Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 315

control grid and the cathode through the external
circuits. This condition is unavoidable because the wires
of the control grid, having a positive charge, attract
electrons passing from the cathode to the plate. It is
important that the control-grid voltage is kept negative,
reducing grid current and distortion.

Grid-current flow in a vacuum tube is generally
thought of as being caused by driving the control grid
into the positive region and causing the flow of grid
current.
The grid voltage, plate-current characteristics are
found through a series of curves supplied by the tube
manufacturer, as shown in Fig. 12-6.
The curves indicate that for a given plate voltage the
plate current and grid bias may be determined. For
example, the manufacturer states that for a plate voltage
of 250 V and a negative grid bias of –8 V, the plate
current will be 9 mA, which is indicated at point A on
the 250 V curve. If it is desired to operate this tube with
a plate voltage of 150 V and still maintain a plate
current of 9 mA, the grid bias will have to be changed to
a –3 V.

12.1.10 Plate Efficiency

The plate efficiency (Eff) is calculated by the equation:

(12-7)

where,

watts is the power output,

Epa is the average plate voltage,

Ipa is the average plate current.

The measurement is made with a load resistance in

the plate circuit equal in value to the plate resistance

stated by the manufacturer.

12.1.11 Power Sensitivity

Power sensitivity is the ratio of the power output to the

square of the input voltage, expressed in mhos or

siemens and is determined by the equation

(12-8)

where,

Po is the power output of the tube in watts,

Esig is the rms signal voltage at the input.

12.1.12 Screen Grid

The screen grid series-dropping resistance is calculated

by referring to the data sheet of the manufacturer and

finding the maximum voltage that may be applied and

the maximum power that may be dissipated by the

screen grid. These limitations are generally shown

graphically as in Fig. 12-7. The value of the resistor

may be calculated using the equation

(12-9)

where,

Rsg is the minimum value for the screen-grid

voltage-dropping resistor in ohms,

Esg is the selected value of screen-grid voltage,

Ebb is the screen-grid supply voltage,

Psg is the screen-grid input in watts corresponding to the

selected value of Esg.

12.1.13 Plate Dissipation

Plate dissipation is the maximum power that can be

dissipated by the plate element before damage and is

found with the equation

(12-10)

Figure 12-6. Grid voltage, plate-current curves for a triode
tube.

15

14

13

12

11

10

9 8 7 6 5 4 3 2 1 0

Grid voltage (Eg)—V

Plate current (

Ip

)—mA

18 

16 

14 

12 

10 

8 

6 

4 2 0

300 V

250 V

150 V

100 V

A

200 V

Eff watts

EpaIpa

=----------------u 100

Power sensitivity

Po

Ein

--------- 2 -=

Rsg

Esgu Ebb–Esg

Psg

=------------------------------------------

Watts dissipation=EpIp