Vacuum Tubes - Glossary
A.C. – Abbreviation for “alternating current”. This is electricity that goes one direction and then
reverses and goes the other direction. Your guitar signal is A.C. and so are the 120-volt wall outlets you
plug your amp into.
ATTACK & DECAY ENVELOPE – The rise and fall of volume levels that occur inherently in an amplifier
when a note is played. This shapes the overall sound and is what gives the “breathing effect” of certain amps.
BIAS – To adjust the operating level of a vacuum tube, much the same as you would adjust the idle on your car;
The negative voltage relationship comparing the grid to the cathode.
CATHODE – This is the part of the tube or semiconductor diode where the electrical current flow enters.
CATHODE-BIASED – One of several ways a tube could be biased. This design uses a resistor between the cathode
and ground to set the biasing. This cathode-biased design is almost always used on preamp tubes and sometimes used
on output tubes. Though not as efficient as fixed-bias, cathode-biased output tubes generally “sing” more.
CATHODE RESISTOR – This is the resistor that goes from the cathode to ground in a cathode-biased circuit.
CAPACITOR – A device made from two conductors separated by a non-conductor. Whatever is between these
conductors is called a dielectric. These devices store electricity.
CUT OFF – A condition in which no current flows through a vacuum tube. This is achieved by having the grid
potential so negative (with respect to the cathode) that current “cuts off”.
D.C. – Abbreviation for “direct current”. This is electricity that goes one direction only. The tubes in an
amplifier use “direct current”. All batteries are D.C.
DIELECTRIC – The non-conductor substance that separates the two conductors of a capacitor. Paper, air, electrolyte,
mica, Mylar, polyester & ceramic are some of the dielectrics that are used.
DISTORTION – The difference between what goes into an electronic device & what comes out.
DYNAMICS – The loud/soft quality of music that gives it character.
ENVELOPE – The changing dynamics of a vacuum tube circuit that can be heard as an attack, decay & sustain volume
level when a note or a chord is played through the amp.
E.Q. – Abbreviation for equalizer”.
FIXED-BIAS – A way to achieve bias in a vacuum tube in which a fixed amount of negative voltage is placed on the grid of a tube.
This is almost done on output tubes and almost never done on preamp tubes.
GAIN – The amount of voltage amplification in the preamp section of an amplifier. This voltage amplification ultimately drives
the power tubes, which do not add any more gain. The power tubes add current (power).
GRID – One of the internal parts of a vacuum tube. This is usually where the input signal connects.
HARMONICS – The frequencies that are related to the fundamental frequency by being multiples of the fundamental frequency.
HEADROOM – The volume level that is attainable before clipping occurs.
IMPEDANCE – Sum of all resistance, capacitive reactance and inductive reactance.
INTERLEAVED – A way of winding a transformer in which a little bit of the primary is wound, then a little of the secondary is wound,
then a little primary, then a little secondary, etc. until the transformer is wound. The more interleaves, the better the efficiency.
K – Abbreviation for one thousand.
MEG – One million ohms or 1000K.
MICROFARAD – One millionth of a farad (.000001 farad). The abbreviation is “uF”.
MICROPHONIC – The tendency for a vacuum tube to pick up sound like a microphone.
NEGATIVE FEEDBACK – A small portion of signal that is taken from the output of an amplification stage and then reinserted to a
previous stage at a point that is 180 degrees out of phase. This would phase cancel some of the louder frequencies and there would be
little effect on the not-so-loud frequencies, thus the frequency response would become more even. Leaving a cathode resistor unbypassed
can achieve the same effect.
NEGATIVE FEEDBACK LOOP – A circuit in which a small portion of the amplified signal is fed back to an earlier part of the circuit
in which it is 180 degrees out of phase. This has the tendency of flattering out the frequency response.
OHM – Unit of measure of resistance.
OUT OF PHASE – If you consider that a vibration such as guitar note changes from positive to negative in its vibrations, then a
signal is said to be 180 degrees out of phase if it is going positive at the same time that the original signal is going negative and of
course it will be going negative when the original is going positive. This is also referred to as an inverted signal.
PENTODE – A vacuum tube that has five electrical components. They are: cathode, grid, screen grid, suppresser grid and plate.
The heater doesn’t count.
pF – The abbreviation for picofarads.
PHASE INVERTER – The circuit in a push-pull amplifier that feeds the power tubes signal. It feeds one power tube an in-phase
signal and the other power tube an out-of-phase signal. The tube for this circuit is always located next to the power tubes.
PI FILTER – A filter, called the pi filter because of its resemblance to the Greek letter Pi, is a combination of the simple
capacitor input filter and the choke input filter.
PICOFARAD – A very small amount of capacitance that is equal to a trillionth of a farad (.000,000,000,001). Sometimes also
PLATE – The part of the vacuum tube that has high voltage on it. Except in a cathode follower circuit, the output is always
taken from this part of the tube.
PLATE VOLTAGE – The voltage that is applied to the plate of a vacuum tube. Higher voltages give more headroom and high end;
lower voltages give more breakup and a “browner” tone. All tweed amps had relatively lower plate voltages.
POTS – Slang for potentiometer, which is a variable voltage divider used in volume controls, tone controls, etc.
PREAMP – The section of an amplifier whose function is to add gain. Preamps are always before the output stage.
PRESENCE – Another type of high end, similar to treble that gives a biting edge to the sound. On Fender & Marshall amps and
many others, the presence control is actually a tone control on the negative feedback loop.
RECTIFIER – A vacuum tube with no grid, whose purpose is to change alternating current to direct current. Current flows from
the cathode to whichever of the two plates that happen to be positive at the time.
RESISTANCE – That which impedes the flow of electrons is said to have resistance. Resistance accounts for the fact that different
conductors will allow more or less current to pass, given the same voltage present.
RESISTOR – A device used to add resistance to a circuit. This device is always used to either create a voltage drop or to limit current.
REVERB – A sound in which sound waves are reverberated. Passing sound through two or more long springs of different lengths and
amplifying the resultant sound does this. The springs are in a small enclosure that is called a “reverb tank”.
RING MODULATOR – An electronic device that generates the sum and difference of two or more frequencies and in doing so creates
non-harmonically related sounds.
SAG – The amount of decay in the envelope before a played note is sustained at a constant volume level. It’s caused by the resistance
of the power supply and that resistance would include the rectifier tube resistance as well as the internal resistance of the power transformer.
SATURATION – A condition in which maximum current is reached and no more current can possibly flow.
SCREEN RESISTOR – The resistor that is placed in series with the screen of a vacuum tube. It is used to limit screen current.
SELF-BIAS – A type of biasing arrangement in which a resistor is used to create a positive voltage on the cathode; this now
makes the grid negative with respect to the cathode.
SOCKET – The female conductor that a vacuum tube inserts into.
TRANSFORMER – An electrical device made from two or more windings of wire wrapped around an iron core. These devices are primarily
used to create power supplies and to match impedance’s between output tubes and speaker.
uF – The abbreviation for microfarads.
TRIODE – A vacuum tube with three internal components not counting the heater. The three components are cathode, grid and plate.
VACUUM TUBE – A device whose major components would include an anode and a cathode in an evacuated envelope. It performs as an amplifier,
oscillator or rectifier in audio circuits.
VIBRATO – The effect of pitch varying slightly higher and slightly lower than the fundamental signal.