Wireless Broadcast

Star (Christchurch), Issue 17833, 29 April 1926, Page 10

Wireless Broadcast

CONTRIBUTED BY

“AERIAL.”

MILLI-AMPERE METER. ITS USE IN THE RECEIVER. '1 he average radio fan, after setting ■up a receiver in his home, seldom worries about what happens in the electrical circuit of his receiver, as long as it pushes out some music from the loudspeaker. But he does sometimes wonder how long the B battery will hold up, especially when three or rnore valves are used. Some more wondering starts when the loud-speaker gives out plenty of volume, but badly distorted music. The use of a milli-amp meter in a receiver may be considered superfluous and unnecessary by the great majority of radio fans, and in certain cases it may be, but to the owner of a'set using three or more valves, such a meter will be found very useful. We are informed that any amplifier tube, whether in a high or low-frequency circuit, should be operated on the straight portion of its characteristic curve to give the best results. If an amplifier tube is operated otherwise distortion will occur.

Let us imagine that an amplifier tube is connected up ready for work with a milli-ammeter having a scale reading of 0-10 mills, in the plate circuit. When the filament is lit a cur-

rent flows from the B battery through the tube, the exact value of current being indicated by the meter, and for the purpose of this article let it be. say, six milliamperes. Now if a steady negative voltage be applied to the grid of the tube, the plate current will drop slightly. Suppose the grid voltage is three, and the plate current drops from six milliamperes to two milliamperes. Now if connections to the grid battery be reversed, another change of plate current will be noticed. Applying a positive bias of three volts to the grid has raised the plate current to, say twelve milliamperes.

Our milli-ammeter has now shown us something, and that is that the tube is not being operated on the correct portion of its characteristic curve, since a negative bias on the grid resulted in a decrease of plate current of four milliamps, and the same amount of positive bias resulted in an increase of six milliamps. If an incoming signal, which swings the grid potential by the amount given above, it will be evident that one half of the waves will be amplified more than the other, and the result will be .some distortion. If, the needle of the milliameter swings about when signals are being received. it is a sign that the grid bias is not correct.. With a correctly adjusted C battery, the grid of the amplifier tube is affected by the two halves of the incoming waves many times per second, so fast, indeed, that the needle of the milliammeter could not possibly move fast enough to show the rapid rise and fall of the plate current, but would, instead, simply indicate the mean or average current flowing through the tube. In the case of an R.F. amplifier, the alternating potentials impressed on the grid by the incoming wave are at a very high frequency, varying between 1500 and 500 kilo-cycles, assuming that the tuning range is from 200 to 600 metres. There is little wonder, therefore, that an indicating device, consisting of a movable coil and pointer, which constitutes our milliameter, is unable to vary its position at such a rate.

In the case of an audio-frequency amplifier, the frequency is much lower, laying usually between 100 and 6000 cycles per second, but this is still too high a frequency for the meter to respond to. A milliameter may be made to register the current drawn from the B battery by all the receiving valves at once, by placing it in the negative B battery lead. Placing it in the positive lead will not show the total current drain unless all the tubes are connected to the. same positive terminal. It is usual in ordinary receivers to apply about 45 volts to the plate of a radio frequency tube, about 22J or so to the detector, and anything up to 150 volts on the audio tubes. To have the meter indicate the total- current drawn from the battery under such circumstances it will be necessary to place it somewhere in the circuit where all the current will go through it, and that place is in the negative lead. Besides showing the amount of current drawn, some faults in the circuit may also be readily traced. Should the meter indicate a rapid rise and fall of current, or the pointer become very erratic, the cause will probably be a loose connection somewhere. By lift-

-ing aIL the tubes froiri the sockets, j and' then placing them in one at a j time, it will be easy to see which part of the circuit, is causing the trouble. Jt may be a bad contact in a socket, or a wire hanging loosely to a terminal somewhere. In addition to the uses mentioned above, there are a dozen and one other uses in and about a receiver to which ' a milliameter can be put. A meter which registers up to twenty-ftve milliamps is about, the handiest range to have, and the instrument, to be of any ! use. should be a good one. such as a j Weston or Jewell. PROTECTING THE LOUD SPEAKER.

Burning out the. fine windings of a loud-speaker is very exasperating, and also rather expensive. When strong signals are received, and are put through several amplifier tubes, there is always the risk tKat the fairly heavy currents flowing will damage the speaker windings or burn them out altogether. There are two good ways of using plenty of amplification, with no risk of accidents to the speaker or headset,

if one is used. The first method is to use a choke coil and a condenser. For the choke coil, the secondary winding of an ignition coil will serve, but the iron core must be in its place, as the choke is useless without it. The plate of the last valve is connected to one end of the choke and to one side of the condenser, which should not be smaller than a quarter of a microfarad. The other end of the choke goes to the positive terminal of ' the B battery. One loudspeaker terminal goes to the other side of the condenser. and the other speaker terminal goes to the negative of the B battery. This is a capaqitv-coupling arrangement. and prevents, the currents from the B battery from flowing through the loudspeaker windings. The second method is to eouple the output from the last tube to the loud speaker through a transformer, as in the usual push-pull transformer arrangement. If a push-pull output transformer is not available, a low- ratio amplifying transformer may be pressed into service. Tts secondary terminals go to plate and B battery respectively, while the primar\ T terminals go directly to the loudspeaker. There is little to choose between the two as regards efficiericy, But the choke coil and condenser arrangement has the advantage of being considerably cheaper than the transformer.

JOTTINGS. Next Sunday, May 2. at 10 p.m.. station 0-IXM will transmit some Standard Frequency Signals for the benefit of Australian and New Zealand amateurs who wish to obtain calibrated waves. The transmission-will last over a period of two hours. Each transmission will occupy ten minutes, then five minutes will intervene while the transmitter is being tuned to a new frequency. Each ten minute period will be divided up as follows: —Three minutes: QST, QST. QST, U, IXM. IXM. IX.M, etc. ' Seven minutes: Repetition of characteristic letter, broken occasionally by the call sign, U—IXM, and a statement of the frequency. The approximate accuracy of these signals is within about 0.1 of 1 per cent. Below is a schedule of the transmissions.

U-IXM is the experimental station of the Massachusetts Institute of Technology Radio Society. The diagram on this page is in reference to the article on a radio-frequency unit described in the “ Star ” last Thursday. The Radio Society of Christchurch, besides running a broadcast set on 266 metres, also transmits telephonv at odd times on the short wave outfit, which uses one fifty watt tube on 35 metres. Very little experimenting has been done so far. but good distances have been covered.

Time Kilo-cvcles Frequency Wave Characteristic Letter 10.00 p.m. 6500 46.1 A 10.15 7000 42.8 B 10.30 7500 40.0 C 10.45 sooo 37.5 D 11.00 S500 35.3 F 11.15 9000 33.3 G 11.30 12000 25.0 J 11.45 14000 21.4 K 12 p.m. 16000 1S.7 L