Chapter 10

VACUUM TUBE AMPLIFIER RECEIVING SETS

The reason a vacuum tube detector is more sensitive than a crystal detector is because while the latter merely _rectifies_ the oscillating current that surges in the receiving circuits, the former acts as an _amplifier_ at the same time. The vacuum tube can be used as a separate amplifier in connection with either: (1) a _crystal detector_ or (2) a _vacuum tube detector_, and (_a_) it will amplify either the _radio frequency currents_, that is the high frequency oscillating currents which are set up in the oscillation circuits or (_b_) it will amplify the _audio frequency currents_, that is, the _low frequency alternating_ currents that flow through the head phone circuit.

To use the amplified radio frequency oscillating currents or amplified audio frequency alternating currents that are set up by an amplifier tube either a high resistance, called a _grid leak_, or an _amplifying transformer_, with or without an iron core, must be connected with the plate circuit of the first amplifier tube and the grid circuit of the next amplifier tube or detector tube, or with the wire point of a crystal detector. Where two or more amplifier tubes are coupled together in this way the scheme is known as _cascade amplification._

Where either a _radio frequency transformer_, that is one without the iron core, or an _audio frequency transformer_, that is one with the iron core, is used to couple the amplifier tube circuits together better results are obtained than where a high resistance grid leak is used, but the amplifying tubes have to be more carefully shielded from each other or they will react and set up a _howling_ noise in the head phones. On the other hand grid leaks cost less but they are more troublesome to use as you have to find out for yourself the exact resistance value they must have and this you can do only by testing them out.

A Grid Leak Amplifier Receiving Set. With Crystal Detector.--The apparatus you need for this set includes: (1) a _loose coupled tuning coil_, (2) a _variable condenser_, (3) _two fixed condensers_, (4) a _crystal detector_, or better a _vacuum tube detector_, (5) an A or _6 volt storage battery_, (6) a _rheostat_, (7) a B or 22-1/2 _volt dry cell battery_, (8) a fixed resistance unit, or _leak grid_ as it is called, and (9) a pair of _head-phones_. The tuning coil, variable condenser, fixed condensers, crystal detectors and head-phones are exactly the same as those described in _Set No. 2_ in Chapter III. The A and B batteries are exactly the same as those described in