ANSWERS TO CORRESPONDENTS

Radio Record, Volume I, Issue 20, 2 December 1927, Page 13

ANSWERS TO CORRESPONDENTS

(By

Megohm

Crystal Queries,

"M.I,.? (Wellington) says: "I started on the ‘‘Record" crystal set and added the one-valve amplifier, which is great," This reader now wishes to know how to add a second valve. You will require another audio transformer, and if yon are wise it will be a Ferranti. Grid bias should be applied to each yalye by imeans of a C battery made for the purpose. You do not say what filament voltage you are using, but let the last yalve be as much a power valve as you are able to run, bearing in mind grid bias required, which may ‘~ six or nine volts for this valve, and, perhaps, 44 for the first. Grid bias required increases as_high-tension current is increased. To give your londspeaker a chance of delivering good tone, a choke filter should be provided for the output, and you will then have a fine amplifier. Various particulars have recently been given of this filter. The following diagram shows the ‘two-valve amplifier complete with ontput filter :-

. ~~ The input terminal marked 1 is connected to the side of the crystal that is not connected to the aerial, and 2 connects to earth. . A power valve is recommended in the last stage, but if: not used the choke filter may all be cut ont at points PP, and the ’nhones or speaker connected directly to these points, the marked positive tags to the B positive side. The fixed condensers shown must all be included to get best results.