"Circuit Sentry" Protects Tubes in P.A. Amplifier
J. LEVITSKY - Chief Engineer, Fanon Electrobic Industries
Audio September 1960, Vol. 44, No. 9

Simple protector circuit added to conventional amplifier prevents damage to output tubes in case of shorts on the loudspeaker line.

In most commertial and industrial public address systems utilizing fairly high power amplifiers, break-down of the amplifier output tubes often results from a short or a severe overload of the speaker line. In many such systems,  the amplifier feeds power via the 70-volt line to numerous speakers distributed over wide areas, each speaker being provided with its own separate matching transformer. Under such conditions, due to the long runs of line and a large number of components connected across it, partial or complete shorts may occur rather frequently.

The severity of the problem can be seen by a glance at the data in Table 1. This data was taken with the Fanon 70-watt amplifier (model 3370), employing two EL-34 power output tubes, operating in class AB1. Columns 1 and 2 show the audio power output for different levels of input under normal trouble-free conditions. Column 3 shows the corresponding power dissipations per tube under the same conditions. Column 4 shows the tube dissipations for the same levels of input signals, with the 70-volt line shorted to ground. Since the average audio power output of a P.A. amplifier may be somewhere between 25 and 30 per cent of its peak output, when signal is being applied, the data in column 4 indicate that if a short occurs in the speaker line, each tube is dissipating roughly three times its maximum rated power. Even if a high-resistance short takes place, say about 25 per cent of the rated load, the dissipation in each tube is much higher than the maximum allowable, as shown in Fig. 4.

Table 1

Power Output
Input Signal
Normal Plate Dissipation
Shorted-Output Dissipation
0.08  19.98  57.2 
10  0.13  21.8  73.7 
20  0.19  23.0  81.5 
30  0.24  25.0  81.8 
40  0.29  28.3  82.1 
50  0.31  30.2  84.8 
60  0.34  27.3  83.5 
70  0.38  24.5  82.2 

The reason for this very high dissipation in a class B or AB type amplifier under conditions of a speaker-line short are two-fold. One reason is that with the speaker line shorted, the plate-to-plate impedance of the output transformer primary is very low, and therefore the signal plate voltage variation is quite small. The plate dissipation is therefore much higher than normal, since under normal conditions the plate voltage varies over a wide range.

The second reason is that most amplifiers employ negative feedback in order to improve stability, minimize distortion, and so on. With the speaker line V1, V2 and V3 constitute part of the regular amplifier. V1A is a voltage amplifier, V1B is the phase inverter and driver, and V2 and V3 are the push-pull output tubes. The new circuitry is that associated with V4 and V5. V4 is a duodiode triode 6FM8, and V5 is a 6AU6 pentode.

The operation of the circuit is based on the balance achieved under normal operation between voltages derived from the signal voltages at points (B) and (C) in Fig. 1, and the fact that this balance is upset when a short occurs at (C).

An equivalent schematic is shown in Fig. 2. When E1 = E2 and R1 = R2, the d.c. voltage at (A) is zero with respect to ground. If E1 is greater than E2. the d.c. voltage at (A) is positive, and with E2 greater than E1, the d.c. voltage at (A) is negative.


Fig. 1. Schematic of final stages of typical P.A. amplifier to which the "Circuit Sentry" has been added (enclosed in dotted lines) to remove signal in case speaker line becomes shorted

It is desirable, of course, that under trouble-free conditions, E1 = E2 over the whole range of input signal levels to the amplifier. E1 and E2 are derived from voltages at points (B) and (C) of Fig. 1. Since these voltages are within the negative feedback loop of the amplifier, the ratio Ec/Eb remains constant over a wide variation of input signal levels, tube characteristics, line voltage variations, and the like.

The circuit of Fig. 2 is now modified by connecting the resistor R3 to a negative supply voltage, as shown in Fig. 3.


Fig. 2. Simplified comparator circuit arrangement.


Fig. 3. Adding a fixed negative bias to the ci rcuit of Fig. 2 provides a control voltage to the pentode shown.

Now the d.c. voltage at (A) is negative for E1 = E2 for all levels of E1 and E2. Under conditions of a short or a heavy overload at (C), E1 is much greater than E2, and the negative bias voltage is of such value as to make the resultant voltage at (A) either zero or slightly positive. The d.c. voltage at (A) is connected to the grid of the 6AU6 and its plate is connected to the plate of the amplifier driver V1B, (Fig. 1) . Under normal conditions, the negative voltage at (A) is sufficient to hold the 6AU6 well beyond cutoff so that its effect on the operation of the amplifier is nil. With the output shorted, the grid of the 6AU6 is at zero volts with respect to ground, the tube draws a relatively high plate current and is practically the same as it is with zero input signal.

Since under normal conditions the 6AU6 tube of the protective circuit is operating in the cutoff region it has no effect on the operation of the amplifier, and therefore no adjustments whatsoever are required when the protective circuit is added to the amplifier.

If a fault in the speaker line is intermittent, and if continuous signal is being fed to the amplifier, as from a phono or radio, for example, the circuit will locking in the blocked condition (no signal on grids of output tubes), thus protecting the amplifier against recurring faults. After the fault is cleared, it is necessary to hold the volume control at zero for approximately ten seconds for the circuit to restore itself to normal.

Fig. 4 is a plot showing the effect of the protective circuit for varying degrees of overloading. The data for these curves were taken with a constant input signal - the input required to drive the amplifier to maximum output under normal rated-load conditions. The curves show the dissipation in the output tubes as a percentage of maximum allowable dissipation versus the load as a percentage of rated load - with and without the protective circuitry. The curves show that when the load across the output falls to approximately 30 per cent of the rated load, there is severe danger of overdissipation. With the protective circuitry, there is no possibility of overdissipation under any conditions.


Fig. 4. Curves showing effect of sentry circu it on dissipation of output tubes. Data taken with Fanon Model 3370 amplifier and with constant input signal.


Fig. 5. Complete "Circuit Sentry" is housed in small plug-in unit.


Fig. 6. Fanon Model 3345 amplifier with "Circuit Sentry" module plugged into socket provided.

The circuit discussed above is offered in the form of an optional plug-in assembly unit for the Fanon P.A. amplifier models 3335, 3345, and 3370. Fig. 5 is a photograph of this plug-in unit. As mentioned above, no adjustments are necessary. The unit is merely plugged into a socket provided for this purpose, as shown in Fig. 6.