VR Tubes Connected in Parallel

One might expect that connecting VR tubes in parallel as shown in figure 3-47 would increase the current handling capacity of the network. Although this is true for some gas-filled tubes, it is not true for VR tubes. In figure 3-47, two VR tubes are constructed in exactly the same way. The only difference will be a slight variation in their ionization potential. For the purpose of this discussion, VR tube VR1 will have a lower ionization potential than VR2. The potential that must be reached before a VR tube ionizes is considerably higher than its normal operating voltage.

Figure 3-47. - VR tubes connected in parallel.

When voltage is applied to the circuit of figure 3-47, as soon as the correct potential is reached, VR1 begins to conduct and the potential across it decreases to its operating voltage. The potential across VR2 never becomes high enough to cause it to ionize. Therefore, placing the VR tubes in parallel accomplishes no useful purpose. When greater current handling capacity and better regulation are desired, electronic (vacuum tube) regulator circuits are used.

Several conditions may either indicate or cause problems with a VR tube regulator. Initially, you can get some indication of the trouble associated with a gas-tube regulator circuit by visually inspecting it to determine the presence of the characteristic glow from the ionized gas within the tube. When current through the tube is near its maximum rating, the tube is highly ionized. When the current is near its minimum rating, the tube is lightly ionized, Therefore, the intensity of the gaseous discharge within the tube is an indication of tube conduction. If the tube is not ionized, however, this does not necessarily mean that the tube is defective. The same indication (lack of characteristic glow) may also result from the following conditions: the series resistor (R_S) has increased in value, the dc input voltage (E_S) is below normal, the load current is below normal, or the load current is excessive. You therefore need to make dc voltage measurements at the input and output terminals of the voltage regulator circuit to determine whether the problem is inside the regulator circuit or outside of it.

You can check value of the series resistor (R_S) by using ohmmeter measurements to determine whether any change in resistance has occurred. If the maximum current rating of the regulator tube is exceeded for a considerable length of time, the tube may be damaged and lose its regulation characteristics; therefore, you can suspect the regulator tube itself as a possible source of trouble.

Although VR tubes are used extensively in electronic equipment, there are circuits that require a greater degree of regulation than a VR tube can provide. For these circuits, an electron tube voltage regulator is used.

Electron Tube Voltage Regulator

An electron tube may be considered a variable resistance. When the tube is passing a direct current, this resistance is simply the plate-to-cathode voltage divided by the current through the tube and is called the dc plate resistance (R_p). For a given plate voltage, the value of R_p depends upon the tube current, and the tube current depends upon the grid bias.

Refer to figure 3-48, view (A). The resistance of V1 is established initially by the bias on the tube. Assume that the voltage across the load is at the desired value. Then the cathode is positive with respect to ground by some voltage (E_L). The grid can be made positive relative to ground by a voltage (E2) that is less than E1. The potentiometer R2 is adjusted until the bias (grid-to-cathode voltage), which is E2 - E1, is sufficient to allow V1 to pass a current equal to the desired load current. With this bias, the resistance of V1 is established at the proper value to reduce the rectifier output voltage to the desired load voltage.

Figure 3-48. - Electron tube voltage regulator using a battery for the fixed bias

If the rectifier output voltage increases, the voltage at the cathode of V1 tends to increase. As E1 increases, the negative bias on the tube increases and the plate resistance of the tube becomes greater. Consequently, the voltage drop across V1 increases with the rise in input voltage. If the circuit is designed property, the increased voltage drop across V1 is approximately equal to the increase in voltage at the input. Thus the load voltage remains essentially constant.

The resistor (R1) is used to limit the grid current. This is necessary in this particular circuit because the battery is not disconnected when the power is turned off. However, the battery can be eliminated from the circuit by the use of a glow tube (V2), as shown in view (B)of the figure, to supply a fixed bias for the grid of the tube. The action of the circuit in view (B) is the same as the action of the circuit in view (A). The output voltage of the simple voltage regulators shown in the figure cannot remain absolutely constant. As the rectifier output voltage increases, the voltages on the cathode of V1 must rise slightly if the regulator is to function.

The voltage regulators shown in the figure compensate not only for changes in the output voltage from the rectifier, but also for changes in the load. For example, in view (B) if the load resistance decreases, the load current will increase. The load voltage will tend to fall because of the increased drop across V1. The decrease in load voltage is accompanied by a decrease in bias voltage on V1. The bias voltage on V1 is equal to E1 - E2. Thus the effective resistance of V1 is reduced at the same time the load current is increased. The IR drop across V1 increases only a slight amount because R decreases about as much as I increases. Therefore, the tendency for the load voltage to drop when the load is increased is checked by the decrease in resistance of the series triode.

Q.40 In an electron tube regulator, the electron tube replaces what component?