RESISTANCE COUPLED AMPLIFIERS

Radio Record, Volume I, Issue 45, 25 May 1928, Page 13

RESISTANCE COUPLED AMPLIFIERS

LTHOUGH resistance coupling is generally looked upon as a reliable means of obtaining pure reproducduction in an amplifier, there are points to be observed in order to ensure that this desirable quality is fully secured. The loss of low notes is avoided to a great extent by choice of suitable coupling condenser and leak. A more frequent trouble is the loss of high notes, and this defect has in many factory-built receivers been accentuated by sacrificing the upper musical scale in order to obtain general high amplification on the remainder of the scale. TRAY capacities in the amplifier circuit contribute to the loss of high note amplification. These capacities are between the plate and filament of the valves, .between the plate resistance and its holder, the joint gridfilament capacity of the following valve and across its grid leak and holder, and finally the grid-plate capacity of this valve. ° The total of these stray capacities has the effect of a by-pass condenser of appreciable value, and according to the frequency of the note conveyed by the signal, has more or less effect upon amplification. This unwanted capacity has a very high impedance to the low notes, so that all the available voltage is transferred to the grid of the next valve, giving high amplification. This is assisted by keeping the value of the resistance as high as possible, that is, large compared with the internal resistance of the valve. AS8 the frequency, that is the pitch, of the notes rises, the stray capacities offer less impedance to the total yoltage, so that an increasing proportion is short-circuited or by-passed,

so that the higher the note the less it ig amplified. In consequence of this, an amplifier in which the amount of unwanted capacity is large, will greatly weaken or even tend to cut off the higher notes, and the general effect will. be lowtoned or "mellow," which is often preferred for musical items, Dut in the reception of speech the lack of high notes is appreciably evident, the consonants, particularly the "s," being slurred and difficult to distinguish. T is thus seen that if the internal resistance of the valve and the plate resistance are increased, the stray capacities remaining at their original value, then amplification of high notes will be further decreased, and any unavoidable increase of stray capacities must be balanced by a decrease of the plate resistance and the internal resistance of the valve. This is all intended to show the wisdom of reasonable rather than extreme resistance values, and in the construction of an amplifier to watch the matter of capacity wherever it can be controlled or kept down. TWO POINTS FOR IMPROVEMENT. [zt is -usual to recommend a ,1 megohm resistance for all plate couplings, but in many cases this value will be too high for the resistor coupled to the detector, especially when the 200A type valve is employed. If less than 180 volts B is available this value may be as low as .02 meg. The best way to obtain maximum efficiecny in this position is to use a variable resistance in place of a fixed one. Similarly, the .25 meg. usually recommended for the grid circuit of the power valve is higher than necessary except in the case of 112 and smaller ‘types of valve. For the 112, 171 and 210 types the value should be from 25,000 to 100,000 ohms. A convenient way of finding the best value is by using a variable resistance of good make, capable of carrying the ¢omparatively heavy current without noise or change of value. If fixed resistanees are used, it pays to try