Two-valve Amplifier

Radio Record, Volume III, Issue 27, 17 January 1930, Page 27

Two-valve Amplifier

From "All About AllElectric" FIRST constructed "Pentode’s" original crystal and one-valve electric set, and obtained excellent results, writes Mr. G. Trevor Hill, of Christchurch, I departed from his specifications only in the turns: per volt in the power transformer, He gave 2500 and 2100 for primary and secondary respectively, but I only wound 1880 and 1200. I.used cheap 201A type valves for rectifier and amplifier and a erystal set I had in use before. Reception of, 8YA was excellent (I am about one mile from the station), but I should have liked more volume. There was absolutely no hum. When "All About the All-Dlectric" came out. I added 1.50 and 2.50 windings to the original transformer and constructed the two-stage amplifier, following the. instructions exactly in every way, except that I continue to use my original crystal set and the 400 — potentiometer I had, and got a 200 — potentiometer for the second valve. I use a 226 valve as first A.F. and a 508X as power. I have not seen this valve advertised and the characteristics do not seem to be published in the maker’s literature, but it is identical with 112A type and gives excellent results. These can be bought from a Christchurch firm for 8/6. I have used a full-sized unit horn speaker all through, and am obliged to detune considerably with the 2stager, as the volume is far too great for-comfort. I get a fairly loud hum which does not affect local reception, but is, I think, excessive, Later I shall try a 227 valve in place of the 226, to try and cut this hum down. In common with a large proportion of listeners, I expect, the finangial side of the receiver is the deciding factor in construction. 226 valves'can be bought for 10/6 and the 227 cost 15/-, so I. use 226 and tolerate the hum. I still use the 201A (R.F.) as rectifier, and it gives 15 m.a. without any apparent overload. These valves can be bought here for 3/6, so if the life of them is not long used in this way it does not matter. I have no means of measuring my rectified plate: voltage, but have measured the current and find the 226 is drawing nearly 6 m.a. and the 112A‘ 9 to 10 m.a., so the voltage must be very close to 160. I use 1500 ohm. resistance for the 226 bias, and about 1200 for the 112. My smoothing choke is constructed according to "Listeners’ Guide" specifications-1} Ib. 30 S.W.G. wire on 1}in. x lin. core, with two gaps. I don’t think it’s possible to obtain 1}in. stalloy in Christchurch, but lin. can be had, and I accordingly used that, making the piles ijin. high, instead of using 1iin. to lin. high. This would help anyone who cannot obtain the 1}in., and save them paying postage through having to send to Auckland or Wellington for it. Two other tips that may prove useful to other constructors: 1. I made a strap out of 20 g. brass sheet to join the pjate and grid terminals on the rectifying valve holder, simplifying the wiring of the set. 2. Instead of putting the terminal strips on the power transformer across the tops of the wooden clamps, I made

an ehonite panel 6in, x 3in., and screwed it vertically to the clamps at one end, keeping it out from the clamps with a distance piece on each of the four screws. I'arranged the terminals as on the commercial transformers, 11 of them; two 160 v., two 2.5 v., two 1.5 v., two 5 v., and for centre filament winding. It was then a simple matter to make a neat wooden case, open at the bottom and one end, that slides down over the whole thing when it is screwed down to the baseboard. This gives a finish to the component, keeps the dust out, and deadens any buzzing of the laminations. . I have had the set running for ten hours without switching off, and the wooden case has just got perceptibly warm to the touch. If shielding is required it would be easy to cover the box with thin sheet iron. Made out of tin, mahogany and polished, the transformer in it’s case looks like a factory job. The 230 v. lead,can be taken through the. baseboard inside the case, and I considered it a good plan to keep it off. the panel. I have engraved the value of each pair of terminals on the panel, and there is no fear of connecting the wrong filament voltage to either valve, In addition to BYA, 1 get 3ZC at very satisfactory strength on the speaker, and when both 38YA and 3ZC are off the air I can get Wellington on the speaker, but very faint (I can distinguish the announcements ‘and understand what is being said.) The hum spoils this reeeption, though. [The hum can be lessened by the usé of a-half-wave rectifier in place of the 201A -Tee, Ed.]