Handbook for Sound Engineers

316 Chapter 12

where,
Ep is the voltage at the plate,
Ip is the plate current.

12.1.14 Changing Parameters

If a tube is to operate at a different plate voltage than
published, the new values of bias, screen voltage, and
plate resistance can be calculated by the use of conver-
sion factors F 1 , F 2 , F 3 , F 4 , and F 5. Assume the
following conditions are specified for a single
beam-power tube:
Plate voltage 250.0 V
Screen voltage 250.0 V
Grid voltage –12.5 V
Plate current 45.0 mA
Screen current 4.5 mA
Plate resistance 52,000.0:
Plate load 5000.0:
Transconductance 4100.0μS
Power output 4.5 W

F 1 is used to find the new plate voltage

(12-11)

For example, the new plate voltage is to be 180 V.
The conversion factor F 1 for this voltage is obtained by
dividing the new plate voltage by the published plate
voltage Eq. 12-11:

The screen and grid voltage will be proportional to the

plate voltage:

(12-12)

(12-13)

In the example,

F 2 is used to calculate the plate and screen currents

(12-14)

(12-15)

(12-16)

In the example,

The plate load and plate resistance may be calculated

by use of factor F 3 :

(12-17)

(12-18)

(12-19)

In the example,

Figure 12-7. Typical graph for determining the maximum
power dissipated by the screen grid.

100

80

60

40

20

0

0 20 40 60 80 100

Maximum operating conditions

Area of permissible operation

Grid-No. 2 input expressed as percent

of max. grid-No. 2 input rating

Grid-No. 2 voltage expressed as percent

of max. grid-No. 2 supply voltage rating

F 1

Epnew

Epold

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

F 1 180

250

---------=

=0.72

Eg=F 1 uold grid voltage

Esg= F 1 uold screen voltage

Eg=0.72u 12.5

–= 9 V

Esg= 0.72u 250

= 180 V.

F 2 =F 1 F 1

IP=F 2 uold plate current

Is=F 2 uold screen current

F 2 =0.72u0.848

=0.61

IP=0.61u45 mA

=27.4 mA

Isg=0.61u4.5 mA

=2.74 mA

F 3

F 1

F 2

----- -=

rp=F 3 uold internal plate resistance

RL=F 3 uold plateload resistance