An Amplifier for Quality and Volume.

Radio Record, Volume II, Issue 16, 2 November 1928, Page 26

An Amplifier for Quality and

Volume.

Details of ‘‘Push-Pull’’

By

Pentode

= I’ is not so very long ago that the wireless enthusiast built a receiver with the only available comee oe| ponents, the best in those (oem, days, and set and listened in awe to the barely recognisable speech and music issuing from throaty horn loudspeakers. Given the same components, it is doubtful if we could do much better to-day, except that the greatly improved transmission might assist to some extent. Dealing pure-. ly with the amplification of received signals, practically no difference exists between the circuits used in the very latest products and those used some years ago. , Power Valves. QE development which has greatly assisted radio to take its place in the home as a soure@ of music rather than just a hobby, is the use of power valves with their associated high plate voltages. It has been very truly said that, using but one valve in the last audio stage, real music cannot, be obtained with less plate voltage than some two or three hundred. . The inconvenience of supplying this voltage and the danger associated with it militates very strongly against its use, however, and we are forced to look for some alternative means of obtaining the required power output. By "power output" it does not necessarily mean volume. Taking the range of audible musical frequencies from thirty to ten thousand cycles per second, very much more power is required to produce notes of, say, sixty cycles, than a note of, say, a frequency of two thousand, with the same intensity. To obtain faithful reproduction of either music: or speech, none of the lower frequencies must be suppressed, as they certainly would be if there was insufficient power available to operate the speaker at the low frequencies under discussion. Certain types of

speakers tend to accentuate the low notes and try by this means to add what the amplifier will not give. The best procedure by far is to utilise an

amplifier that will faithfully amplify the low notes and not to rely for the bass notes on the booming and druinming effect produced by these speakers. Push-Pull Amplification. RETURN ING to the design of the am. plifier suitable for the amateur’s construction and use, it has previously been mentioned that the dangers involved do not warrant the use of high-plate voltages. There is another alternative circuit which can be used to give exactly the same effect as the other system using high-plate potentials. This is known as the push-pull method of amplification. Using tw valves in push-pull. the voltage input which can be accommodated (and it is the ability to handle the input rather than give the output that denotes a "nower valve") may in fact be as much as between three and four times what each valve would accommodate singly In the first place. she input is dividen equally between the two ‘valves, and in the second place, each valve can, if i very great output is desired. be so biased to the extreme bottom of the characteristi¢ curve that it becomes quite safe to use larger maximum plate voltages than those specified by the maker of the valves in question. However, this is a technical point which need not trouble the constructor. Although ninety volts will operate the amplifier about to be described, it is advisable to: go up to 135 or 180 if

possible. The results obtainable will be equal to if not better than those obtained by using a single valve in the last stage and two or three hundred volts on the ate, Some suggestion regarding suitable valves and correct grid bias potential will be given later. The in evitable circuit diagram is given in figure 1, A small fixed condenser is inserted across the input terminals: In the transformer used by the writer (Ferranti) this condenser is ineorporated in the shell of the transformer and is omitted in the construction, This condenser helps to by-pass high frequency currents from the detector as, if these currents pass into the audio side, grave distortion is liable to be set up, very often evident in the form of a high pitched whistle. The ‘Transformer. A much longer article than this would be needed to explain the theory of push-pull amplification, but on referring to the diagram it will be seen that the primary of the second transformer is used in the normal. fashion. The secondary is divided into two equal halves, the two ends joining to the grids of two separate valves, while the centre tap is connected to C. battery negative, An output transformer is necessary. The primary in this ease is centre tapped which joins to B bat-

tery plus, while the two ends are cohnected to the plates of the two valves. The secondary connects directly to the speaker terminals and thus it will be seen that the speaker is isolated completely from the B battery. No direct current will be flowing through the windings and therefore it is immuterial which way the speaker leads are connected.

Constructional Detz"'s. BELOW is given a list of the parts to be used. Because these components are mentioned, it does not necessarily mean that they are the only ones suitable. Practically all pre-sent-day manufacturers of repute produce components that can be relied upon, but one word of warning would not be amiss. It is not good policy to buy cheap material, There is a saying, "Why spoil the boat for a hap’th of tar." if the constructor decides on making a good amplifier, why spoil it by using even one transformer that happens to be a bit cheaper than one made by a firm of repute. The present case may be likened to a pipe through which a stream of water is passing, One obstruction anywhere along the tube will influence and retard the even flow of water from the outlet. . ' The baseboard, 15in. x 6in, x 5-8in. can be planed and stained any dark colour. No cabinet or front panel is used in this set, but a box in the form of a lid could easily be constructed to protect the whole from dust. . Parts required :- »

1 transformer (Ferranti) AF4 or A¥F3. 1 Push-Pull Transformer AF40 or A¥8C (Ferranti). 1 Output Push-Pull Transformer OP6G or OP38C (Ferranti). 2 yp’ -e sockets. 1 Ingranic porcelain rheostat, 6 ohms. Terminal strip, 6in. x 17in. 6 terminals. Baseboard 15in x 6in. x 5-8in. Busbar, sundry serews, etc. — The layout is quite symmetrical. On the left of the baseboard is screwed the first transformer AF4 or AF3,-with the input (marked primary) terminals on the left. In the cenire of the board the second AF8C or AF4C is screwed into place with the three terminals towards the right of the board. On the extreme right the output transformer is fastened with the speaker or output terminals at the opposite end to the input terminu!s on the left of the poard. At the bavs between the first 2ad second transformer, a valve socket is screwed with the grid and plate terminals or soldering lugs at the back as shown. In front of this is the rheostat. The Ingranic rheostat used can ee adapted to baseboard mounting and ‘the dial fastened by the rub screw so that it can be operated from the top.. Between the second. and third transformers are fixed the two remaining sockets with the grid and plate terminals at the back as before, Six terminals are arranged equidistant along the top edge of the ebonite strip, so that when this is screwed into place on the back of the board in the centre, the termin-

al nuts will clear the wooden base- ~« board. | Wiring. . O much for the layout. ©The next procedure is to wire up the comjonents. . .For this purpose it is preferable to used covered wire. Glazite is quite suitable although by far the easiest to handle is No. 18 SWG bare tinned copper wire and use pieces of spaghetti covering cut to the ‘quired length to insulate wires that are likely to touch. If valve sockets provided with terminals are used, then the whole assembling can be done withiut the use of a soldering iron. Connections to

the terminals can be done by two nuts at the back of each binding post, although a soldering lug and a small touch of solder makes a far more substantial job. — By referring to th diagram it is ‘quite a simple matter to follow the wiring. A. point to point connecting scheme will, howeve , make any Ppossibility of error very small. The filament wiring should be completed first. Join the. second to the left hand ter-. minal at the back, marked. A-B-C-, to the right hand terminal on the rheostat, The wire is cut to length, and a length of insulating sleeving slipped

over before the terminais are tightened up. ‘The same procedure is necessary with all the wiring, and-.even if two wires touch, there is no fear of a short.cireuit. The left hand t minal of rheostat is connected tc one filament terminal of each socket. The remaining three filament terminals are connected together by one piece of. wire which also is run to the first terminal on the strip on the left. This ‘completes the filament wiring. The third terminal marked C-1 runs direct to terminal «-arke’ "7 bias or ¥- on the first transformer. In the same way C-2 is connected to the centre tap. on the secondary side of the secohd transformer. The fifth terminal marked B-+-1 is connected to

B+- on the primary. side of the centre transformer. The centre terminal on the output transformer joins. directly to B+2 or the sixth terminal on the extreme right of ebonite strip. Lastly, the grid and plate wires are connected to their respective places on ihe transformers. Grid of first transformer to grid of first valve. Plate of. first valve joins to terminal marked P on primary side of centre transformer. The two terminals marked G on the secondary side of the centre transformer are joined to the two grids of the last two valve sockets. Finally, the plate terminals of these two sockets are connected to the two

terminals marked plate on the primary side of the last output transformer. This completes the construction of the. amplifier, and it is all ready to connect to the batteries and have a trial run on the local station as received in a crystal receiver. This amplifier, it may be added, will vive good distortionless volume. when used in conjunction with any of the numerous gramophone pick-ups avail. able. Join the pick-up with necessary volume control to the input side of the amplifier. Providing the speaker will reproduce faithfully, then it will be astonishing to hear notes during a piece of music that were never heard before

on the record as played on the ordinary gramophone, Kither two, four or six-volt valves can be used in this amplifier, and below is a table showing the recommended grid bias and plate voltage for the pumerous valves on the market. If ninety volts alone is to be used, then B-++-1 and B+2 should be joined together and run to B+90. Grid bias must always be used with this amplifier.

First Stage Plate voltage PMI LF A209 PM3 B409 BAl5 PM5 B609 B615 UX201A 90 volts 90 volts 90 volts 90 volts 90 volts 90 volts 90 volts 90 volts 90 volts Grid bias C-1 Second Stage (Push-Pull) Plate voltage 3 volts PM2 44 volts B205 B203 Four Volt Series 44 volts PM4 44 volts B406 3 volts B4038 Six Voit Series. 44 volts PM6 4} volts BG6O05 4}: volts B603* 44 yoits UR20IA UN112 90-150 90-150 150 90-150 90-150 150 90-150 90-150 90-150 90-180 90-180 Grid bias C-2 12-20 12-20 Bi) 12-20 12-20 40 12-20 12-20 16-40 12-25 12-30

Two Volt Series.