gumbi - I was thinking maybe we could create a thread similar to GBnC - The Ask Your Own Questions That Don't Deserve Their Own Thread, Thread. (maybe that would keep it more active)
Hello interested party! (by gumbilicious)
I will take a brief moment to try and explain why this is here before plunging head first into the mundane complexities of guitar amplifiers. I have been playing guitar since '96, my intense interest in the subject matter of guitars over a number of years has actually netted me quite a bit of knowledge about amplifiers. recently i have been asked to share some of this knowledge to provide content for an 'honest-debate' thread.
i plan on splitting the information into segments, starting with fundamental concepts of electronics. i also plan on making the content as accessible to the 'guitar-noob' as much as it is to the 'guitar-collecter'
subjects: -introduction to amplifiers -history of amplifiers -explanation of voltage, current, and impedance -explanation of tubes -explanation of transistors -other amplifier components (diodes, pots, speakers, transformers, etc) -speakers -cabinets
Introduction to Amplifiers (by gumbilicious)
an amplifier is an electrical circuit that takes an alternating current (AC) as input signal and usually increases the amplitude of the AC signal. amplifiers are used for MANY applications, but the type of amplifier we will be focusing on is a guitar amplifier. this is important to note mainly because
-there are many principles of amplifiers that are important for an electrical engineering student to understand but will be beyond what the average guitarist will need to know.
-narrowing the application of the amplifier will allow the talk to be more specific.
a guitar amplifier has to be able to accept a very weak passive AC signal that is produced by a guitar pickup and needs to be able amplify that signal many times to an AC signal powerful enough to drive a loudspeaker cabinet. a guitar amplifier does this by dividing the amplification process into 2 main sections: the preamp and the power amp (aka power section).
Harmonics (by mmolteratx and CECamps)
Harmonic overtones are extremely easy to hear. Why do you think instruments sound different? It's all about the harmonic series. That and their attack. Synthesizers work by imitating both of those.
Even order harmonics are octaves of the fundamental note. Odd order harmonics are notes out of tune with the fundamental note. Both are present in tube amplification with those created in the preamp primarily being even order and those created in push-pull output sections being almost completely odd order. Single ended output sections produce a more balanced harmonic series.
Harmonic overtones are notes (overtones) that become audible in the guitar tone which are not the fundamental note. For instance, the open A string on guitar is A4 with a fundamental frequency of 440Hz. However, when played, what you hear is not only the fundamental frequency. That would make for a pretty boring and sterile tone.
Appearing with that fundamental note are harmonic overtones like the octave for instance, which is A5 at 880Hz. The presence of these harmonic overtones makes the sound seem richer to the human ear without it being blatantly obvious that there are other notes intermingled with the fundamental.
Now, the guitar already produces harmonic overtones before it even hits the amp. When you strike a note, you don't just hear the fundamental in other words. Some guitars produce more than others based on the design.
An amplifier produces even more harmonic overtones on top of those found in the raw guitar signal by distorting the electronic waveform produced by the guitar or changes the shape of the waveform even if it doesn't clip it. This is called harmonic distortion. As the guitar signal travels through an amp, the way it is changed and manipulated produces harmonic overtones.
Some amps add more harmonic content to the guitar tone than others. It is dependent on so many factors, but it is quite noticeable to a person who knows what to listen for. This is the very reason why some amps are coveted and some are forgettable.
Clean tones have more even order harmonic content which creates more of a warm and lush sound with very little perceptible dissonance. Distortion tones are heavy on odd order harmonics, which have a discordant/dissonant character. The mix of odd and even order harmonics is the foundation of any guitar tone.
-the preamplifier section specializes in amplifying low-level passive signals that are generated by pickups or microphones.
-preamps are usually run in a 'class a' operation with 'dual triode' tubes like 12AX7's or 12AT7's or transistors performing the amplification.
-the signal is actually sent through a series of amplifying stages to amplify the signal in small sequential steps, this allows for particular tubes/transistors to perform particular roles in processing the input signal (for example, the second preamp tube is often used as a 'driver' tube as well as amplification).
[QUOTE=gumbilicious]as far as i know 12ax7's have a gain factor of ~100. i have heard them shipping within tolerance of ~95 to ~107 (you can buy hotter JJ's cuz eurotubes tests them and separates higher and lower values for special purchase, as i am sure more companies who perform a QA inspection do as well). a 12ax7 is also the highest gain dual triode i know of (of course i don't know them all, but i know quite a few), so i don't know what 'higher gain tubes' would be unless its a hot 12ax7 and i wouldn't worry about running a bunch of 107 gain 12ax7's in my preamp.
as far as i know, you can replace higher gain tubes with lower gain tubes in amplifiers, but it is not advisable to run them vise versa. so if your preamp has 2x12ax7's and 1x12at7 then you wouldn't just wanna replace them all with 12ax7's. from what i have heard the problem is not 'too much gain man', it's more to do with current draw. i have heard 12ax7's don't last as long with the current draw a 12at7 requires and has a shorter life because of it, evidently i have heard you can change a resistor value on one of the pins in order to keep the 12ax7 from getting burned out.
as far as i know, a power tube's designation implies it meets a particular spec (socket design, output pins, nominal specs) and as long as these tube meet specs of what you run, then they can be used. like, if you run a class A/B pushpull amp with a sylvania 6CA7 (it's an american EL34) and a mullard EL34, then as long as they are a 'matched set' (their outputs are close enough so they can be biased in push/pull) then you are good. in my class A, singled ended THD i son't worry about tolerances at all, i can run any 2 EL34-style power tubes together regardless on how the 'match' as a set (i can actually run any 2 type of tube together with the bivalve, i run a sylvania 6L6GC and a mullard EL34 now).
as far as i know, big amp makers tend to ship with one brand of vacuum tube (with the trend going toward 'rebranded' 3rd party tubes with the amp maker's company's logo on the tube). the reason for doing this is consistency, they want to know their product will be stocked when needed and going through 1 supplier is easier and more efficient for a number of reasons (supply chain, ordering, availability, cost for bulk buying).
thats just what i heard though[/QUOTE]
-the 'volume' or 'gain' knob is most often wired in after the first gain stage of an amplifier (this is why it sounds different when running you amp at low volume with the volume knob on the guitar all the way up; as compared to turning up the amp and turning the volume knob on your guitar down).
-preamps also have to prepare the input signal for the power section, so at times a preamp tube must be used as a phase inverter (PI) for class A/B push/pull amps.
an important section in the preamp is known as the 'Tone Stack'; you may recognize the interface for the tone stack as the EQ knobs (bass, mids, treble). the job of the tone stack is easy: to rid the signal of unwanted frequencies while being able to adjust frequency response in a pertinent ranges. the reason a tone stack is desirable or needed is quite a bit more complicated, but it has much to do with undesirable frequencies/noise being amplified with the input signal and how your ear/brain hears sounds differently at low volumes and high volumes. the actual implementation of a tone stack consists of a series of RC (resistor/capacitor) and CR circuits and potentiometers.
************************************************** ******* POWER AMP -the roll of a power amp is to take a line level signal (usually provided by a preamplifier) and to amplify that signal to a level that can be sent to a loudspeaker for conversion to an audio signal
-power amplifiers in guitar amps can come in many different modes of operation, but the most common is some type of class A/B push/pull configuration. when an amp is called 'class a' or 'class A/B', then this is the part of the circuit they are referring to.
-depending on the mode of operation, the power section may amplify the signal many different ways; but power sections will generally provide amplification in 'one stage' as opposed to the multiple stages you see in preamps. in tubes amps, much more powerful beam pentodes and kinkless tetrodes are commonly used for power amplification.
-the 'master volume' is normally wired into the circuit after the preamp and before the power amp. this is why you can't get power tube saturation with the master down.
after the power section the signal is usually sent to a transformer to convert the final signal from the amplification into something appropriate to drive a loudspeaker. most guitar amplifiers have certain impedance requirements from the speaker attached to the amplifier (aka: The Load).
-tube amplifiers usually require a 'matching impedance', meaning the outputs of the amplifier are label and a speaker cabinet with corresponding impedance must be matched to the output.
-solid state (SS) amplifiers use impedance bridging instead of impedance matching, so usually they require a minimum impedance from a speaker cabinet, so any speaker cabinet above the minimum setting may be used. solid state amplifier output is usually dependent upon the impedance of the cabinet (example: 100 watts @ 16 ohm, 200 watts @ 8 ohm).
sometimes speakers are included with a guitar amplifier (a combo) and sometimes the speaker and head components are separated (piggy back or stack). the benefits and merits of these setups will be covered in the 'speakers and cabinets' section.
Explanation of voltage, current, and impedance (by gumbilicious)
I am gonna take the easy way out on this one, simply because there is already really good content about this concept all over the web. i thought wikipedia did a pretty good job.
The important thing to keep in mind is how a fundamental idea like 'resistance in a DC circuit' can turn quite complicated just by taking the same principle in AC circuits (impedance has a bit more baggage). a guitar amplifier uses both direct current (to run plate voltages in tubes for one) and alternating current (in the signal path).
It is important to be able to understand basic mathematics concerning calculating values in a circuit. you should be able to
-Calculate values using V=IR (introducing inductance is optional if you're up to it) and total resistance run in parallel or series
-Know what a capacitor, resistor, diode, potentiometer, and transistor/triode is
-Understand the hydraulic analogy and fundamentals of both direct current flow and alternating current flow.
100dB sensitivity: 1 speaker rated at 100dB driven at 1 watt = 100dB volume at 1 meter 1 speaker rated at 100dB driven at 10 watts = 110dB volume at 1 meter 1 speaker rated at 100dB driven at 100 watts = 120dB volume at 1 meter
4 speakers rated at 100dB driven at 1 watt = 106dB volume at 1 meter 4 speakers rated at 100dB driven at 10 watts = 116dB volume at 1 meter 4 speakers rated at 100dB driven at 100 watts = 126dB volume at 1 meter
95dB sensitivity: 1 speaker rated at 95dB driven at 1 watt = 95dB volume at 1 meter 1 speaker rated at 95dB driven at 10 watts = 105dB volume at 1 meter 1 speaker rated at 95dB driven at 100 watts = 115dB volume at 1 meter
4 speakers rated at 95dB driven at 1 watt = 101dB volume at 1 meter 4 speakers rated at 95dB driven at 10 watts = 111dB volume at 1 meter 4 speakers rated at 95dB driven at 100 watts = 121dB volume at 1 meter
So you can see that ONE 100dB speaker driven at 10 watts is just as loud as FOUR 95dB speakers driven at 10 watts.
wow, holy crap. i had a list started... it was like 5 threads... how long you spend on this? yeah, sounds good to me. been a little busy recently but answering guitar related questions is how i relax evidently.