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 (RS) has increased in value, the dc input voltage (ES) 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 (RS) 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 (Rp). For a given plate voltage, the value of Rp 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 (EL). 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.