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1. Speaker connections, if done wrongly, can DAMAGE your amplifier. Some amplifiers have some sort of 'protection circuitry', to protect against common problems. Even the amplifiers with such circuitry are not foolproof; or, cannot protect against all faults.
As a general rule, vacuum tube amplifiers (or, amplifiers with vacuum tube output stages) are quite tolerant of abuse that might kill 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, but are immune to opens...as a general rule.
Vacuum tube amplifiers usually have limited protections, typically those might be a speaker output jack (especially SOME types of 1/4" output jacks) that have a built-in switch, so if no plug/wire/speaker is connected, the jack ...hopefully ...does the necessary safety-shorting. Some tube amps do NOT have that sort of jack, and may be highly susceptible to damage from 'speaker circuit opens'. Other protections in vacuum tube amps include diodes at the output stage that can HELP prevent very high voltages from being produced in the output stage transformer. These are NOT typically 100% effective ...but do help.
Transistor output stages are 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 will be destroyed, usually doing lots of other damage at the same time. Transistorized amplifiers usually have, and NEED, MUCH more sophisticated protection circuitry, compared to vacuum tube amps. A lot of such circuitry is not fully protective.
If you wish to connect speakers in parallel, series, or have other requirements, please ASK.
2. Speaker Phasing...what is it? Why is it important?
A loudspeaker (except for some solid-state tweeters) is, at its most simplified explanation, simply a coil of wire immersed in a strong magnetic field. 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, moving magnets, and so on. For our purposes here, the important thing is that in most situations, (excluding some more complicated Public Address or other systems with two or even many more speakers, that could 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.
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 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 lower mid-range to bottom bass. That the effect is very much less at higher frequencies, and almost non-existent at near the high frequency limits of human hearing, is an effect of how humans hear things. But, the lower mid-range to bottom bass effects can be PRONOUNCED. It is even possible, for "Effects" units that are popular with musicians, to sound 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 a type of noise cancelling. 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 main 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.
As you may now think, if the battery is connected backwards to marked speaker terminals, then the cone moves INwards (towards the magnet). Yes, that is true. 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, is almost always 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) volume.
The battery test is USUALLY the easiest, and most accurate method of identifying speaker terminals. Of course, you have to be able to see the 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 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. A battery with a voltage under 3 volts is not likely going to move the speaker cone enough to easily see the direction it moves. 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 the more sophisticated tests and connections that are usually needed.
The other method of checking speaker phasing 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 simply tune your FM radio to a place BETWEEN stations, if necessary turning off any muting (sometimes selecting MONO or stereo OFF is needed), you will want to be listening to HISS NOISE. This can also be done on common AM radio, although the sound will not be a hiss. Any mono, not stereo signal will actually work. It is FAR easier using hiss, than program material.
Example: you are sitting in a room, equal-distant from two speakers, which are typically 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, have more bass, than the reverse position. If you have trouble hearing this, then, repeat, closing your eyes. The correct position of that one speaker's wires is when the sound appears in the middle of your head, the wrong position is when you hear things separately in your individual ears. If more than one speaker, 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. Thus, almost all signal wiring has standardized color coding. 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 desirable.
3. Amplifier biasing. Both far too much, and far too little, is discussed about amplifier biasing ...and, for tube selection and biasing.
In 99% of instances the talk is about vacuum-tube biasing. For transistorized output stages, bias is set once, and then not again ....unless repairs are needed. 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 hot with no signal input. Transistor output stages may have other adjustments, for such as DC output minimalization, balance, etc.
For vacuum tube output stages, the situation can be simple ...or, complicated. There are designs of several types, including "cathode biasing" that have no adjustments for bias. That type of circuit is used almost exclusively for lower wattage amplifiers, with some notable exceptions. For higher output vacuum tube output stage amplifiers, the type of biasing used is supplied by a negative power supply output, and the method is called "fixed bias". That, simply-put, means that the bias on the tubes is adjustable and there is no cathode resistor (or, the one used is very low in ohms, and is there primarily to allow certain measurements across it). Typically there are one or two or three adjustments, and there may be connection places for a meter, which may be on the chassis available to the owner/user ....or, inside. The adjustments allow for bias amount, balance between the tubes, etc. While 'standard' settings may be set by the manufacturer, usually an oscilloscope and a sine wave oscillator is used, 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. There is sometimes also a balancing adjustment.
When talk is about 'higher bias', that usually really means LOWER bias VOLTAGE than normal or optimal, including tube life especially. The result of so-called higher-bias is higher current through the tubes with no or very little signal being present. The 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 is for near maximum power output, so that at overload conditions, the sound is a type of clean tube amplifier overload, and not a raspy lousy sound. Sometimes the biasing checks are done at high output or low output, sometimes both. Tube amplifiers are OFTEN designed so that the amplifier has one or more clean pre-amplifier channels, and same for channels that have the capability of front panel adjustments for such as distortion, etc. There may be both a master volume control and channel amplifier volume controls, and proper use will produce clean sound well 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. Later large Marshall amps are often like that. Some amplifiers have switches that change bias for different types of sound. There may be switches for maximum power output adjustment, or, for Triode 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.
****Vacuum tube bias ALSO affects maximum clean and maximum dirty power output. GENERALLY, if the output level capability is increased (this is measured with a clear tone signal input, from an oscillator) beyond specifications, tube life will suffer. USUALLY the output wattage capability is not increased very much by this "over-biasing".
Another thing that happens, that is rather subjective, is that the TONAL QUALITY of the amplifier will change, with a change of vacuum tubes (particularly if not matched, or if of other 'selected' grades than the amplifier is designed-for). In a few instances, amplifiers can be designed to be used with more than one type of output tube, to get the 'British sound', and many others. Marshall 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 or resistor change, or both. An amplifier using 6550, or 6L6 or 5881, or EL34 is common. I have seen some that allowed, also, KT88 and KT66 tubes. In some instances, with 4 or 6 output tubes, we can advise SIMPLE, NO COST methods of getting 'your tone', such as by removing certain tubes. Experience or consultation with an audio repairman who knows his stuff, is advised here.
SO....In some instances, a tone more to your liking is available on amplifiers with HI/LOW switches, and with so-called MODELING amplifiers.
I won't get into this in much more depth, just to say that we 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 as 'important', the specific order of use for your musician's amplifier's power and standby switches?
Some, not all, musician's amplifiers have both a power switch and a standby switch. The standby switch turns off the high voltage to the output tubes.
It would be better for business profits to NOT tell you this, as NOT telling you this might bring more vacuum tube sales for me (or other shops) ...and more over-all repairs .... But:
While this applies to all filament type vacuum tubes, it is much more important for the large audio power output tubes for technical reasons. At the very center of most power-type vacuum tubes is a central tube, usually round, but sometimes flattened in shape. The central tube contains a spiral or up and down tungsten thread/ribbon that is heated to glowing reddish-orangish-hot, which then heats the central tube to the same color. The outside of this central tubing is generally coated with a substance that likes to emit electrons when hot, if proper voltages are applied to other elements 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 switch at the same time, or the standby switch is turned on much too soon, then the high voltage that appears usually instantaneously on these tubes at main power turn-on with Standby in PLAY (on) position, will 'suck' electrons from anyplace it can. While a large current flow can damage fine-wire structures inside the vacuum tube, the discussion here is for the electrons-emitting central tube inside the vacuum tube.
During the time that the central tube is slowly still heating-up, the 'sucking' (sorry!) will try to find microscopic faults in the central tube coating, as the major portion of the coating is not yet hot enough. So, fairly substantial current flow will be had from very tiny micro-size-pinpoints. These pinpoints, which are otherwise OK faults, are not designed to handle the large electron flow that occurs from such a tiny area, so the area gets too hot...that is, the outer bonded material will have VERY high heating, at microscope points. This is akin to electric wiring with too high a current for the size of the copper wire. In the vacuum tube under discussion here, perfectly OK faults will be made larger and coating material will likely flake off. Damage can occur from short circuits by this flaking material if the flakes are large enough, and it 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.
The short circuits can ruin the tube, smoke a resistor or two (or more), damage power supply diodes, and do other damage. If you are using an oversize fuse, it can do a huge amount of damage. NOTE that the effect can also happen when turning the amplifier 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 (the 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 finger testing tubes for pending problems.
When turning the amplifier completely OFF:
A. Turn the standby switch to the non-function, or OFF position.
B. After about 10 seconds or more, turn the mains power switch off.
5. Why is using an oversize fuse so bad for your equipment, and when is 'wrong', actually usably OK? :
Fuses are incorporated for more than one purpose. PRIMARILY, fuses are incorporated to prevent fires; and, to protect the circuits and components from serious, or excessively serious damage.
Fuses are rated 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. The wrong voltage rating fuse is very rarely going to be a problem.
Using a SLOW-BLOW fuse is a bit more of a problem, if the equipment was not designed for a slow-blowing fuse. However, in many instances, one can substitute a slo-blow fuse of LOWER than equipment manufacturer standard no-blow specified current, but you need to consult with your electronics advisor (our advice is FREE). DO NOT substitute a slow blow fuse of higher current value unless I have approved this, specifically for your unit.
The most common problem or potential problem that I see ....is using a fuse at a higher amperage rating than the equipment is designed-for (some few amplifiers, including many tube type Marshall's, have a panel specification for amperes that IS OK to exceed, so ask me). I will try to give you a simplified explanation of why generally 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:
A piece of equipment is rated for a 2 ampere fuse. You are on a gig, the fuse blows, and you install a 3 ampere fuse. The fault that occurred, perhaps a tube shorted, whatever, would normally be inexpensive to repair, just replace it, and the fuse, and rarely some resistor or diode failure which is cheap. Replacing the original 2 ampere fuse with a similar fuse might have led to a quite modest to even no repair beyond the cost of the fuse and maybe a shorted tube. The 3 ampere fuse MIGHT lead to a rather more expensive repair. A substantial increase in fuse size CAN be much worse. WHY?
ONE of the specifications of 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, the 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 (or, metal foil) installed in equipment. The damage done causes the owner to bring the gear to my shop. It often is messy, burned-up, and sometimes quite expensive to repair.
If you have to substitute a fuse, and cannot immediately get the correct one, no damage is likely to happen if you use a LOWER value fuse ....but you only MAY be safe-enough if using a 50% greater amperage one. It is mixed, however, in what may happen, if using a slo-blow fuse where only a standard-blow was specified. Some amplifiers rated for standard (fast) blow fuses will be OK with a slo-blo fuse. NOT ALL. It is nearly always OK to substitute a same value fast blow for a specified slo-blow.....although the fuse may blow irregularly with nothing wrong with the tubes.
You can also USUALLY substitute a regular fast-blow rated fuse for the slow-blow type of fuse, at the same, or SLIGHTLY increased amperage rating.
Future articles being considered:
6. Effects of removing half of the output tubes from a 4 or 6 output tube amplifier.
7. Substituting amongst 12AX7, 12AU7, 12AT7, ETC. AND, what about ECC81, ECC82, ECC83..etc?
8. Setting basic bias yourself.... how-to, pitfalls, etc.
9. 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.