Laboratory Jottings

Radio Record, Volume III, Issue 10, 20 September 1929, Page 20

Laboratory Jottings

Osram Power Valves, Two interesting valves have been given us for test by the British General Electric Company, Wellington. They are. Osram PS625 and P625A. The former ig am'‘excellent.example of the 171A type of valve with the following characteristics, the valve is eminently suitable forthe last stage of a medium receiver. Used in push-pull it gives very fine household strength in a suitable amplifier, such as that described | in other ‘pages of’ this issue. P625A isa representative of the 245 -type of valve, and is one'of the first ‘of ifs type to-appear on the New Zealand market. It has the decided advantage of giving a volume step-up as. well as being capable of handling considerable voltage swing without any. trace of distortion. The writer was _ impressed by a very simple experiment made with a three-valve set. ‘This set had ‘shown itself capable of very fine performances, and it was known just what it could accomplish. The last valve was removed shortly before 3° o'clock one. afternoon, and P625A substituted, the requisite 250 volts being applied to the.plate. 3YA’s carrier was tuned, in and .at 3.o’clock the chimes and the following announcements came over with wondérful strength. It wasimpossible .to. believe that this was Christchurch and not Wellington. Disbelieving that a power valve could make such a .wonderful improvement, the writer replaced it by an ordinary pover valve. Volume immediately dropped. Th: 245 valve is enjoying wonderful -popularity in the States. It is being featured in most of the newest radio receivers, Unfortunately the writer .was unable to test these valves in the power amplifier. | Undoubtedly they would give very fine results if a bias resister of about 750 ohms were used. Pilot Radio Products. ‘ABEL SMEETON LTD., Auckland, have submitted a very comprehensive range of Pilot products. These have‘ been given an extensive test both from the mechanical and the electrical point of view. The general conclusion is that the line is first quality. A constructor who uses these products may _be certain that as far as the products themselves are concerned, trouble can-. not arise, for most of the trouble eneountered by constructors has its origin in defective parts. Undoubtedly the best proposition is first grade components, and Pilot are first grade. Pilot Centra Line. Condensers. Constructed of brass plates, supported by.an aluminium frame, Pilot condensers are both accurate in their. movement and of good appearance, They have three terminals, two to the fixed plates, and one to the moving. This latter is connected to the frame, and this to the moving shaft by a rat-. tail connection, ensuring perfect noiseless operation. The condensers are of the straight line frequency type, and are made in the following capacities :- .00016, .00025, .00085, and .0003 mfd. maximum capacity. The minimum capacity of each condenser is about .00001. By the use of a special coupler, the condensers: may be ganged. Illuminated Drum Dials. With the modern tendency towards, single drum dial control, in view, Pilot, Electrie Company have brought out. a full range of illuminated dram dials. When mounted.on a panel, the covering

plate looks very much as though handtooled from a solid -bleck of metal, A ‘gingle artistic knob about the size of a shilling, is mounted on this, and this controls the drum. The movement is very smooth and fine. A six-volt lamp "hidden from sight illuminates the dial. Pilot Transformers. _ A wide range of these is manufactured. They include inter-valve transformers, output filters and transformers, push-pull transformers, and B eliminator chokes. A special-.feature is thatthe whole winding and laminations are ‘encased in a moisture proof bakelite ease, One of the tests that these trans:. formers haye to go through is an immersion test, in which, for a period of thirty days they are imunersed in water. The transformers under’ test. were inadvertently left in the wet, and then put- back into an amplifier with ‘no loss to tone or quality. A very true over-all reproduction results through the use of these, A Battery Eliminators. WE have tested, with quite considerable success, an A_ eliminator the "Wellmayde" trickle charger with one of the new type,electrolytic condensers shunted across the output terminals. . These condensers have a capacity of approximately 2500‘ mfds., and in consequence have a great smoothing effect. The. A _ eliminator thus constructed will deliver about .35 amps of reasonably smooth A current. This is suitable for a three-valve set. It was found that there was a slight ripple which disappeared when an audio frequency choke wound with 22 D.C.C. wire was placed in series with the positive lead from the terminals; this slightly lowered the voltage, and it was necessary to use 4-volt valves, with the six-volt tapping. A variable resistance allowed the final output to be accurately adjusted. This small charger and:the electrolytic condenser is an excellent combination for supplying: power for the field windings for a 6-volt dynamic speaker, the condenser smoothes out all tendency to ripple, giving an almost perfectly silent background. Ceco Valves. OND and Bond, Ltd. Auckland, have submitted to us for test types of their "Ceco" A.C. valves. Type N27 is a valve of the indirectly heated cathode type, with the following characteristics: Heater voltage, 2.5; I. current, 1.7 amps.; plate current, 2 milliamps. ; 8000 ohms; amplification factor, 8; anode voltage, 45. This type of valve which we have selected as a first audio stage, for the’ A.C. amplifier described this week, -has been designed to eliminute hum. This may be claimed’ for the Ceco valve that we have tested, for in all respects, it has come up to the very high standard that has been set for it. It is interesting to note that this type of. valve is coming very much into prominence in A.C. seis. The Ceco valve is of robust construction, and almost without hum, gives distortionless reproduction. M26: This is also an A.C. valve, but has a directly heated filament>which requires 1.05 amps at 1.5 volts. With

an ‘impedance of 7400, amplification factor of 8.2, this valve compares fayourably with others of its type: 135 volis.are needed on the plate, with 5 volts grid bias. ‘The directly heated type of valve is much simpler in its application to use than the 227, but there is slightly more hum. With the valves tested, hum was very indistinct, and well to the background. J71: This valve is of the familiar 171A type, the characteristics of which are well-known to all, as a final stage power valve. J71 will be found equal to anything of its type. It can handle a wide voltage swing without trace of distortion, and does not drop volume more than any. other power-valve. To obtain maximum results when using 180 volts on the plate, grid bias of 40 volts should be employed. A.C. or D.C. ean be applied to the filament with equally good results,’ . yO Our test has convinced us that Ceco valves can justly claim position among first quality valves. Radiokes Coils. . (ort winding with the amateur is usually a difficult task, and very many ‘shirk from set-building because of this, but with the wide range of commercial coils now obtainable this need no longer be a hindrance. Abel Smeeton, Ltd.,. have sent us three types of Radiokes coils for use in our circuits, and they have given every satisfaction. Mechanically they" are excellent pieces of work, wound in green’ double silk-covered wire and mounted ‘by narrow strips of bakelite on the low loss, principle, they are électrically perfect. The wide range of coils allows the constructor to build all the usual types of circuits-the Neutrodyne set neutralising from the primary is probably the safest for the amateur: constructor to use. The construction of a neutrodyne is, by the use of commercial coils, . rendered a very simple operation. The Browning-Drake- coils have a diameter of 23 inches, and when widely spaced, render: shielding. unnecessary. Neutralisation is.carried out in the latest style devised by Glenn Browning, the originator of this circuit, that is, by an extra winding to the secondary. This elminates a great deal of the trouble formerly encountered in the Browning-Drake. A set of screen gyid coils enables an amateur with a little experience to construct a highly efficient screen grid receiver, using two stages R.F. In addition, full ranges of shortwave coils are manufactured and ‘these can be used in the ordinary shortwave set. Pilot Resistograds, THIS is a high-value variable resist- ' ance ranging from 40 ohms to 10 megohms. This piece of apparatus is able to handle 20 watts, that is, at 200 volts it can pass 100 milliamps. ‘This makes it highly suitable for eliminators designed to handle heavy current at high voltage. While on test, the resistograd was used in an apparatus of this type, and at 250 volts the resistograd passed 105 milliamps." It is thus. perfectly suitable for all types of eliminators. .As a volume control Pilot Resistograd functions perfectly.