Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 313

'Ep is the change in signal plate voltage,
'Eg is the change in the signal grid voltage.

If the amplifier consists of several stages, the amount
of amplification is multiplied by each stage. The gain of
an amplifier stage varies with the type of tube and the
interstage coupling used. The general equation for
voltage gain is

(12-4)

where,
Vgt is the total gain of the amplifier,
Vg 1 , Vg 2 , and Vgn are the voltage gain of the individual
stages.

Triode tubes are classified by their amplification
factor. A low-μ tube has an amplification factor less
than 10. Medium-μ tubes have an amplification factor
from 10–50, with a plate resistance of 5:–15,000:.
High-μ tubes have an amplification factor of 50–100
with a plate resistance of 50 k:–100 k:.

12.1.6 Polarity

Polarity reversals take place in a tube. The polarity

reversal in electrical degrees between the elements of a

self-biased pentode for a given signal at the control grid

is shown in Fig. 12-3A. The reversals are the same for a

triode. Note that, for an instantaneous positive voltage

at the control grid, the voltage polarity between the grid

and plate is 180° and will remain so for all normal oper-

ating conditions. The control grid and cathode are in

polarity. The plate and screen-grid elements are in

polarity with each other. The cathode is 180° out of

polarity with the plate and screen-grid elements.

The polarity reversal of the instantaneous voltage

and current for each element is shown in Fig. 12-3B.

For an instantaneous positive sine wave at the control

grid, the voltages at the plate and screen grid are nega-

tive, and the currents are positive. The voltage and

current are both positive in the cathode resistor and are

in polarity with the voltage at the control grid. The

reversals are the same in a triode for a given element.

12.1.7 Internal Capacitance

The internal capacitance of a vacuum tube is created by

the close proximity of the internal elements, Fig. 12-4.

Unless otherwise stated by the manufacturer, the

internal capacitance of a glass tube is measured using a

close-fitting metal tube shield around the glass envelope

connected to the cathode terminal. Generally, the capac-

itance is measured with the heater or filament cold and

with no voltage applied to any of the other elements.

In measuring the capacitance, all metal parts, except

the input and output elements, are connected to the

cathode. These metal parts include internal and external

shields, base sleeves, and unused pins. In testing a

midsection tube, elements not common to the section

being measured are connected to ground.

Input capacitance is measured from the control grid

to all other elements, except the plate, which is

connected to ground.

Vgt=Vg 1 Vg 2 }Vgn

Figure 12-3. Polarity characteristics of a vacuum tube.

Figure 12-4. Interelectrode capacitance of a triode.

180° 180° 0°

Esig

Rsg Rp

B+

0°

E 0

+^

Esig

Rsg Rp

B+

E 0

+^

Ip +

Ep

Isg +

Ik +

Esg

Ek

Rk

Rg

A. Polarity reversal of the signal between the

elements of a pentode vacuum tube.

B. Polarity reversal of the current and voltage

in a pentode vacuum tube.

Cg-p

Cg-c

Cp-c

G

P

C