The Shield-Grid R. F. Booster Unit

Radio Record, Volume I, Issue 48, 15 June 1928, Page 12

The Shield-Grid R. F. Booster Unit

' ADAPTING THE 8625 VALVE wee

YE shield-grid R.F, unit, described on June 1 for use with the UX222 valve is equally well suited for use with the $625, the British type of screen-grid valve. The only difference necessary in connection with the use of the 625 is i. the actual connections to the valve, and the matter of grid-bias. In the case of the 222, which only requires 3.2 volts on the filament, the small necessary grid-bias is obtained by the "drup" across @ resistance or the rhe«stat in the negative filxment leg. In the case of using a 6-volt battery a 10-ohm resistance is xy laced in the negative leg, and the rheostat in the positive lex. With a 4-volt battery the rheostat is placed in the negative leg, and gives 2 small amount of negative bias, if ete 625 requires from 5.5 to 6 volts on the filament, so that the necess.ry grid bias should be provided by @ ii-volt flashlight cell, which may conveniently be one of the sinallest site, suitable fo1 inclusion within the phield, It might be said here that in ease of using the higher plate voltage of If. volts on the 222, extra grid bias may be provided on a 4-volt circuit by inserting a dry-cell between the roter o£ condenser and the rheostat.

‘ ‘ ‘ VALVE CONNECTIONS, TPHERE are five prongs on the 625, two at one end and three at the other, The two are the sereen grid and plate connections, the latter marked with an A for "anode." At the opposite end are the two filament legs, the third being the ordinary control grid connection, in its usual relative position. bd A® ordinary British socket is re- : quired, to be mounted in place of the UX type. Only the plate and grid connections will be made to this, the grid terminal in this case beings the screen grid, Instead of the two filament wires connecting to the socket, they must be sufiiciently long {to reach well above the top of fhe valve, and each wire must terminate in the smallest socket procurable, and these slip over the respective filament prongs, und are easily removable at any time, The control grid conneection from the top of the coil may be made by means of : small battery elip, just as in the ease of (he 222, IIEN experimenting, the writer placed the valve in the Klosner UX socket already in place, with plate in usual position, and by slightly closing the spring on the opposite filament position and making the screen (grid connections to this, the holder

auswered quite well, so that this type of holder with a flat grip each side of the prongs can be used, All connections are as shown in the theoretical diagram on June 18, exeept that the 10-ohm resistance is omitted, and a grid-bias cell is placed in the grid lead at the point marked X. The plan diagram of wiring herewith shows. the addition of the gridbias cell between moving plates of condenser and filament negative. This means that in the pictorial view the dotted lead marked 4y. is omitted, and the 14-volt cell is placed in the dotted lead marked Gy., which then continues under the board to join filament negative lead. TROM the battery cable the A positive goes to one side of rheost..f, the other side to one of the filament prongs at top of valve. The A _ |

negative lead from battery goes direct to the other filament prong, and is joined under the board by the lel from stator via grid-bias cell. fs PLATE yoltage of 90 volts is shown on the original diagram, but any voltage up to 185 volts may be used on the 222 and 120 on the 625, aid the latter a maximum of 80 volts on the sereen grid. The grid bias on the control grid is from 0 to 14 volts negative. Referring to the plan wiring diagram. the wires running down through holes in baseboard are numbered and connect as follows:-1 up through 6 to filament positive; 2 to A positive battery lead; 3 under board to connect to wire coming up through 5 to filament negative; 4 to B battery low yoltuge lead in cable; 7 to B battery high voltage in cable, IlXl three connections to the top of the valve are shown with arrow points, the dotted line from top of coil being the control grid connection with small battery clip or socket. The A battery must always be switched off when connecting «» disconnecting filanent vires. The $625 is retailed at 30s. The supply of all shield-grid valves is at present:very low in New Zealand, but good stocks will -rrive at an early date. Amalgamated Wireless, Ltd., imported as an initial stock what was considered to be a large quantity of the 222, but so popular has the valve proved that the stock was soon cleared, and those at present available are any that remain in the retailers’ hands. The Marconi 8625 shipment is due to arrive shortly, and the same applies to the Osram 8625, for which the British General Electric Co. are wholesale distributors,