The Two R. F. Browning-Drake Unit

Radio Record, Volume I, Issue 49, 22 June 1928, Page 14

The Two R. F. Browning-Drake Unit

THE FIRST COMPLETED —

THE first letter to hand announcing completion of the two R.F, and @etector unit for the Browning-Drake is from Auckland, and reads as follows :- 7 HAVE completed, and have in operation, the 2.R.I". shielded Browning-Drake as described by you in the ‘Radio Record’ I have finished the shielding by oxidising and lacquering, and the completed receiver is worthy of a place in any home. I am using a separate two-valyve amplifier, or perhaps I should say that I am using one stage of a two-valve amplifier because it is impossible to use the two owing to excessive volume, even on Australian stations with reaction at zero. Selectivity leaves nothing to be desired when I can get 38YA Christchurch or 2BL Sydney with Auckland working }-mile distant only, and no background. Iyven with the foregoing praise of this receiver I do not think that I am getting the results that the set could deliver with a little adjustment, and in that particular I would be obliged if you could advise me on the following points... ." HIS constructor then asks three \ questions concerning the working of the unit. The first deals with the matter of filament voltage, which requires adjustment for each station, and the other questions refer to primary turns and neutralisation. The fact is, that this- constructor is obtaining too much amplification per RF. stage, which is a thing to be avoided in all receivers with more than one R.F. stage. In commercial re-

-eeivers, R.F. amplification is kept low, in order to obtain undoubted stability under any conditions, with perhaps a different valve, and unskilled operation, and to make up for the comparatively low amplification per stage, an extra stage will be added, and the advantage of greater selectivity gained, for each tuned radio frequency stage increases selectivity. UPPOSE we have one R.I. stage working on a veceiver of any type, and from this stage the highest possible amplification is being obtained. We will give this amount of amplification the value of 10. Now we cannot add extra stages and get an amplification of 10 from each, and at the same time preserve stability. But suppose that we have a stable circuit with an amplification of 8 from each stage, then the total R.F. amplification is Gt in place of 10, and with three stages it might be 7 per stage, giving a total of 348. | This is given to show the principle upon which R.I. amplifieation is to be worked. Stabilisation of R.F. stages is the suppression of any tendency to self-oscillation of the yalves, Which is chiefly met with on the lower wave-lengths. Other condi-. tions being suitable, neutralisation forms an effective means of stabilisation by balancing out the stray capacities of the circuit, and shielding reduces to 2a negligible minimum the in-| teraction that would otherwise be pre--sent between one stage and the next. T is a recognised fact that an amateur constructor expects to obtain greater amplification per RI. sfage

than is usual in a commercial receiver, and there is no harm in this, provided the gain per stage is not overdone, The commercial proposition is a receiver that will suit all conditions and unskilful operation, whereas the constructor knows, or soon will know, sufficient to enable him to handle his receiver quickly and efficiently, correcting any tendencies that might prove troublesome to a less experineed operator. The construetor knows all that his set contains, and is, or should be, keen upon learning the purpose of each part, and how it should be handled. , It is impossible for two constructors to build their sets exactly alike, even from the specifications published, as there may easily be slight differences in sizes of coils, and placing of parts, and then there is the choice of valves, which, of course, leaves a loophole for considerable differences. Nervertheless, the writer does not anticipate that constructors will experience any greater difficulty in this work than on any other circuit, especially if they haye already had the experience of constructing the four-valver. A WARNING was given to constructors to look over the coils when in position in order to ensure that adjacent turns are not touching anywhere, as such a happening causes’ great instability, and was the experience of the writer at the outset, a fine hair-like shred of solder making a loose connection between the two bot- | tom turns in this detector stage, a varying contact that would sometimes | change with the rotation of the tickler, and so insigniiicant as to be diflicult to locate. Apart from this trouble, the unit neutralised with ense, and functioned well from the start, using PM3's (of which PMO is the six-volt equivalent) with 16 primary turns. ‘OMMERCIAL Browning-Drake sets with two R.F. stages haye been turned out of the factory for some time, and the writer is of opinion that bogeys raised by some people about the diffieulty of adding an RF. stage to the Browning-Drake have been the outcome of half-hearted attempts to earry it out, One correspondent states that his dealer told him he would gain nothing by carrying out the method advocated by "AMegohm." This is sheer nonsense, and if the dealer in question took the trouble to construct the unit, adhering faithfully to the specifications, he would find that his ideas would require revision. Hill extra RE. stage gives a great improvement in quality of reception over the four-valyer, becuuse its erenter reserye gets signals well above the noise level, so that the — background of rushing noises, usually present, is minimised when receiying New Zealand or Australian main stations. Our correspondent is to be congratulated upon his construction, and when he has cut down the amplitication to a reasounble amount by reducing primury turns, he will still be well satistied with his receiver and its performnee, [It is as well to remind constructors of the two RI. unit that something more than ordinury in the way of an audio amplifier is required to efHciently handle the heavy volume which the unit is capable of delivering, and this matter will have early attention. ecae cmencacnen eenee*@.ase a*aeane arene