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A Dynamic Mutual Conductance Valve
Analyser

G
A French. The Radio Constructor, June,
1956.

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extras

As an alternative to a professional valve tester this
project aims at accuracy through a bridge circuit. G A French
provided a 'Suggested Circuit' in every Radio Constructor I
have seen.

Although too much reliance should not always be placed on
such instruments, it cannot be denied that valve testers and
analysers are extremely useful items of equipment to have,
both in the amateur and in the professional workshop.
Unfortunately, commercially made valve testers tend to be
rather expensive, the more elaborate models, in particular,
incurring quite heavy outlays. Cheaper valve testers
frequently check emission only, whereupon the information they
provide concerning the valve being tested is not of great
practical value.

The process of constructing a valve analyser, instead of
purchasing a commercially made instrument, is quite a feasible
proposition if carried out with a reasonable amount of care. A
simplification is provided by the fact that a multiplicity of
different valve-holders is not nowadays as necessary as it was
before the war. Similarly, the provision of a large number of
filament or heater voltages does not raise as many
difficulties as have been apparent in the past. This is due to
the fact that most modern valves which do not employ 6.3 Volt
heaters or 1.4 Volt filaments are intended for connection in a
series heater string. Such valves may then be accommodated by
feeding them, from a suitably isolated AC supply of some 200
Volts or so, via series dropper resistors; the latter being
switched to feed 0.15, 0.2 or 0.3 amp heaters as desired

The more difficult problems, so far as home constructed
valve analysers are concerned, consist of working out a
suitable type of test and of calibrating the instrument after
it has been completed.

Mutual Conductance Test

Probably the most realistic check of the 'goodness' of a
valve is given by measuring its mutual conductance. The mutual
conductance, or gm of a valve, defines its change in anode
current given by a change in grid voltage, the potentials of
all other electrodes, other than the grid, remaining constant.
In this country mutual conductance is expressed in terms of
mA per Volt. In American literature mutual conductance
is expressed in micromhos, the mho being a unit of
conductance. (A mho is the reciprocal of an Ohm: a circuit
having a resistance of 5 Ohms would have a conductance of 0.2
mho). A gm of 1,000 micromhos is equivalent to one of 1mA per
volt.

This months circuit illustrates a very simple means of
measuring the mutual conductance of a valve under dynamic (ie
non-static) conditions, and has the advantage of requiring no
complicated or expensive gear for calibration. There are the
incidental factors that the instrument can also provide checks
for microphony and emission.

Principle of Operation

The analyser functions by applying an AC voltage to the
grid of the valve under test and of converting the resultant
AC current at the anode to a voltage; the voltages at grid and
anode being then applied to a bridge in order to determine
their ratio. The conversion of anode current to voltage is
carried out by feeding the anode through a resistor, the value
of this being kept very low.

To understand the functioning of the device more clearly,
it might be worth while to commence by examining the
simplified circuit shown below.

In this diagram the series anode resistor has a value of
1,000 Ohms; with the result that a change in anode current of
1 mA causes a change of 1 Volt to appear across this
resistor. In consequence of this fact, if the valve under test
has a gm of X mA per volt, we can assume for the moment that a
change in grid voltage of 1 Volt will cause a change in
anode current of X mA, together with a change of X Volts
across the anode series resistor. These voltages are depicted
in the diagram. Since the grid T and anode voltages are 1800
out of phase it then becomes a very simple matter to apply
them to a bridge potentiometer, as is done in the diagram,
whereupon the arm of the latter can be adjusted for minimum
output or null. The mutual conductance, in mA per volt will
then be equal to the ratio, R9 / R10 It
may now be seen, incidentally, that there is no real necessity
to apply an input AC having a precise voltage to the grid of
the valve, since the ratio R9 / R10 will
hold good for any reasonable input voltage. If Y volts AC were
applied to the grid, XY volts would appear across the anode
resistor. In practice, however, it would be preferable to use
as low an input voltage as could be detected by the indicating
device coupled to the bridge, since this would prevent the
possibility of swinging the grid over any non-linear part of
the IaVg characteristic of the valve under test.

There is one disadvantage with the circuit above, this
being the fact that the 1,000 Ohm series anode resistor has
too high a value. If the circuit were used for testing a power
valve with a high gm, the AC voltage appearing across the
anode resistor would be excessive and would not allow the
requirement that gm be measured with constant anode potential
to be met.

This trouble can be obviated by making the series anode
resistor small in value, say 25 Ohms. The AC voltage appearing
across a resistor of this value would then be one-fortieth
part of the voltage appearing across the original 1,000 Ohm
resistor. If we next applied to the lower arm of the
potentiometer R9, R10 one-fortieth part of the input AC
applied to the grid of the valve under test, we would once
more be able to balance the bridge, and the expression
R9 / R10 would once again define the gm
of the valve in mA per Volt. This is what has been done in the
circuit which is discussed this month, and it is assumed that
the error introduced by the presence of the 25 Ohm anode
resistor is negligible for all valves normally encountered in
radio and television receivers.

The Analyser

The full circuit

The full circuit of the valve analyser is given above. In
this diagram the AC voltage is obtained from an audio
oscillator or similar device which functions at a frequency
around 4 kHz. (The reason for this rather high frequency
will be explained shortly). The 4 kHz tone is applied to
the volume control R1 and, thence, to the cathode
follower V1. V1 may be any low-mu triode. The cathode load of
V1 is 1,000 Ohms, and one-fortieth part of the voltage built
up across this load is applied, via C6, to the
lower end of the potentiometer R9, R10.
(R9 R10 is a single component, of
course, two circuit references being quoted in order to
simplify explanation). The total output of the cathode
follower is applied to the grid of the valve under test, V2,
via C3. The anode of V2 is connected to the HT supply via
R6, this being the 25 Ohm series anode resistor
already discussed. The upper end of R6 is decoupled
to chassis by C4. C4 is an important
component in the bridge circuit and must have a low reactance.
At 4 kHz a 50 μF capacitor (the value specified for
C4) has a reactance of 0.8 Ohm, and this should be
sufficiently low for our purposes here. The necessity for
ensuring that C4 has a low reactance is,
incidentally, the reason for employing an operating frequency
around 4 kHz.

The voltage appearing at the anode of the valve under test
is applied to the upper arm of the potentiometer
R9, R10 via C5.
R9, R10 is kept at chassis potential via
the resistors R7 and R8, these being
included to prevent noise as the potentiometer is adjusted. As
will be seen, both ends of R9, R10 are
connected to chassis via almost identical impedances.

The slider of R9, R10 is coupled to
the grid of V3, this being a standard high-gain AF pentode.
The voltage appearing at the anode of V3 is next applied to a
conventional output valve and loudspeaker. The output circuit
can follow normal practice and is not, in consequence, shown
in the diagram.

Calibration and Use

The analyser is initially set up by fitting the
potentiometer R9, R10 with a scale, and
calibrating this directly in terms of the ratio R9
/ R10. The latter may be determined with the aid
of an Ohm-meter. When used, the valve to be tested is plugged
into the V2 position, the correct HT and grid bias potentials
applied, and R9, R10 adjusted for
minimum tone in the speaker. The volume control R1
should be adjusted during this process to as low a position as
is consistent with an accurate determination of the null
point. Mutual conductance can then be read directly, in mA per
volt, from the scale of the potentiometer.

Tests for microphony may be carried out by switching off
the 4 kHz tone, setting R9, R10 to the
top end of its track and tapping the valve under test. An
emission test can be carried out by inserting a meter between
R6 and the anode of the valve. This meter should be
short-circuited whilst measuring gm. It will be apparent that
it would not be possible, with this circuit, to supply the
filaments of battery valves under test from a 50 Hz AC supply;
this being due to the consequent introduction of hum. The
simplest method of heating such valves would be by the use of
a battery, all other voltages being provided, of course, from
the mains power pack of the unit.

Little else concerning the practical aspect of the circuit
needs to be emphasised, apart from the fact that the null
point on the bridge will be masked if stray capacities exist
between the bridge circuit and amplifier, and the circuit to
the grid of the valve under test. However, it should be
possible to keep stray capacities to a low value by employing
normal sensible layout techniques.

Calculated Errors

Before concluding, it would be of interest to calculate the
errors inherent in the circuit design. The main possible
causes of error are the presence of C4 and of
R6. The capacitor C4 has a reactance of
0.8 Ohms at the operating frequency of 4 kHz. Thus the
impedance between the anode of V2 and chassis is equal to
(252 + 0.82)0.5 = 25.03 Ohms;
and the phase shift at V2 anode is approximately 20. The
resultant unbalance introduced into the bridge should be
negligible and could be further reduced, if desired, by
increasing the value of C4.

Inserting R4 in the anode circuit of V2 causes
the dynamic mutual conductance of the valve to be gm / (1 + (R
/ Ra)) instead of being purely gm. Under the worst
conditions, such as could be given by, say, a triode-connected
6F6 with an Ra of 2,600 Ohms, the dynamic mutual conductance
becomes:-

gm / (1 + (25 / 2600)) or approximately gm / (1 + (1 /
100)) = gm + 1 / 1.01.

In this case, the error in reading given by the analyser
would be approximately 1%. With most valves, the error would
be far less than this
figure.

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