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502
RADIO BROADCAST
OCTOBER, 1926
FIG. 4
Plate-voltage plate-current curves, or, as the tube technician would say
"Ep-Ip" curves. From the data plotted here and in Fig. 3, all of the
important tube constant may be calculated
RADIO BROADCAST Photograph SOME DELICATE WESTON METERS
These are standard meters and naturally are more expensive than those used by the average radio fan. These meters, although of the d. c. type, may be used for a. c. by using them in conjunction with thermo-couples, two of which are shown in front of the meters
MUTUAL CONDUCTANCE CALCULATION
THE mutual conductance of a tube is an expression for the effectiveness of the grid voltage in controlling the plate current. It may be obtained from the Eg-lp curves since this factor is defined as:
Cm =
plate current change grid voltage change
and since, in the case above, we found that ten milliamperes change resulted from shifting the grid voltage ten volts, we have:
Gin = .010 = .001 mho, or 1000 micromhos
The following table will aid in keeping the units straight for the above series of calculations:
.1 milliampere = .0001 ampere
i.o = .001
10.0 milliamperes = .01
100.0 = .1
1000.0 = i.
Now let us assemble our data and see if we cannot work out a system by which we may arrive at the important factors quickly. Our procedure in which any other values of plate and grid voltages may be used may be as follows:
1 . Set plate Collage at 90.
2. Read plate current at Eg = positive 3.
3. Set Eg = negative 3 and read plate current.
4. Divide plate current change by grid voltage change (here it is 6 volts). This gives the mutual conductance at an average of zero grid voltage.
5. Bring plate current back to what we read at positive 3 by increasing the plate voltage.
6. Divide the plate voltage change by the plate current change. This is the plate impedance.
7. Divide the plate voltage change by the grid voltage change (6 again). This is the amplification factor.
There is an important relation between the amplification factor and the plate impedance. From our expressions given above but here set down in our short hand language,
,. P
Kp = =— ; On
IP
EC
divide the amplification factor by the plate impedance:
rP
Ep . Ep
Ei ~ rf
Ep Ip _ IP
~"
whence we see that the mutual conductance is the ratio between the amplification factor and the plate impedance. Within the limits of error in measuring the respective currents and voltage — and providing one only uses values on the
straight parts of curves — the value obtained by this division will check that obtained from the curves.
There are several important points to be considered in measuring tube constants. It must be remembered that the factors vary at each point on the various curves. For each value of grid voltage and plate voltage, there is a value of plate impedance, and amplification constant, although the latter does not change
60 80
PLATE VOLTS
100
120
140
we may operate by simple algebra. Let us
FIG. 5
This diagram of an " Ep-lp" curve shows the difference between the a. c. "impedance" and thed c.
"resistance." The d. c. resistance is merely the plate voltage divided by the plate current, and may
be represented by the reciprocal of the line joining the origin and the point under consideration.
The a. c. impedance is the reciprocal of the slope o! the curve