KyteLabs InfoBase - Electron Tubes & Valves Data Last modified: 2000-08-27 (12716)

B
Electron Tubes' History and Basics

B.8
Glossary - Explanation of Tube & Valve Specific Terminology

Absolute Maximum Ratings  -  must not be exceeded under no circumstances. Power supply voltage variations, component tolerances, etc. should be carefully taken into account. Exceeding only one of these maximum ratings can seriously damage the tube.

Continuous Commercial Service (CCS)  -  is defined as that type of service in which long life and reliability of performance under continuous operating conditions are the prime considerations.

Controlled Heater Warm-up Characteristic  -  A controlled heater warm-up time ensures dependable performance in equipment employing series-connected heater strings, like in television receivers. Heater warm-up time is measured in a circuit as follows: The heater is placed in series with a resistance having a value 3 times the nominal heater operating resistance (R = 3 x Uf / If). A voltage having a value 4 times the rated heater voltage is then applied. The warm-up time is the time required for the voltage across the heater to reach 80% of the rated value (U = 0.8 x Uf). The average value has been fixed to 11 seconds in the USA and to 14.5 (11 ... 18) seconds in Europe.

Conversion Transconductance (gc)  -  This characteristic gc (or Sc) is associated with the mixer function of tubes (frequency converter) and is defined as the limiting value of the quotient of the intermediate-frequency (IF) current in the primary of the IF transformer divided by the applied radio-frequency (RF) voltage producing it, as the RF voltage and IF current approach zero. When the performance of a frequency converter is determined, conversion transconductance is used in the same way as control-grid to plate transconductance in single-frequency amplifier computations.

Design Centre Ratings  -  are given for receiving tubes to design equipment where nominal component values can be used, and normal supply voltage variations of ±10% do not affect proper tube-circuit functioning.

Design Maximum Ratings  -  must not be exceeded using a tube with the indicated ratings (bogey tube) under worst case operating conditions. Circuit designs based on these ratings should consider all component tolerances, supply voltage fluctuations, and signal variations which are to occur while operating the circuitry.

Dynamic Plate Resistance  -  The resistance ra of the path between cathode and plate to the flow of alternating current can be derived from the tube characteristics (plate-characteristic family of curves) and is defined as the quotient of a small change in plate voltage divided by the corresponding change in plate current, under the condition that the control-grid voltage remains unchanged (partial derivative):

ra = dUa / dIa while Ug = constant

There is a relationship between transconductance, plate resistance, and amplification factor:
ra = µ / S


Effective Plate-Supply Impedance  -  Ra is effective in rectifier circuits limiting the peak plate current. Topologies with filter-input capacitor mostly need an additional protective series resistor in the plate circuit, by which the effective plate-supply impedance consists of the protective resistor Rs, the DC resistance R2 of the transformer secondary, and the transformed resistance R1 of the primary:
Ra = Rs + R2 + (n2/n1)2 x R1   (view figure 1)

The minimum effective plate-supply impedance Ra is generally specified by the tube manufacturer at particular operating conditions (AC supply voltage, load capacitance), whereas an eventually necessary protective series resistor may be determined in consideration of the inductive components' DC resistance. Generally higher values of capacitance than indicated may be used, but the effective plate-supply impedance may have to be increased to prevent exceeding the maximum rating for peak plate current.
Using a choke-input filter, normally no protective resistor is required, since a reasonably high inductance in series with the filter capacitor prevents large current peaks. In addition the choke is providing significant DC resistance. This results in an effective plate-supply impedance consisting of the choke's DC resistance RsL, the DC resistance R2 of the transformer secondary, and the transformed resistance R1 of the primary:
Ra = RsL + R2 + (n2/n1)2 x R1   (view figure 2)


Efficiency - Plate Efficiency  -  eta (lower-case greek letter) of a power amplifier tube is the ratio of the AC power output Pout to the product of the average DC plate-supply voltage Ub and DC plate current Ia at full signal, or:
eta(in %) = Pout / (Ub x Ia) x 100


Hydrogen Thyratron  -  These tubes are used as drivers for pulsing magnetrons and other oscillators and as high-speed switches. Hydrogen-filled thyratrons have extremely low de-ionization times. They are zero-bias tubes, triggered by a positive grid pulse. Maximum pulse repetition frequency (fp in pulses per second) will depend on the peak forward anode voltage (Ua[pk] in volts) according to formula (Ua[pk])2 x fp = 2.6 x 1011 max.

Intermittent Commercial and Amateur Service (ICAS)  -  is defined as that type of service where minimum size, lightweight and maximum power output are more important than long life. Intermittent operation implies that no 'on' period exceeds 5 minutes and an 'on' period is followed by an 'off' period of the same or longer duration.

Power Output  -  The power output Pout of a tube at proper impedance matching, if necessary by tuning and neutralization of radio-frequency stages, is given as the difference of the plate input Pba and the plate dissipation Pa. The actually available output is reduced by losses in the output transformer or the output resonating circuit.

Resistance-Coupled Voltage Amplifier  -  A tube circuit using only resistors and capacitors to create a voltage amplifier is called resistance-coupled. This circuit design can be used with triodes and pentodes, whereas a particularly high voltage gain can be achieved by using pentodes.
In a resistance-coupled amplifier, the load resistance of the tube is approximately equal to the resistance of the plate resistor RaL in parallel with the grid resistor Rg1' of the following stage. Hence, to obtain a large value of load resistance, it is necessary to use a plate resistor and a grid resistor of large resistance. However, the plate resistor should not be too large because the flow of the plate current through the plate resistor produces a voltage drop which reduces the plate voltage applied to the tube. If the plate resistor is too large, this drop will be too large, the plate voltage on the tube will be too small, and the voltage output of the tube will be too small. Also, the grid resistor of the following stage should not be too large, the actual maximum value being dependent on the particular tube type. This precaution is necessary because all tubes contain minute amounts of gas which cause a minute flow of current through the grid resistor. If the grid resistor is too large an additional bias voltage will be developed that can disturb the proper function of the amplifier stage. A higher value of grid resistance is permissible when cathode-resistor bias is used than when fixed bias is used. With cathode-resistor bias, a loss in bias due to gas or grid-emission effects is almost completely offset by an increase in bias due to the voltage drop across the cathode resistor. Typical values of plate resistor and grid resistor for tube types used in resistance-coupled circuits, and the value of gain obtainable, are shown in chapter  3  - Resistance-Coupled, Audio-Frequency Voltage Amplifiers.

Static Amplification Factor  -  The amplification factor µ can be derived from the tube characteristics (plate-characteristic family of curves) and is defined as the ratio of the change in plate voltage to a change in control-grid voltage in the opposite direction, under the condition that the plate current remains unchanged (partial derivative):
µ = dUa / dUg while Ia = constant

There is a relationship between transconductance, plate resistance, and amplification factor:
µ = S x ra

The static amplification factor represents the best-case voltage gain that could be obtained if the ratio of load resistance to the tube's plate resistance can be increased to any large number. This means virtually a load resistance line running horizontally on the plate-characteristic family of curves, corresponding to an ideal constant-current source. Using a practical load resistance, a voltage gain only less than µ is available that can be calculated as V.

Transconductance or Mutual Conductance (gm)  -  The control-grid to plate transconductance gm (or S) can be derived from the tube characteristics (transfer-characteristic curve) and is defined as the quotient of a small change in plate current divided by the change in the control-grid voltage producing it, under the condition that the plate voltage remains unchanged (partial derivative):
S = dIa / dUg while Ua = constant

There is a relationship between transconductance, plate resistance, and amplification factor:
S = µ / ra

The unit of transconductance for commonly used tubes can be described simply as milliamperes per volt (mA/V). The American literature is using the unit of conductance "mho", named by spelling "ohm" backwards. For convenience, a millionth of a mho, or a micromho (µmho) is used to express transconductance.

Voltage Amplification - Gain  -  V is the ratio of the voltage variation (AC) produced in the load resistance to the input signal voltage and is expressed by the following convenient formula using the amplification factor µ, plate resistance ra, and load resistance RaL:
V = µ x RaL / (ra + RaL)

With pentodes it is better to use the product of transconductance S and the plate resistance ra in parallel with the load resistance RaL:
V = S x (ra x RaL) / (ra + RaL)

From the first formula, it can be seen that the gain actually obtainable from the tube is less than the tube's amplification factor but that the gain approaches the amplification factor when the load resistance is large compared to the tube's plate resistance. View also Resistance-Coupled Voltage Amplifier.

Seitenanfang - Top of PageInhalt - Table of ContentsStart - Top of Document

Copyright © 2000 Mail to Franz Hamberger, Berlin, Germany