speakers) to any place BETWEEN stations, if necessary turning off any muting (sometimes selecting MONO or stereo OFF is needed).  The idea is that you will want to be listening only to HISS NOISE.  This can also be done, but not as well, with a common AM radio station.  Any FM mono, not stereo, signal will also work. It is easier using hiss, than program material.

Example:  Sit in a room, approximately equal-distant from the two speakers, where you are are centered.  LISTEN, then have a friend reverse ONE ONLY speaker connection.  That is, your friend will swap + and - connections, never allowing a short circuit (for amp protection reasons).  The position of the leads will definitely cause one position of leads to be louder, than the reverse position.  If you have trouble hearing this, then, repeat, closing your eyes:   The correct position of that speaker wires is when the sound appears to come from the middle of your head, the wrong position is when you hear things separately in your individual ears.  If more than one speaker pair, use ONE as the master, for each test. ((For large public address systems, the testing is done and adjusted, for echo effects minimalization)).

In the same way that phasing of speakers is important, so are the connections to such as stereo phonograph/turntable cartridges, and for the SAME reasons. The reasons are far less important, however, because there is usually no big reason to have Left and Right interchanged by not having the phono cartridge properly wired.   For phonos/turntables, signal wiring has standardized color coding.  Sometimes the wires are push-on, not only to the phone cartridge (which are color or function or both, marked), but to the cartridge holder...which are almost always NOT MARKED, and need to be properly identified by YOU, or ME.   What is of much lesser importance is the LEFT-RIGHT connections for amplifiers and speakers ....except for home theatre ...and where absolute authenticity of listening position is usually desirable.

3. Amplifier biasing.   Both far too much, and far too little, is discussed about amplifier biasing ...and, for tube selection and biasing:

In 90% of instances 'the talk' is about vacuum-tube biasing for musician's amplifiers; much of this is due to biasing effect on tube life,  but other things are also affected, such as the power output and the tonal quality.  For transistorized output stages, bias is set once, and then not again ....unless repairs (including new tubes on tube amplifiers) are done.  Transistor output stages have the bias current set for a value that is high enough to eliminate distortion at very low volume levels, and low enough so that the output stage does not get excessively warm with no signal input; AND, to a specified voltage or current in some area of the circuit, that is often adjusted on an oscilloscope, using a sine-wave tone from an oscillator.  The adjustment is often made to the point that noticeable distortion disappears.  Many transistor output stages have an adjustment for a specified current flow, which may be measured in several different ways, including the voltage drop across known resistor values (1 ohm or less) in the circuit.  Transistor output stages may have other adjustments, for such as DC output minimalization, balance, etc. 

 Vacuum tube output stages may have balance adjustments besides bias adjustments.

For vacuum tube output stages, the situation can be simple ...or, slightly more complicated.  One type of design circuit uses "cathode biasing" that usually has no adjustments for bias.  That type of circuit is used almost exclusively for lower wattage amplifiers, with some notable exceptions.  For high output vacuum tube output stage amplifiers for musicians, where power output of the tube(s) is of importance (to get the most power, within reasonable tube life, and more or less a certain tonal quality) the type of biasing usually used is called fixed bias, and the needed negative voltage is supplied by a negative power supply output.  In these amplifiers, bias on the output tubes is nearly always adjustable and, as noted, there is no cathode resistor (or, the one used is very low in ohms, such as 1 ohm and exists to allow voltage measurements that, via Ohm's Law, one mathematically converts to current flow; or, for connection to an oscilloscope, or, other reason).  Some of these amplifiers have balance adjustments, and some old amplifiers from the sixties (such as from Fender Musical Instruments Corp.) are designed to incorporate a "sort of" balance and minimum hum...and, MAYBE bias adjustment ...all via just one control.   Depending on the amplifier design, there may be connection places for a meter, which may be on the chassis available to the owner/user ....or, inside, and several types of adjustments. Some have lamps to indicate proper adjustments.   Adjustments allow for bias amount, balance between the tubes, etc.   Standard settings are usually specified by the manufacturer, usually an oscilloscope and a sine wave oscillator is needed and one or two of the adjustments are made at or quite near-to, maximum clean signal output....this means that the output is watched on the oscilloscope, while adjustments are made.  A special resistor load is used instead of a speaker.   Some amplifiers are not stable, with certain loads.  This is seen now and then, and the instability is typically at a frequency much higher than the signal used for the testing.  A truly good job is completed by careful analysis of load, power, frequency, and such as burst signal or other pulse signal.

Adjustable power stages may have bias set at certain published values, often this is actually a suggestion only, and applies only to brand new vacuum tubes that, themselves, are tested on a special tube tester; for specified characteristics.   When the tube(s) are installed into the amplifier, the adjustment is usually BEST made for a minimum amount of current flow that still minimizes distortion, especially second harmonic type.   Note the difference, in that transistor power stage bias is usually set at very low signal (if any) output, while tubes bias is usually set for just before any more volume creates an overload while looking at the waveform on an oscilloscope (I do transistor stage testing at both signal situations, all on a 'scope, but I mostly do it as a check on the published settings).

Re-saying some things:
When talk is about 'higher bias', that is almost always for vacuum tubes and usually means LOWER negative bias VOLTAGE than normal, and it can result in a lowering of tube life, due to the HIGHER continuous current flow, with only a modest amount of extra power output being available from the higher bias.  The amplifier manufacturer selects bias for a certain amount which relates to a certain current flow; or, more often, selects the bias so that with whatever particular tubes are installed, the tubes are operated under conditions that reduce output distortion, yet provide a good compromise for maximum power output.  USUALLY, the important biasing for vacuum tubes is set with near maximum power output, so that at just barely into overload conditions, the sound is a type of 'clean' tube amplifier overload, and not a raspy lousy sound.   Bias that is set higher (lower voltage) is sometimes used by musicians who want a certain type of sound at and through the loudest overload (maximum power/signal output) condition.  These folks will pay the price for this, in replacement of tubes and sometimes other things.  Sometimes the biasing checks are done at both very high output & very low output.  I do this.  There are some types of vacuum tube output amplifiers that are specifically biased for extreme LIFE, those are typically for public address use in factories, offices, etc.   Quite often these amplifiers run lower B+ voltages together with cathode bias.  This can give extreme tube life, and have no drawbacks for Public Address purposes.

Amplifiers for musicians are OFTEN designed so that the amplifier has one or more 'clean' pre-amplifier channels, and also one or more 'dirty' or purposely distorted channels that have the capability via front panel adjustments for such distortion/type/etc, etc.   There almost always is both a master volume control and channel amplifier volume controls, and proper use will produce clean sound somewhat into the OUTPUT STAGE OVERLOAD POINT ...OR, the master volume can be turned down some, and the pre-amplifier gain control turned up, which will cause, on purpose, a hard overload sound (more or less).  Some musicians amplifiers have all sorts of combinations of these sorts of things, even multiple IN SERIES pre-amplifiers, which enable, on purpose, high distortion and headroom, at even low volumes.  Some amplifiers are purposely designed to have extreme control over the amount of amplification/gain. Some later large Marshall amps are like all of these things.  Marshall is NOT the only one that has produced such amplifiers...Ignater did, and others too.

Some amplifiers have switches that change bias and/or something else, for different types of sound.   There may be switches for maximum power output adjustment, or, for Triode Connection sound, etc. Some amplifiers have front panel controls for a dozen or so different type of main amplifier sound/tone.   There is a UK/England "British" sound, and many others, some called by the particular type of circuit design, or even by vacuum tube number.   Example:  EL84 or EL34 or 6L6 sound....etc.   Some settings have other descriptive names.  Some available switch settings may modify the bias, or a long list of 'other things'.  Transistor output stage amplifiers have other methods, in general, for producing tonal change.  Some amplifiers have switches for power output at the overload point, and some also have switches to change the output circuit from 'beam power tube types' to triode type.

The TONAL QUALITY of the amplifier will change with a change of vacuum tubes and bias setting and/or beam power to triode.  In many instances, amplifiers are designed to be used with more than one type of output tube, to get the 'British sound', ETC.  A few amplifiers were designed to have two types of vacuum tubes in the output stage at the same time.  That is not the same as Marshall, who is relatively famous for building amplifiers that will accept several types of output vacuum tubes, fitting into the same octal socket with the same wiring....and the only change needed is a bias adjustment, by control setting ...or resistor change, ...or both.    An amplifier using 6550, or 6L6 or 5881, or EL34 is common, with many even allowing allowing KT88 and KT66 tubes. I've even seen EL37 tubes ($$$ these days).   In some instances, with 4 or 6 output tubes, we can advise SIMPLE, NO COST methods of getting 'your tone', especially if that means you prefer a two tube output stage instead of a 4 or 6 tube output, this brings about distortion more quickly, and easily done by such as simply removing certain tubes.  Experience or consultation with an audio repairman who knows his stuff, is advisable.

Re-saying some of the things in earlier paragraphs:  In some instances, a tone more to your liking is available on amplifiers with HI/LOW switches, and/or with so-called MODELING amplifiers. 

I won't usually 'over bias' your amplifiers, even if requested, unless there are special circumstances.  Large amounts of over-biasing will reduce tube life, and, in some instances, can lead to $$$ repairs.

4.  WHY do I advise a specific order of use for your musician's amplifier's power and standby switches?

Many, not all, musician's amplifiers (especially ones over 10 watts or so) have both a power switch and a standby switch.  The standby switch turns off the high voltage to the output tubes.  It's really a good idea to have both of these switches.  I can usually add such a switch if you do not have one.  The cost is typically paid for by lengthened output tube life, fewer blown fuses and other parts, etc.   These standby switches extend your output tube's life for two reasons:

While this description/advice applies to most vacuum tubes, it is more important for the larger audio power output tubes for technical reasons.  At the very center of power-type vacuum tubes is a central tube, usually round, but sometimes round and flattened in shape.  The central tube contains a spiral (or up and down) tungsten thread or ribbon that is heated to glowing reddish-orangish-hot, which heats the surrounding central tube to approximately the same color.   The outside of this central tube is coated with a substance that likes to emit electrons when hot, if proper voltages are applied to other items in the vacuum tube.  The electrons can be thought of as flowing towards the big grayish surrounding metal plate structure.  

If you turn ON the power and the standby (high voltage) switch at the same time, or the standby switch is turned on much too soon, or, there is no such switch, then the high voltage that appears instantaneously, with exceptions for slower to heat up vacuum tube rectifier tubes which don't need a standby switch much, will 'draw' concentrated areas of electrons from anyplace it can, towards the gray surrounding and at high positive voltage 'plate' metal. With most power output tubes, this happens also with a certain wire-grid inside the tube. While a large current flow can damage fine-wire structures such as that wire-grid, the discussion here is for the electrons-emitting central tube.  If you do not have a standby switch, then the following effect happens at every turn-on, particularly with solid-state rectifiers type of power supply, but also tends to happen with rectifier tubes that heat up rather fast:

During the time that the central metal tube is still slowly heating-up, the intense attraction of electrons from the central tube towards the surrounding metal plate structure will try to find microscopic faults in the central tube coating, as the major portion of the coating is not yet hot enough for mass electron flow from every part of the surface.  So, fairly substantial current flow will be comparatively had from very tiny micro-size-pinpoints.  These pinpoints, which are otherwise generally OK faults, can not 100.00% handle the comparatively large electron flow that occurs from such a tiny microscopic area, so the tiny pinprick sized area gets very hot...that is, the outer bonded material coated onto the central tube will have VERY high heating, at microscope points.  In the vacuum tubes under discussion here, otherwise OK faults will be made larger and coating material on the outside of the central tube will likely eventually begin to flake off.  On some types of tubes, this also happens to the particular grid-wire I mentioned. Damage can occur from short circuits by this flaking material if the flakes are large enough, and the particular grid wire can also fail....and these things CAN happen on vacuum tubes that are either mounted with the top of the glass UPwards........or; DOWNwards.  Damage can also come from warping of the central tube and other structures in the vacuum tube.  Basically, several types of damage can occur, and add to each other, causing lowering of tube performance and eventual failure, at a faster rate.

The short circuits can ruin the tube, damage a few resistors, damage power supply diodes, and do other damage.  If you are using an oversize fuse, a large to very large amount of damage is possible.  NOTE that the effect can also happen (vastly less though) when turning the amplifier switches OFF in the wrong order.

The correct use of the two switches is:

A. To turn the amplifier ON, first be sure the STANDBY switch is OFF (this is the switch position for NO SOUND).  This may be confusing to you, as some amplifiers have the standby switch position for OFF, called Standby-ON.   Re-stated:  The true OFF position is the one such that the amplifier would not produce speaker output.
B.  Turn the MAINS or A.C. Power Switch to the ON position.
C.  Wait at least 30 seconds...I prefer 1 minute ....before turning the standby switch to the ON, or play, or, let's just say the functional position.
D.  It is perfectly OK, when on a gig, to turn the standby switch OFF, between sets, etc., leaving the mains power switch ON, if you wish.

Ask me about my finger-tapping test for tubes for oncoming problems.

When turning the amplifier completely OFF:
A.  Turn the standby switch to the non-function, or OFF position.
B.  After a few seconds, turn the mains power switch off.

5.  Why is using an oversize fuse so bad for your equipment.....ETC! 

Fuses are incorporated for more than one purpose.  PRIMARILY, fuses are incorporated to prevent fires; and/or, to protect the circuits and components from excessively serious damage.

Fuses have several specifications, one is for maximum safe voltage to be applied ....otherwise, a very damaging ARC of current might occur, for instance, if you use a 25 volt rated CAR fuse, and not a proper 125 or 250 volt rated fuse.  Any such ARC will allow current to flow through the fuse for longer than desirable, as the fuse is blowing.  A wrong voltage rating fuse is, however, rarely going to be a problem.

The most common problem or potential problem that I see ....is someone using a fuse of higher amperage rating than the equipment is designed-for.   It is not particularly unusual for me to find a 30 ampere car fuse in a musicians amplifier!

In GENERAL, it may...MAY...be OK to use a substitute fuse that is up to 30% higher than originally specified, but this is very seldom so for a slow blow specified fuse.  In almost every instance, using a lower than specified amperage fuse is OK, whether SloBlow, or not.

I will give you an explanation of why using an oversize ampere rating for the fuse can be so very damaging to your wallet, and thereby good for my business:

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:

ONE of the specifications (that Ohms-law sort-a stuff) is:  the power that can pass through a fused circuit, before the fuse opens, is a function of the SQUARE of the fuse current rating.  Thus, the 2 ampere fuse can pass 4 units of heating power, or however you would like to think about it.   BUT, a 3 ampere fuse will be able to pass 9 units of power.  If you installed a fuse of twice the original specification, that is, a 4 amp rated fuse, now 16 units of power can be passed.  I have ... many times ... seen 20 and 30 ampere car fuses (typically, not always, 32 volt rating, which is even worse) or even metal foil installed in equipment.  The damage done causes the owner to bring the gear to my shop; usually because there is no or improper operation, perhaps a strong burned-up smell, etc.  The repair job is often messy, and sometimes VERY expensive. 

If you substituted a slow-blow fuse, whether of correct amperage, or not, more damage than simply a tube replacement is more likely.  That's because the overload current flow lasts longer.   THERE ARE INSTANCES WHERE I HAVE SEEN THE DAMAGE FROM USE OF VASTLY OVERSIZED (IN AMPERAGE) FUSES CAUSING THE EQUIPMENT TO NOT BE WORTH REPAIRING!!

If have to substitute a fuse, and cannot immediately get the correct one, NO damage is overly likely to happen if you use a LOWER value fuse OR in many instances if a slo-blow fuse, if not specified.  If using a larger ampere fuse, bad expensive things might happen.    Occasionally, the use of a 25 or 32 volt car fuse, even in correct amperage, has caused $$ problems, that would not happen with a 125 or 250 volt fuse, due to the ARC of electricity possible with the design of the lower voltage fuse, compared to the higher voltage fuse.

You can USUALLY substitute a regular standard blow rated fuse for a specified slow-blow type of fuse, or even SLIGHTLY increased amperage rating.   A lower amperage rated fuse is usually safe, but every time you replace a fuse, with some fault going on, you are stressing some component or other in your amplifier, potentially causing extra damage.

6a.  Think those old NOS (new old stock) vacuum tubes are way better than modern manufactured tubes??
Well, they are sometimes not made as well as newly manufactured tubes from SOME manufacturer's.   Problem is, there is way too much hype and miss-representation of the quality and specifications of old and new, including old brand names that were purchased and now being made poorly....by some company outside of the USA.   MANY modern tubes are not made by the name on the tube.  Yes, this includes Groove Tubes, which was purchased by Fender.   There are not a lot of actual tube manufacturer's, and most make tubes under various brand labels.  It's true that reproductions have steadily improved over the last 15 years or so.  We can advise what specific tube numbers and brands are better than others, for one or another purpose, including just being longer-life or lower noise, etc.  I can also supply information on who is building what tube.  Often it is New Sensor.  DO NOT be swayed by advertising or word-of-mouth 'expertise'.   Some much-hyped tubes are not worth anything near the selling price.  
Did you know that Electro-Harmonix (EHX) is the largest tube manufacturer in the world?
SOME OF THE ELECTRO-HARMONIX BRANDS ARE:  Tung-Sol, Electro-Harmonix, EH Gold, Genalex Gold Lion, Mullard, Svetlana & Sovtek.
Yes, the same Electro-Harmonix makes stomp pedals, and other things.

6b.  >>>...here's something that tube manufacturer's don't want to admit to, and very few users know about, a phenomena called Heater Flash or Filament Flash (have been other words too).    NOTE that this problem is NOT common to USA-built vacuum tubes, as the manufacturing process was usually different.    While this problem is fairly common to NOS (new old stock) tubes like the ECC81, ECC82, ECC83, it has been seen in all sorts of vacuum tubes ...especially the small glass tubes, but even some power tubes have exhibited the problem.   First, I will discuss what it LOOKS like....because a very simple visual test will tell you if your vacuum tube has the potential problem, with just a LOOK at the tube when turning on the equipment:

When you turn the power ON, power is instantly sent to the vacuum tubes to heat the filament (heater element part of the vacuum tube).  It is that part of the tube that, within maybe 20 seconds, turns orangey-red in color (maybe a bit longer time for big power tubes).  This is NOT meaning any surround-the-filament tiny tube. 

WATCH the tube(s) near the bottom (base) of the tube glass area.  If the tube is installed upside down, as in many Fender and other makes, you are watching the base area of the tube.  Begin with the power having been off for at least a few minutes.  WATCH, AS you first turn on the mains power switch.  If the filament/heater shows a very sudden flash of light then that tube HAS the problem, and is likely to soon do what we call 'burn out', but is really just the filament/heater breaking its wire.   It is this manufacturing design problem that causes a shorter life, in what otherwise MIGHT be a really nicely made, low-noise, well-performing, often foreign-made, or OLD STOCK, or new stock, vacuum tube.

If you have a choice, use a tube tester, or the amp or other piece of equipment the tube will be used in, as a test instrument...to power-up that tube BEFORE you purchase it.  REJECT such tubes unless you understand the life problem and can deal with it.  Sometimes, in the dual-triode tubes mentioned (12AU7, 12AX7, 12AT7...aka ECC81, ECC83, ECC82.....), only one of the two filaments/heaters will exhibit the problem.  REJECT!

Note, that if you had a way to SLOWLY power the tube filaments/heaters, the problem would not exist.  Circuits and methods have been designed to do that.  Such circuits are QUITE rarely seen.  After all, some of those foreign tubes are usually otherwise very well made, especially the REAL old stock types from Telefunken, Mullard, and others.  Some traveling musicians are knowledgeable about the problem and have asked me to modify their amplifiers. The method I use is simple.  I add a high quality filament switch, and put a power resistor of appropriate ratings across it, and wire that assembly in series with one of the filament connections to the type of tubes that need such protection...which is seldom the output tubes.  The value of the resistor depends on the total tubes filament current.  I have also done the modification with an electrical part that does not need a switch.  When you power up the unit, filament/heater switch OFF, the resistor allows only a modest amount of current to flow.  After maybe a minute, turn the switch ON.   If you have a regular standby switch, THEN is the time to set it to the operating position. There are some very $$$ vacuum tubes that have this problem, and sometimes, after informing the customer of the problem, the request will be for even more reliability, and I use a three way switch.  This is almost never done for anything but certain very expensive tubes used in industrial or commercial equipment.  No, a three way switch is not needed for EL37 audio tubes.  Said another way, on some vacuum tubes designs, and this does not necessarily mean all tubes of a same number, it is the first fraction of a second at 'lighting up'; power on, ...that causes fast wear on the tube filament connections.   The information about tubes with filament flash is really nerdy, 99% of you should ignore it, unless you are purchasing tubes for a mission-critical piece of equipment....or, don't mind the cost of having spares on hand.

7.  Bits of advice/hints is 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.  WD40 tends to harden over time AND has a bad effect on some plastic bodies switches, etc...ETC.
c. For quite-long stored items, DO NOT plug in and turn on your AC powered items to fully powered with line voltage, without FIRST raising the voltage from near zero, slowly, over some hours, even a day or two, 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, after not having any voltage for a long time.  These 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.  Yes, I do it over many hours, usually overnight, raising the voltage a bit at a time.
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 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 a smoother overload or smoother 'drive' mode.  ASK.

8.   Capacitors...what is recapping REALLY about?  What do you REALLY need to know?  Hype versus truth, ...etc.
The homepage, https://fleischerelectronicsservice.com, contains a outline boxed area about not powering up amp's, etc., that have been unpowered for a long time.  Please read it, if you have not already done so. 

There is a lot of talk about replacing capacitors, particularly when they get quite old.   In general, nearly all types of capacitors used on transistorized (solid state) equipment, are operating at quite modest voltages, and are usually not under any serious strained operation.  These capacitors tend to be very long-lived, unless the over-pressure seal fails. 

Old style 'paper' or 'wax' capacitors have a rather serious tendency to fail by their equivalent series resistance decreasing to the point that the circuit as designed will fail to work properly.  We almost never replace these types of capacitors with the original design, unless so requested. 

Occasionally an electrolytic capacitor will start to electrically leak (physical contents leaking is modestly rare) and will then age quickly, or, will upset circuit operation.    These 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...they age more quickly.  We 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.     Coupling capacitors used to pass the AC (sound) signal to the output tubes, even modern capacitors, have a tendency to act as if they have a resistor across them, as they get older.   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 tubes too often, 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.    An failure of one or two of these 'driving' capacitors will cause the output tubes to quickly fail.  Along the way, you may or may not notice serious distortion out of your speakers.  Coupling capacitors are very cheap, compared to new tubes.

If capacitors 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.

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

Capacitors can sometimes be tested with only one wire (of 2) disconnected.  Some 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 parts.
NOTE that capacitors have special ratings beyond temperature, capacitance, voltage, and polarity identification;...and, replacement of some capacitors need to have these special ratings carefully considered.    

Willy-nilly replacing capacitors can cost a lot of money, improve little to nothing, ...ETC.    
On the other hand, a good technician will test, evaluate, etc....and, advise you appropriately. 



Future articles/item to discuss are being considered.....(let's have your ideas/input):
A.  Substituting amongst 12AX7, 12AU7, 12AT7, ETC.   AND, what about ECC81, ECC82, ECC83...etc?
Is there a REAL difference in sound quality/tone, between various tubes, both original numbers and substitutes?   What about such as tall plates versions, etc.?
What's the truth about NOS tubes?
B.  Setting basic bias yourself.... how-to, pitfalls, etc.
C.  How to wire speakers and match them to amplifiers. Series connections, parallel connections, combinations.  Effective ohms loads.  Effects of speaker impedances (ohms) on amplifier distortion, etc.


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