| OCR Text |
Show Obviates Necessity of Excessive "E" R Questions are frequently asked regarding re-garding the use of tubes with a high amplification constant. These tubes are primarily designed for use in resistance re-sistance and impedance amplifiers. We quote a few lines from one of Keith Henney's tube articles in the Radio Broadcast magazine. He writes as follows: fol-lows: "A low-mu tube will not 'load up' a power amplifier unless coupled to it by means of a transformer. For example, the average amplifier to deliver de-liver .06 watts power requires at least 9 volts variation on its grid. A tube with a mu of 8 coupled by means of resistance impedance to the amplifier cannot produce a variation of voltage greater than 8 volts and probably not over 6, so that the amplifier will not deliver its rated quota of power. On the other hand, a tube with a mu of 20, or a tube with a mu of 5 coupled by means of a 2:1 transformer, can easily produce the desired change in input voltage. High-mu tubes can be used as detectors and hence are useful use-ful in vacuum tube voltmeters. Their use in resistance and impedance amplifiers am-plifiers make the latter practical without with-out increasing the 'B' battery voltage beyond reason. It must be said here that the mu of such tubes is not the only important constant. The plate impedance must be considered and, like all other tubes, the usefulness of high-inu tubes increases as their Impedance Im-pedance decreases. "In this question It must be remarked re-marked that tubes of 1926 differ from those of 1925 in the fact that their plate impedance Is less. This is due to the use of better filament wire which has a higher electron efficiency. This low impedance, while It makes good amplifiers, causes trouble when the tubes are used in sets that have been neutralized for high impedance tubes. It is well known that less inductance in-ductance is required in the plate circuit cir-cuit of low impedance tubes to make them oscillate than is the case with high impedance tubes. On the other band, once these newer tubes are neutralized, neu-tralized, the voltage amplification and power output are increased." |
