technical stuff
that guitarists is good to know about
why vacuum tube?
Tube versus transistor sound has long being debated among engineers and musicians. Linear operation of an active electronic component is mostly not our case in electric guitar amplifiers because we usualy drive them hard into break up up to heavy overdrive. Under this condition there is a major difference in the harmonic distortion of tube and transistor circuits.
​The harmonic content of an overdriven tube amplifier consists primarily of 2nd order and 3rd order harmonics with some 4th order harmonics. The harmonic content of an overdriven transistor amplifier is primarily 3rd order with suppressed 2nd order harmonics. 2nd and 3rd order harmonics are the most important from a viewpoint of electronic distortion. Musically the 2nd harmonic is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller. The 3rd harmonic is a musical 12th. Instead of making the tone fuller, a strong 3rd harmonic makes the tone softer. The odd harmonics (3rd, 5th, etc.) produce a "stopped" or "covered" sound. The even harmonics (2nd, 4th, etc.) produce a "choral" or "singing" sound. Adding a 5th to a strong 3rd harmonic give the sound a metallic quality that gets annoying in character as the amplitude increases. A strong 2nd with a strong 3rd harmonic tends to open the "covered" effect. Adding the 4th and 5th harmonics to this gives an "open horn" character. The higher harmonics, above the 7th, give the tone "edge" or "bite."
The basic cause of the difference in tube and transistor sound is the weighting of harmonic distortion in the amplifier’s overload region. Transistor amplifiers exhibit a strong component of 3rd harmonic distortion when driven into overload. This harmonic gives a "covered" sound with a restricted quality. A tube amplifier when overdriven generates a whole spectrum of harmonics. Particularly strong are 2nd, 3rd, 4th, and 5th overtones that give the sound a full-bodied "brassy" quality. Combining this reinforcing harmonic content with the compression and high-frequency roll-off of the output transformer in a tube amplifier is why your tube amplifier will "give it up" while your friend’s transistor amplifier will sound restricted and harsh.
transformers
There are also significant differences in the construction of tube and transistor amplifiers that contribute to the sound. Tube amplifiers require output transformers to match the high impedance of the power tubes to the low impedance of the loudspeaker. Speaker's impedance is reflected to power tubes through the output transformer and a tube amplifier should not be left working without a speaker connected
Power transformer is the one that delivers the required amount of voltage, in order to bring amplifiers in working condition.
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what is bias?
Bias is a procedure of setting the voltage to the control grids of the power tubes so the tubes draw the correct amount of operating current.
On amplifiers that have adjustable bias, it is a potentiometer that is provided for the technician to set the bias voltage to the control grids while monitoring the plate or cathode current of the output tubes.
If the operating current is set too low, the amplifier will sound weak and have a gritty non-musical sound caused by crossover distortion.
If the operating current is set too high, the tube will draw excessive current, which will shorten the life of the tubes. Setting the bias correctly will give the best sound and longest tube life.
Due to the dangerous high voltages encountered in tube amplifiers in addition to the knowledge and equipment required to set the proper amount of operating current, this adjustment is best left to be done by an experienced technician.
Some amplifiers provide a bias voltage to the grids of the power tubes, but do not have a way to adjust the voltage so it is best to use matched tubes.
Cathode-biased amplifiers use a resistor and bypass capacitor in the cathode circuit of the output tubes to provide a positive voltage to the cathode. This sets the grid voltage negative in relation to the cathode and establishes the proper operating current. There is no bias adjustment on these amplifiers.
Preamp tubes are cathode biased, so there is no need to check or set bias on any preamp tube.
when should tubes be changed?
Pay attention to your amplifier’s performance. When you hear the sound begin to deteriorate, it may be time to change the tubes. You may notice that chords sound muddy, the amplifier is losing high or low frequency response, there is poor balance in the level of various notes, or the amplifier lacks punch and sounds weak. If the amplifier is making funny noises such as ringing, popping, or ghost notes or power is fading up and down, these are all indications that it is most likely time to replace the tubes.
