Anode Bend Detector Unit

Radio Record, Volume III, Issue 5, 16 August 1929, Page 28

Anode Bend Detector Unit

Local Reception with Minimum _ Distortion oe

(By

MEGOHM

JSTHNERS who are situated in close proximity to a main broadcast station have a problem forced upon them that is unknown to those who operate at a considerable distance. This problem is the one of avoiding overloading of the broadcast detector valve by the powerful signals from the local station. It is quite possible to detune a multi-valve set so that good local reception is the result, but on the other hand some receivers do not oper-

ee ate satisfactorily under such conditions. _.« Moreover, the local station being the one upon which the average receiver operates for a much longer period than -any other, there is considerable economy with battery-operated sets. when

two or three valves are cut out for local reception. The reduction in yalves will almost invariably result in improved reception. — The most usual method of adapting a-receiver for the local station is to provide a crystal detector in place of the usual grid-leak detector, the rectified signals being fed to the audio am‘plifier.. This system has met with considerable success, though jit is by no means so near perfection as many suppose. .Much depends upon the state of the erystal, and it must be remem; bered that slight distortion of any kind that might pass unnoticed when listening to crystal reception with headphones, would be very evident when amplified. Anode Bend for Clarity. ITH a view to obtaining, if possible, better reception than is. ob-. tained under certain conditions with a erystal, or too many valves, in cases where heavy volume is required from the loca] station-and there are few valve-set owners who do not want, itthe writer has experimented with anode bend detection, and now presents a detector unit that can be absolutely relied upon to give flawless reception of the local station, and which may also be used in conjunction with a gramophone pick-up to give high amplification and quality music. It is, of course, understood that the amplifier to follow the detector is capable of

giving the required volume without distortion. The present. space will not permit the theory of the anode bend detector being dealt with, but it suffices to say

that this system of detection dispenses with the usual grid leak and condenser, and in their stead a negative bias is imparted -to the grid of the valve. Such a detector is less sensitive than the leaky-grid type, and therefore requires stronger ‘signals, but there iseno lack of these near the. transmitter. On the other hand, this detector has the desirable property of being able to handle greater volume without distortion than can the leaky variety. Reaction is not required for local reception. The detector. is presented in unit form for convenience of description, but may easily be adapted for incorporation in any receiver if necessary. Bias is applied to the grid of the valve by a 44-volt dry C battery, the bias being regulated to a nicety by a*potentiometer connected across the filament battery, the arm connecting to the positive pole of the C battery. Aerial and Volume. Control. A. SHORT aerial of only two or three feet of wire may be used, but to get full effect of the lower

register, an aerial of not less than 30 feet is recommended. This. will .bring in more volume than is required, but is reduced: by means of the variable resistance across ‘aerial and earth, the value to be not less than 10,000 ohms. A good value is 200,000. The number of turns in the primary coil is not eritical in any way, and may be regulated according ‘to the constructor’s judgment. Six turns near to the station with a long aerial, and perhaps eight or nine with short aerial, Up to twelve turns or more, as distance from the’station is increased, say over ten miles. It is not possible just now to give the range of this unit, but probably up to 20 miles heavyy glume will be obtainable with a good "uerial and two good stages of audio amplification. ° Construction. AS there is no necssity for making the unit selective, any variable ¢ondenser that will tune the coil may be used, and the shape of the plates is immaterial., Constructors will have many of the parts on hand, the total list comprising variable condenser, tyuning coil, rheostat, potentiometer, resistance, half-microfarad condenser, valve holder, terminals, and connecting wire. A vernier dial is not required. The minimum size to accommodate the components comfortably will be 84 by 5tin. for the panel and 7 by 52in. for the baseboard. \ The small primary or aerial coil of six or more turns, should be slightly less diameter than the secondary or main tuning coil, so that it will slip inside the top end. This coil may be of 18’s or 20’s d.ce,’ and may be wound upon a 1-inch collar of celluloid or cardboard, and secured to it with celluloid cement.’ It. is convenient to cut the ends close to the coil and solder to each one, a length of flexible connecting wire to run to the aerial and earth terminals respectively. It should be noted that this is the only part of the circuit directly earthed. The secondary coil is only earthed by its connection to the A battery. The condenser and 3in. coil td work together are as follow:-.0005 condenser and 56 turns, .00035 and 67 turns, .00025 and 78 turns. The coil should not be space,wound. The above turns are for 20’s wire, but thinner wire will require fewer turns, particulars of such being obtainable on page 136 of the "Yisteners’ Guide." Fewer ‘tus will bring in 1YA and ?YA, ° . Wiring is clearly shown in the diagrams, only two filament leads being placed under the base. Outside’ leads are shown as continuous wires, which may be cabled together, or provided with terminals at suitable positions. The potentiometer actually used was a 2000-ohm variable resistance, altered to provide’a connection at each end of the resistance strip, but'a lower value could be used. The valve-socket must be the usual side-grip pattern, and on no account the old American flat-spring type. The rheostat is to suit the valve and battery voltage-a 30-ohm is a ggod all-round value, In use, note which side of the potentiometer (the right in diagram) is connected to A negative. The torrect position for grid bias must be found upon

that half of the resistance by judgment of quality when listening to good volume ‘on the loudspeaker. Try the effect jot using both the 3-volt and the 44-volt taps on the C battery. Best results have been obtained with a valve of not less than 50,000 ohms impedance, with 90 volts on the plate,. -eonnected to a good make of audio transformer with high inductance primary. This combination gives very fine reproduction, brings ont. the bass well, at the same time giving prominence to the melody, with a general brilliant effect. Much lower plate volt‘age results in 4 weakening of high notes and want of clarity in the bass, The same may be said of iower impedance valves tried, which at their best did not appear to come up quite to the higher. impedance one, especially

when using the pick-up. Too high a voltage tends towards scratchy tone. * Connections for Pick-up. [THE method shown for connecting a gramophone pick-up allows the changing from local to gramophone or vice-versa by merely throwing over a small two-way switch which is situated at the top of the panel. When the pickup is in use the tuning coil is entirely disconnected from the grid circuit, so that there is little chance of hearing the gocal station in the background if x working whilst the pick-up is in usé, Terminals may be provided at the side of the panel for the pick-up tags, one of these connecting to a lower switch contact and the other to A nega- _ tive. If a pick-up is to be used, the wire from coil top to grid is cut at XY, and the two ends are connected over the top of coil to. X and Y respectively on the switch. With the switch

up, the local. station is received, and with it down the pick-up may be used. The lead from’ plate of valve connects to the primary of first audio transformer, and the other primary terminal of transformer to detector B. positive voltage. . Preceding remarks concerning the valve apply equally when using the pick-up, but as the amplification is great when using the detector valve, soft gramophone needles should be used. . The soft tone Tungstyle semipermanent needles are quite suitable, but for use with an amplifier only. the loud tone would be required." A -‘volume control, potentiometer pattern, must be provided. to be placed across the pick-up leads, and room could no doubt be found for this on the panel if desired. . ‘ Connections to the A’ battery must be made so that the main switch for the receiver also switches aff the supply to the unit, so that the potentiometer drain is cut off when the receiver is not in use. The ideas of different individuals vary greatly as to the valve to use for detection, and how to use jt, and though in a‘recent article, the writer made a few general remarks upon the subject, the best advice to constructors is to spend a little time in finding out. which valve and voltage best suits their own conditioris, remembering that small vhanges often have a big effect upon results. . If the use of a scratch filter appears necessary, do not trouble to put one in, but take it as a reminder that the audio. equipment may possibly need a little improvement. It is an easy matter to replace mediocre . transformers by good ones, and is always worth while. Present-day records have very little seratch-perhaps it would . be more correct to say "practically none." Pick-up connections are shown dotted. in the diagram, and if a pick-up is not. to be provided for, these connections and the small two-way switch are omitted. The method of connecting and disconnecting the unit to the amplifier by switching or otherwise must be arranged. to suit the conditions. A jack controlling filament and plate supplies would also cut out the potentiometer, The plug would be connected to the ‘amplifier, and would be inserted in another jack to connect the amplifier to R.F. and ordinary detector. Another method is to connect the.arm of a twoway switch to the plate terminal of the transformer primary, the tickler to one contact, and plate of local detector to the other. The: switch then changes over for local and distance. Batteries are then separately cut off. If the local filament is cut out with the rheostat, then only the A and B supply for R.F. and detector stages of the receiver remain to be provided. for. The writer feels. certain that constructors of this unit will find that they have taken a.good step forward on the path of better reproduction. In our next issue a further article on this topic will appear.