Radio Club

Timaru Herald, Volume CXXXIX, Issue 20077, 6 April 1935, Page 10

Radio Club

TECHNICAL ARTICLE

(BY G.H.S.)

Converter No. 2 For the second section of our converter, we will need —if we decide to have tuning with two condensers—one .00035 condenser and another coil similar to the first one, but without the first three turns at the bottom end of the heavy winding. That is wind 4, tap, wind 4, tap, wind 7, finish; 3-16 inch away wind 8 turns 28ga. wire. We can mount this coil at the side of the 24 valve, and mount the condenser the same distance in from the left of the panel as the other is in from the right. This second condenser is .00035, the other is .00025. In the first converter a 100,000 resistor is mounted from the grid of the 24 and the aerial, the other end of the resistor to earth. If the two condensers are used this particular resistor is not required. This sytem of using a resistor instead of a tuned circuit is known as a periodic or untuned coupling, and is really a tuning system so broad that it responds to any frequency at all, hence any signal in the air will be present at the grid of the 24. By varying the oscillator tuning condenser, it is made to beat with any one of this station’s signals, and so form the beat note. However, any untuned system introduces certain loss, and for those who wish the maximum gain use the second tuning condenser. We advise getting the converter going before completely coupling up the second stage. To couple this up disconnect the aerial from the grid cap, and completely remove the 100,000 resistor. Join the bottom of the fine winding, the bottom of the heavy winding, and the moving plates of the condenser to earth. Join the top of the heavy winding to the grid cap, and the top of the fine winding to the aerial which was disconnected from the grid cap. Fit a clip to the stator plates of the first condenser so it can be clipped to any one of the taps as in the first coil.

The two types of converters embraced in these articles are:—One with one coil and condenser, easiest to get going and simplest to operate; the second with two coils and two condensers, slightly harder to operate but'

having increased gain. At a later date we will explain how the untuned style may be left permanently connected to the b.c. set to which it is coupled, to give increased gain to the lug set. If the constructor wishes a .01 condenser shunted by a 5000 resistor may be fitted in series with the lead from the cathode of the 24 to the three turn winding at the bottom of the oscillator coil, and a grid leak and condenser of .00025 and 100,000 ohms respectively inserted in the grid lead — the top of the heavy winding to the 27. With these inserted. 250 volts may be used as the H.T. supply, without them 45 to 90 is suitable. Now for the drawbacks of converters. Don’t expect them to work on any set. None of them will work when connected to the aerial terminal of a set with a band pass filter or one with an aerial trimmer or tuning arrangement. Practically all five-valve super-hets have band pass filters. In these cases the output must be connected to the grid of the first valve in the set. The top cap in the case of supers, under the chassis where there are no capped valves. Don’t expect it to work on a set of less than five valves. Six are generally required. Don’t expect the short wave stations to rock in like broadcast. They just won’t. And don’t expect to get a station any time you tune in, and tune very very slowly when you are tuning.

Set the condenser clips on the same taps on each coil, say the seventh from the top, and try to find G.S.B. London between 8 and 10 at night. Almost on the same degree on the dial are 2ME and 3ME on Sunday afternoons. D.J.A. Germany is also right in this degree. On the first taps up on the coil GSD London and FVA Paris should come in the former between 8 and 10, the latter on most afternoons. WBXAL should come in on the total turns connection—not very loud though. The converter covers from about 15 to 90 metres. A much more elaborate converter has been built using a 2A7 valve and embodying several features that so far as the writer is aware have never bee^- incorporated in converters. This

would be too complicated to explain in these columns, but the circuit and any required information concerning it and the ones described may be obtained on sending a stamped addressed envelope to the Radio Club.