Adapting the Browning-Drake to the Shielded-Grid Valve

Radio Record, Volume I, Issue 40, 20 April 1928, Page 12

Adapting the Browning-Drake to the Shielded-Grid Valve

worm, ILE March number of ‘‘Ra~ dio" describes a method of using the UX222 shieldfeet} ed-prid valve in the R.F. gea| stage of the BrowningPaommeats, Drake by a few simple changes. The main proviso is that the R.F. and detector stages must both be completely enclosed in copper scteening boxes, the top, bottom, and sides all being of metal. Many constructors will fight shy of the screening, but it is well worth the krouble, ‘even when only ordinary

valves are used, as both quality and selectivity are gained by its use, and if the screens are roomy, so that a space of at least 1; inches exists around and above all coils, and 1 inch at the bottom in the case of vertical coils, practically no damping of the citcuit will result. Those who have constructed the short-wave receiver with shielding as specified will have experience of the benefits it gives, and will have found also that working in sheet copper is a comparatively simple and satisfactory process, and not the difficult operation that it might appear beforehand. "The requisite changes include (1) shorting out the primary of the r-.f. transformer and substituting a 4 mfd. condenser and'r.f. choke, (2) shielding the c.f. stage, (8) inserting a 4 mfd. condenser between tlie valve’s plate and a connection to ground, and (4) putting a 10 or 15 olim resistance in the filament circuit to reduce from 6 te 3.3 volts. A neutralising condenser is used to balance the very small capacity betwen the valve’s plate and control =

grid, thus giving greatest efficiency for the regeneration on the rf. transformer, ‘These changes are shown in the diagrany. "The primary of the r.f. transformer is shorted out because of the valve’s very high impedance, ‘This direct coupling (really an auto-transformer) requires a parallel feed system consisting of the 4+ mfd. condenser -and choke coil, thus keeping r.f. current out of the B supply. The valve’s plate ig connected directly to the stator of the second tuned circuit, .

"As this puts the plate-screen grid capacity across the second tuning condenser, it may be necessary to put a 15 mmfd. condenser across the first tuned circuit, as shown by dotted lines. In many cases, however, especially if the .0001 mfd. series antenna condenser adds enough capacity, the trimmer condenser will take care of any difference between the two sitigly-controlled tuning condenser settings throughout the waveband. A few experiments will determine whether it is needed in a given installation. "Suitable shields and instructions for their use may be secured from the Browning-Drake Corporation. _ ‘The } mfd. condenser between the grid and ground is essential when using: this valve. ‘The 10-15 ohm resistance in the filament circuit not only cuts the 5 volts used with the 800-A or 240 detector to 3.3 volts for the screened grid tube, hut also supplies biasing current. If a 8-volt supply and -99 valve is used as a detecior, this resistance is mot (Continued foot of next page.)