Handbook for Sound Engineers

312 Chapter 12

12.1.4 Transconductance

Transconductance (gm) is the change in the value of

plate current expressed in microamperes (μA) divided

by the signal voltage at the control grid of a tube, and is

expressed by conductance. Conductance is the opposite

of resistance, and the name mho (ohm spelled back-

ward) was adopted for this unit of measurement.

Siemens (S) have been adopted as the SI standard for

conductance and are currently replacing mhos in

measurement.

The basic mho or siemen is too large for practical

usage; therefore, the terms micromho (μmho) and

microsiemens (μS) are used. One micromho is equal to

one-millionth of a mho.

The transconductance (gm) of a tube in μmhos may

be found with the equation

(12-1)

where,

'Ip is the change of plate current,

'Esig is the change of control-grid signal voltage,

Ebb is the plate supply voltage and is held constant.

For example, a change of 1 mA of plate current for a

change of 1 V at the control grid is equal to a transcon-

ductance of 1000μmho. A tube having a change of

2 mA plate current for a change of 1 V at the control

grid would have a transconductance of 2000μmho.

(12-2)

where,

gm is the transconductance in micromho or

microsiemens,

Ipac is the ac plate current.

12.1.5 Amplification Factor

Amplification factor (μ) or voltage gain (Vg) is the ratio

of the incremental plate voltage change to the

control-electrode voltage change at a fixed plate current

and constant voltage on all other electrodes. This

normally is the amount the signal at the control grid is

increased in amplitude after passing through the tube.

Tube voltage gain may be computed using the

equation

(12-3)

where,

Vg is the voltage gain,

Table 12-3. Tube Nomenclature

C Coupling capacitor between stages

Cg2 Screen grid bypass capacitor

Ck Cathode bypass capacitor

Ebb Supply voltage

Eff Plate efficiency

Ep Actual voltage at plate

Esg Actual voltage at screen grid

Eo Output voltage

Esig Signal voltage at input

Eg Voltage at control grid

Ef Filament or heater voltage

If Filament or heater current

Ip Plate current

Ik Cathode current

Isg Screen-grid current

Ipa Average plate current

Ipac Average ac plate current

Ika Average cathode current

Isga Average screen grid current

gm Transconductance (mutual conductance)

mu Amplification factor (μ)

Psg Power at screen grid

Pp Power at plate

P-P Plate-to-plate or push–-pull amplifier

Rg Grid resistor

Rk Cathode resistor

Rl Plate-load impedance or resistance

Rp Plate-load resistor

Rsg Screen-dropping resistor

Rd Decoupling resistor

rp Internal plate resistance

Vg Voltage gain

Figure 12-2. Basing diagrams for popular tubes.

Diode Triode Tetrode Pentode or

sheet-beam

Beam

power

Pentagrid

converter

Eye tube Gas-filled

rectifier

Photo tubeHigh-voltage

rectifier

Duo-diode

triode

Dual-triode Two-section Full-wave

rectifier

gm

'Ip

'Esig

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

gm=Ipacu 1000

Vg

'Ep

'Eg

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