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520 RICHARDSON'S BLUEBOOK OF PROJECTION
tubes tested is under suspicion, and the first step may well be to change both of the doubtful tubes. If the meter reading then returns to normal, the doubtful tubes may be replaced one at a time to find which one is faulty. Be sure to allow a cold tube of this size a minute or so to heat up to full emitting temperature before taking the reading — larger tubes than those here shown may require several minutes of heating before their emission can become normal.
It should be noted that in Class A amplification sound volume has no effect on the meter readings. Sound merely causes the space current to undergo a rhythmic rise and fall around some average value. The extent of that fluctuation represents the volume, but at all volumes the increase is equal to the decrease and the average current value remains constant. The meter cannot respond to current fluctuations as rapid as even the low frequencies of sound ; it reads the average value only.
The same is not true of Class B amplification, where the sound is represented by very large fluctuations in space current and the meter will read the average of those fluctuations. Since they vary with volume, the needle will flicker. The same will happen to a small extent even in a Class A amplifier when driven to extreme volume, because in such cases the circuit is beginning to act as Class AB. Class A and Class AB space current readings are preferablv taken with no sound present.
Push-pull tubes, as VT-3 and VT-4 of Fig. 201, should be matched. This can be done by substituting a new tube first in VT-3 socket and then in VT-4 socket, taking readings each time after the new tube has had a chance to heat. The difference in reading should be the same in each case. If one reading is substantially higher than the other, then the new tube is in a socket which formerly held a tube of low emission.
New tubes should not be paired in push-pull with older ones that, while still usable, have substantially lowered emission. Two new tubes should be used, *and the one that is "still good but weak" marked and saved until it can be matched with another in the same condition.