Questions and answers

Radio Record, Volume III, Issue 3, 2 August 1929, Page 34

Questions and answers

T is regretted that a large number of Questions and Answers have to be held over until next week. This is unavoidable owing to the fact that the. technical staff was engaged "in exhibition duties during the week. . 4 Sensitivity of the Linen Diaphragm Speaker. WouvLpD the linen diaphragm speaker given the same volume as my horn speaker, or does it need more power to work it? asks "G.F.", (Duneam. : While this speaker is very senit cannot equal the sensitivity of a small horn such as "G.F." is using. Almost invariably a delicately-con-structed horn speaker is more sensitive than a cone, but the latter makes up

both in tone and in: the amount of volume it can handle. 2. How does the speaker described May 38, 1929, compare with the linen diaphragm speaker as regards tone and volume? : A.: There is very little ‘difference. Both can reproduce a very wide frequency range at great volume. | "Pentode’s" M.C. Speaker. I have made this speaker, writes "J.B." (Christchurch), and the results are splendid as regards tone and volume, but the field winding takes 14 amps. at 6 volts. _ I have tried. running this from a 2-amp. charger, but only a hum can be heard, and an ammeter connected in series shows. no movement.

A.: Although the current from the charger is.direct, it has a strong ripple owing to the fact that filter devices are not used with battery . chargers. This can be regarded as A.C., and the field of the magnet is acting as a choke coil which will not pass A.C. Insert an electrolytic condenser across the ‘field terminals. . Unit For Linen Diaphragm. Js it possible to use an ordinary speaker unit in conjunction with this type of speaker? asks "N.E. # " (Auckland). -A.: Yes, providing it is of the balanced armature type, that is, the type used for -cone and not for ‘horn speakers. Alternative Construction to L.D. Speaker. OULD the alternative construction be as good, or better, than that supplied in detail? writes "A.S." (Martinborough). A.: It is not‘'as good as the straightout construction, and for this reason full details were omitted. 2. Would it do to have a spring to balance the pull on both sides of a double diaphragm speaker? A.: This would be -all right, but there is a possibility of resonance effects due to the spring. It would be worth trying.

Attaching the Unit to the L.D. Speaker. How would I attach the driving rod of the balanced armature unit to the coupling bolt? asks "H,.G.0." (Seatoun). A.: Tap the rod and use two washers and two nuts, or stick it with glue or secotine, 2.: Will it operate directly from a crystal set without amplification? A.: It is most unlikely.

Points Regarding the L.D. Speaker. "FBG." (Tokomaru) asks :- 1. The address of a Wellington’ firm where the units are procurable. A.: Smythe and Oo., Victoria Street. 2. Will Copal brand varnish te suitable to paint the diaphragm? A.: No, use the preparation specified. 8.; Is the hole to allow the driving rod to pass through to be the exact size or slightly larger?-Slightly larger. Adapting the M.C. Speaker. "WW. "Pp." (Dunedin), in stating that ** "Pentode’s" dynamic coné speaker has "exceeded his rosiest expectatations," mentions that the drain on the accumulator is rather heavy. He ‘wishes to rewind the field magnet in order to adapt fhis to A.C. . A.: An A.C. dynamic cone speaker implies the use of alternating current

rectified and smoothed to the field windings. The effect ‘of A.C. on'these | winditgzs -has been explained in the reply to "T.B." (Christchurch). To operate. the dynamic .cone speaker directly from the mains, a rectifier and a filter system must be constructed. This would comprise a transformer to step the voltage down to about 12 volts, a rectifier to pass. sufficient current, a choke and condensers to filter the output. This would then be ‘broken down to 6 volts by a resistance and applied to the windings such as described by "Pentode." 2. Would a erystal and. three-yalve R.C, amplifier give full volume on the M.C. speaker ?? A.: It would probably give reasonably good results close to ‘the local station, But the result could not be compared with that of a well-construct-ed tranéformer coupled amplifier preferably using power valves in pushpull. 3. If the reaction condenser dial is, turned to "0," I can remove the grid leak without affecting results. Is there anything wrong? ’ A.: It appears that the grid condenser has broken down. ' : 4, I have 22 volts grid bias applied to two 256’s in’ push-pull in the final stage. Is this correct?--Yes,. "J A." (Auckland) asks the same * question, and his question is consequently answered by the above. A 25 to 1 step-down transformer will still be necessary.

Oscillation ‘Trouble. HAVE a 3-valve set made from a popular kit, writes "J.S." (Dunedin), and the set seemed to stop oscillating all at once, so I disconnected the lead from the ney .tive side. of the "QO" battery and connected it to the transformer, thus cutting out the grid bias on the power valve. The set will now oscillate, but I can only get "she local station. . ek A.: Test transformers, chokes and condensers. by the ’phones and cell method, though it would seem that the trouble is in one of the valves. A nearby dealer would test these for you. By employing the final valve without bias, the tone of i.e set is being spoiled. This is a most unsatisfactory compromise. . 2. The spindle of my loudspeaker has broken. Where could I purchasé another? -A.: Write the technical department, Philips Lamps, N.Z., Ltd., Hope Gibbons Buildings, Wellington. 5 The: Tetrode Amplifier. 3 "TC." (Blenheim) has -.constructed * the amplifier described by "Galena,’"’ which uses the tetrode valve. He has had very good results, including 1, 2, and 8YA, He asks if it is possible

, to replace the crystal by a vaive of the same type. . \ A.: It is possible, but it would require the altérdtion of the whole set. The tetrode valve has proved itself an excellent one-valve amplifier for a erystal, but when a wide voltage swing has to be accommodated, distortion takes. place. Overseas. Reception. ® cannot get any foreign stations, ' githough the New Zealand and Australians are very loud, complains "Worried" (Oamaru). c A.: Reception of stations other than ~ these is most uncertain, and very difficult in the case of an all-electric neutrodyne such as used by the correspondent. Almost invariably a reaction. using set is necessary, and aerial and earth conditions have to be favourable. _ "However, if the correspondent wishes to do a little searching, he may find some of the foreign stations in the following places. JONK, very ‘close to 2BL; two other Japanese on either side of 4QG; an American on Auck.land’s wavelength; and one on 2FCs wavelength. 2 Should the lead-in wire touch the walls? . ) A.: No. Even if they are insulated they should be kept elear of the wall for insulation can be worn away and a short circuit take place. 8. What length should be the aerial? A,: The aerial and lead-in should measure about 100ft: Beyond this, signal strength, static, and unselectivity are increased. Below this, signal strength is. lost, selectivity sharpened, / and statie less severe. Trouble With the Two R.F. ° CAN you assist me to get the best from my two.R.F. parallel feed Browning-Drake, writes ‘Constant Reader" (Blenheim): The R.F. valve (PM5) is controlled by a 30 ohm rheostat, and the detector (201A), by a 6 ohm rheostat which. also controls both audio valves (PM6 and PM256). A.: The detector being a 5-volt valve, should have a fixed resistance in series with its filament. It is being burned at 6 volts when it requires no ‘more than.5. This would account for ‘some of the trouble. If a 6-volt accumulator is used there is no necessity for the audio valves to be controlled ’ bv a rheostat.

4 2. I am using 90 volts "B" battery and 44 volts "C" battery. "Is this correct. A.: No;-double the voltage of the "C" battery. 8.-There is a fixed condenser, gipacity .0001 mfd. in series with the . aerial. I have short circuited this, and find that I can get more volume on stations above 300 metres (10,000 kes.). I cannot tune in the stations cn the higher frequencies because the tuning condenser is right. out. A.: It is evident that there are too many turns on the secondary of. the first transformer. If this comprises one tapped coil remove a few turns, otherwise take them from the secondary coil. The effect of a fixed condenser in series is to sharpen tuning and raise the frequency receivable. ‘By bridging it, sensitivity is increased, and the lower wavelengths (higher frequencies) can be-tuned in:: Purpose of Shielding. LT AM r.duilding. my set, writes "T.0.H." (Brooklyn). Would t ere. be any. advantage in using an eld ebonite nanet as hesehoss2.

"A: Yes, it would eliminate any tendency to short circuit, due to the dampness of the wood. , 2. What is meant by, and what is the purpose of shielding?+ = A.: This question was fully dealt with in the issue of the "Radio Record" dated March 15, 1929.. Shielding pre-: vents interaction between coils, and results‘in greater selectivity and stability. / A Push-Pull Problem. I CANNOT eliminate a high-pitched gqueal which variés in intensity with the speech or music received, writes "B.S." (Waiau Pa). My, audio amplifier consists of two valves in push-pull. The squeal can be sileticed py touching one of the grids, but accentuated by touching the other. I have ‘tried reversing the. connections to the primary using by-pass condensers, and ‘earthing the transformers. A.: It appears that the valves are unbalanced, but try the effect of a 2 megohm grid leak in the grid lead to each valve. If this does not prevent the howl test the valves to see if they are ‘balanced. — 2. I reversed the tickler leads. Is this all right?-vYes. 3. Would: connecting two speakers in series double the impedance?-Yes, 4, Can a #B.D." set be connected to the aerial without a condenser in series ?-Yes. 5. Is the 171A a suitable valve for push-pull and what grid bias shouid be needed? -A.: It is quite suitable-with 180 volts use 40 volts grid bias, with 100 volts use about 20 or 224 G.B. Points Concerning the Two R.F. B.D. GHOULD the sides of the box as well as the bottom of the transformer stage of this receiver be sheeted with aluminium? & A.: It is dlmost essential that this receiver be screened. Hach R.F. stage and the detector stage should be encased in a complete shield. Screening boxes such as adopted as standard by the "Radio Record" are equally suit-

able. 2, Where is the best’ place for the 2000 ohm resistance. and condenser? A;: There is no such a combination in the two R.F, Browning-Drake. 8. Should the R.F. chokes be evenly or jumbled wound? e A.: Jumble, wound. 4. How far should coils be from tuning condensers?-At least one inch. 5. Would a 3-plate neutralising condenser be suitable for the R.F. stages? -Yes. : 6. Does the metal tube for cable have to be earthed ?-Yes. 7. Should the 18 turns for the tickler read 18? A.: Try 13 at first and then if this is not sufficient increase to 18. The correspondent asks for discrimination between two transformers, but there is very little difference between any two first-class pieces of apparatus used in the average set. Neutralising Troubles. HAVE had difficulty in reneutralising my Browning-Drake, writes "ALR.S." (Taranaki). A.: Try by-pass condensers between the B positive R.F., the detector B positive, and negative. The correspondent. has asked for a circuit of a D.C. battery eliminator. This. will be sent to him within the course of the next week,

Meaning of G.B. " P.B." (Petone) asks the meaning ™* of GB+ and GB-. . A:: This refers to the "C" battery. GB standing for "grid bias." 2. What-ratio transformer could I use to add an extra valve, and what valve do you: suggest? A.: A transformer with 3) to 1 fadio is O.K. There are many valves suitable for this position. ~Your best plan is to consult a dealer. Peculiar Voltmeter Effect. WHENEVER the tickler is advanced to a certain point, the music stops, writes "L.C.S." (Trentham). When # voltmeter is placed across the "B" battery, everything is right again. 2. The set goes into oscillation with a sharp plop, and although I am using a variable grid leak, it ,does not improve matters. How can I make "the set go into oscillation smoothly? I have 24-turns on the tickler. A.: Both these questions point to a defective grid leak. It seems that the charge is being stored by the condenser and on the grid, and untila surge is set up by the current passing through, the voltmeter, the set cannot function. The violent oscillation may be the result of too many. turns on the tickler coil; reduce to 22. Generally speaking, variable grid leaks are unsatisfactory. 8. I cannot tune below 250 metres. Would a fixed condenser in series improve matters? A.: A very few coils designed for the broadcast band will tune below 250 metres. A series condenser capacity over .0001 mfds. will certainly lower the tuning a little, but there is very little below this wavelength to tune in. Eliminating the Local’ Station. "DEMUERA" complains that even with a wave trap he cannot cut out the local station. > A.: A loosely-coupled first radio transformer will greatly improve semew

lectivity. By loosely coupled we mean a transformer of two distinct portions. A small coil of about 25 turns is connected to the aerial and the earth, A secondary coil of about 50 turns connects with the grid and the filament. By arranging this so that the primary coil may be varied in its relation to the secondary coil, selectivity can be. varied at will. A series gpndenser in the aerial. lead will sharpen selectivity.