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.

Class  -  The operating conditions of amplifiers are classified by letters A, B, C, and D. More specific and derivated operating classes are A1, A2, AB, AB1, AB2, B1, and B2. Subscript 1 indicates that grid-No.1 current does not flow during any part of the input cycle, while subscript 2 indicates that grid-No.1 current flows during some part of input cycle.

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):

There is a relationship between transconductance, plate resistance, and amplification factor:

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:

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:

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:

Heater  -  To obtain a significant electron emission, it is necessary to heat up the cathode to an elevated temperature. Usually the heater is a thin wire or filament that is brought to incandescence by an applied heating current. If the filament itself is the electron emitting medium, we call this a directly-heated or filamentary cathode.
It is not essential that the heating current flows through the emitting material. The heater can be electrically insulated from the cathode. This type of heater-cathode construction is called indirectly heated and has the advantage, that the heater circuit is separated from the cathode circuit. This fact allows a more flexible circuit design. Due to the thermal inertia of an indirectly-heated cathode, the heater may be supplied by alternating current (AC) without introducing noise or hum to the cathode circuit. However, the magnetic field induced by the AC-operated heater might introduce unwanted noise to the control-grid circuit. To decrease the magnetic influence, a bifilar-wound heater wire is used.
Filamentary cathodes of receiving tubes are preferably heated by direct current (DC), while power output tubes with thick filaments and low heater voltages can be heated by AC. To minimize hum, the heating transformer should have a center tap that can be grounded.

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 to include the many applications where the transmitter design factors of minimum size, light weight and considerably increased power output are more important than long tube life. In this service, life expectancy may be one-half that obtained in Continuous Commercial Service.
Under the ICAS classification are such applications as the use of tubes in amateur transmitters, and the use of tubes in equipment where transmissions are of intermittent nature. Intermittent operation implies that no operating or 'on' period exceeds 5 minutes and every 'on' period is followed by an 'off' or standby period of at least the same or longer duration.

Intermittent Mobile Service (IMS)  -  is defined to include those applications, such as aircraft, where the transmitter design factors of minimum size, light weight and exceedingly high power output for short intervals are the primary requirements even though the average life expectancy of tubes used in such transmitters is reduced. Tube ratings for IMS service are established on the basis that the transmissions have maximum 'on' periods of 15 seconds followed by 'off' periods of at least 60 seconds, except that it is permissible to make equipment tests with maximum 'on' periods of 5 minutes followed by 'off' periods of at least 5 minutes provided the total 'on' time of such periods does not exceed 10 hours during the life of any tube.
Although the use of tubes under IMS ratings involve great reduction in tube life, such use can be justified as economical practice in applications where high power is intermittently desired from small tubes.

Maximum usable operating frequency of a wideband amplifier  -  is defined as the quotient of mutual conductance and total capacitance S / (2 x CT) of the operating-biased tube circuit. This value represents a figure of merit of the amplifier. The total capacitance is the sum of input capacitance at operating bias, Cin', output capacitance Cout, and circuit or stray capacitances Co (normally supposed at 5 pF), i.e. CT = Cin' + Cout + Co.

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.

Single-Tone Operation  -  refers to that class of amplifier service in which the grid-No.1 input consists of a monofrequency RF signal having constant amplitude. This signal is produced in a single-sideband suppressed-carrier system when a single audio frequency of constant amplitude is applied to the input of the system.

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):

There is a relationship between transconductance, plate resistance, and amplification factor:

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):

There is a relationship between transconductance, plate resistance, and amplification factor:

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:

With pentodes it is better to use the product of transconductance S and the plate resistance ra in parallel with the load resistance 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.

Copyright © 2016Franz Hamberger, Berlin, Germany

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