QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 30, 5 February 1932, Page 11

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Editor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three--for more than this a shilling fee is charged, and a similar Supplying layouts. fee is payable for queries answered by post. circuits and solutions of intricate theoretical problems is beyond the seope of this service. any all requests for information. NonA eoupon must accomp appearance of the coupon in any issue cannot be regarded as @ reason for its not being used. Address ali queries, The Techniegl Kditor, Box 1032, Wellington.

i" "s\ICHRO" (Dunedin): Why does biasde ing the cathode in an indirectlyheated a.c. valve equal direct bias in the grid cireuit of a d.c. valve? I cannot understand how negative bias in the cathode lead should affect the electron flow from eathode to anode in an ac. valve. A.: The bias on the grid of any valve is determined by the voltage difference between the grid and the filament or cathode of the valve, irrespective of the remainder of the circuit. DWxamine an ordinary d.c, bias circuit and you will find that the grid is negative with re spect to the filament, or, alternatively, the filament is positive with respect to the grid. Examine now the bias cireuit of an a.¢c, valve, forgetting in the meanwhile about the electron stredm, and consider the current as flowing from the plate to the cathode in a conventional manner. In that resistor between cathode and earth or B- there is the plate current flowing from the cathode to complete the circuit to B-, As this current is flowing from the cathode portion of the circuit. the cathode is positive with respect to B- or earth. It will be seen that the grid is connected to earth also, therefore the eathode is also positive with respect to the grid, thus giving a negative potential to the grid (with respect to the cathode), of a value determined by I equals © x R, where ‘C" is the plate eurrent of the valve, and ‘‘R’ the resistance between cathode and B-. 2. I am so disgusted with the overwhelming preponderance of heavy classical musical from 4YA that I am seriously considering not renewing my license in March. Should I do this, what action, jf necessary, can the authorities take? A.: Should you operate your set without a license you will most certainly be

prosecuted and fned. If you do not wish to listen your installation must be digmantled. 3. Ig there any license fee payable on a short-wave receiver? If so, why A.: Yes, and for reasons too obvious to outline. U* (Te Kuiti): I wish to couple the output of an oscillator direct to the aerial terminals of receivers, and also to use an attenuator. : A.: We would suggest the use of a potentiometer as attenuator in the manner shown by the accompanying diagram.

2. What size coil should I use in my oscillator with condensers as specified? A.: A 250-turn honeycomb coil answers this purpose quite well. 3. Some time ago a description of the Loftin-White amplifier using specified parts appeared. If I halve the value of the resistors in the main chain ecan‘I parallel the output valve with another A.: This change is not advised, as it would necessitate numerous other changes to provide for the increased load. _ 4. Can you give me the plate current of the better-known American valves when tested with the usual a.c. set tester? A.: This depends on the make of tester, and for full details you should apply to the agents. 5. Will brass gauze do for shielding the oscillator or will I have to use aluminium or brass sheet? A.: You will probably find metal sheet necessary for screening, although it might be worth while trying the gauze if you have this on hand, . (N.B.-You appear to realise that the maximum number of questions which can be answered without payment of a shilling, is three, as you have suceessfully kept your number down to this by including extra questions as sub-clauses, ete. Play the game.) pms (Otago).-Would the enclosed circuit radiate? The wiring shielded as in the super six.-No, 2. Can I couple a dynatron oscillator to a sergen-grid first detector in a battery operated super-het? A.: Coupling to the grid coil of the detector by means of a pick-up coil is likely to be the most effective method. 8. The LE. is to be 175 kc. Would gemi-yariable .0008 mfd, condenserg tune

the grid coil of the 1.F. transformer ?- es. 4, I propose making this receiver an allwave one by cutting the stators of the .0005 mfd. variable condensers into two sections, say, .0001 and .0004 mfd., with means of coupling both sections in parallel. Plug-in coils will be used for both bands. A.: This is a good idea if the separation and mounting of the sections of the condenser can be done satisfactorily. The switching could be automatic by using the eontacts of the plug-in coils to effect it. y t.xrf. sereen-grid set has one stage of resistance coupling after the detector. After switching on there is a time lag as with a.c. receivers, which is not pregent with the stage of r.c.c. cut out. A.: This trouble may be overcome by reducing the value of either the coupling condenser or the grid-leak. Use a mica POOR IO APP ARAAS Sal OC OO OL

condenser. J.c.c. amplifiers are subject} to this fault, 6. I use YO volts on the plate. What é& the best plate resistor to use? A.: Not higher than 100,000 ohma, Wire-wound types are best. "QIDAR" (Auckland).-I am forward ing a sketch of a dynatron oscilla» tor which I propose using for sérvicing, What would be the approximate number of turns, and gauge of wire, wound on & Pilot "super-wasp" coil former, required to cover (a) the 1800 to 550 kc. band, and (b) to cover the 185 to 120 k.c. band A.: (a) 100 turns of No. 82, d.s.e. (b) You will have difficulty in accommodating enough turns on one of these formers {0 intermediate frequencies, If you are pry (Concluded on page 22.) BOOK OK KK KK KK OH

--- Questions and Answers

(Continued from page 11.) pared to bank wind about 400 turns on to the former, you will not be far ont with the condenser you specify. What will be the point of attachment for coupling the oscillator to a receiver to be tested? . ey A. Couple to the or any grid circuit on high frequency, and to grid circuits on intermediate frequencies. J ©.S. (Christchurch).-I enclose a dia- ° gram of my "B" eliminator, What ia the correct value for the voltage divider? A.: The value of 20,000 ohms, which you are using at present is quite O.Ks it regulation will possibly be improved by reducing the value to, say, 15,000 ohms. 2. The eliminator delivers only 14 mils. Would my choke be the cause of this low output. . , A.: No You however, find that if ou replace the 2 mfd. fixed condenser before the choke with a 4 mfd., a slight increase in output will result. The small output obtained may be due to a poor rectifying valve, which should be of the 280 type. Have the rectifier tested. 8. It makes no difference whether the earth is connected or not to the set, even on weak signals. Why is this? A.: Blectrie receivers generally work just about as well with no earth connescion, as the set is actually earthed through the capacity between the secondary and primary of the power transformer. Ineidentally, in connection with your eliminator, we would suggest the addition of 2 mfd. condensers between the 50 and 100 volte tappings on the voltage divider and A TH, (Khandallah).-Could you tell *- me where I could obtain a good circuit for an all-electric one-valve erystal amplifier? A.: A one-valve would not be powerful enough, while a two would not cost much more. A suitable circuit was published in the "All About the All-Hlectric." As 2QZW has commenced transmissions since this booklet was published, however, you will. need to modify the crystal circuit, making it more selective. This could be easily accomplished by adopting the eircuit of the "Rejecta Two" crystal set described several months ago in the "R.R." However, we think your best proposition would be the "Direct-Coupled Two," a simple and cheap local station receiver

-- hich will give high quality results. Tais ‘be W R.K. (Pahiatua)-My commercial * five-valve neutrodyne works quite well on medium volume, but immediately itis switched to loud the programme is drowned out by a loud, roaring sound. _ A.: More than probably a faulty transformer. We would suggest that you have the one in the second stage tested, as it sounds as though the primary is faulty. Pp*450¢ (Dunedin).-What value fixed condenser do I need to reduce (a) 10005 to .000125; (b) .00035 to .00015? A.: About .00015 and .00025 mfd. 2. I enclose a circuit of my home-built "B" eliminator. Where is the best place to pe a 12 mfd. electrolytic condenser? Will it reduce the hum I now get? _A.: Substituting this condenser for either "C2" or "C3" in your skeéch will probably reduce the hum. Also, if "B--" 45 volts operates the detector valve, put a 2 or 4 mfd. in place of the 1 mfd. If the hum still persists, connect .1 mfd. condensers of at least 1000 volts test between = ends of the secondary and centre tap of same. "QPARKS" (Carterton).-Could you ~ give me the number of turns for primary, secondary and reaction, on Colvern formers to tune between 20-85 metres? Tuning condenser is a .0001 mfd. double-spaced, and the reaction .0002 differential. we 80-metre band; pri. 6, sec. 25, react. 40-metre band; pri. 6, sec. 15, react. 20-metre band; pri. 2, sec. 5, react 4. 20 or 22-gauge wire will be suitable for primary and secondary, and 28 or 30 for reaction, either silk or cotton covered. When winding the coils, space the aerial and secondary windings in every other groove on the Colvern formers. "QSUNLIGHT" (Stratford): Which is the more powerful-the "Super Six" or the "Cathode" super-het.? A.: The latter easily, as it has one more valve, is a.c.,, and is intended for operation on an outdoor aerial. 2. Which is the easier to build? A.: The "Super Six"; the construc tion of the "Cathode" super should not be attempted except by the more advaneed amateurs. {N.B.-We cannot make the comparison you ask for.]

Ww J.W. _(Bizkenhead) : "To get the * maximum amount of power from a erystal set, both the aerial and the erystal should be tapped into the tuned circuit." ("Radio Record," January 8, page 27, "Tips and Jottings.") What is meant by this? A.: Ip instead of taking the aerial and erystal to the top of the coil they are tapped part of the way down the latter, as shown in the accompanying circuit diagram, volume and selectivity will be

noticeably improved. For example, if an 80-turn coil is tapped every ten turns, it will be found that reception is best when the crystal is tapped down approximately half-way, and the aerial a little below that. 2. How is a battery and potentiometer used with a carborundum type semlpermanent detector? . oo, A.: See the accompanying | MU (Mamakau).-My five-valve commercial set is very hard on "A" current. Should I replace my valves with 221’s and a B605? . A.: These would be quite and also much more economical on "A" current, 2. Would the B605 work satisfactorily ibe 90 volts "B" or would it require A.: Reproduction will be fairly satisfactory using 90 volts "B" and 9 volts bias, but you would notice a distinct improvement when using 135. 3. How many Daniells Cells would be required to keep the "A" battery fully charged? A.: Not less than six. ONSTANT READER (Seatoun): The 23 plate midget condenser wi have a capacity of .0001 mfd. and the 13 plate .00005. We would suggest 20-24 turns on reaction. Reception will be slightly weaker with the smaller conenser. R H.C. (Raetihi): Which shortwave ¢ set would you advise a beginner to build? A.: The "Night Owl Three," which was described in the "R.R." issue dated Nov. 138, 1931. The "Kestrel Three" was described in the issues dated July 24 and 31, 1931. 2.’ Would the trees in the accompanying diagram interfere with reception? A.: Not to any appreciable extent. M.T. (Murchison) : I wish to replace ¢ the valves in my five-valve commercial set. What would be.the best types? _A.: In rf, detector, and 1st audio stages use type 221. Power valve may be of type B605.