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How to Make a One-tube Reflex Set That's a "Knock-Out"
2 I
such as the WD-i i, WD-I2, all the "C" tubes and UV-20I-A. The resistance of both types varies with the current in such a manner that slight fluctuations above and below the normal battery voltage do not produce a corresponding change in filament current. This property of varying resistance is the chief asset and, strange to say, the greatest drawback of this type of ballast resistance. For, when a battenis applied to a circuit containing a fixed resistance (such as the filament of a vacuum tube) and a varying resistance such as an "Amperite" the initial current is goverened solely by the sum of the value of the fixed resistance and the Amperite — when "cold." And, unfortunately, the resistance of an Amperite is much lower when "cold" than when heated by passage of current — therefore the initial current is in excess of the proper value and is quite harmful to the filament of a tube.
For this reason and also because ballast resistances are not made for all types of tubes we have for some time been using fixed wire resistance strips which may be slipped into the regular mounting. They are easily made, and
a
MOUNTING FORT3
AMPERITE M'TlG.
Q
a
ANGLE BRACKET
FIG. 8
Plan for the sub-panel. This is in correct proportion,
but less than actual size ({ J). For a holes, use No. 27
drill; for b holes, use No. 52 drill
o
a
ANGLE BRACKET
5 Id) HOLES *27 DRILL
a O
FIG. 9
This is the bracket (half actual size) that fastens the sub-panel to the main panel
with the proper length and size wire may have any value of resistance.
The first few were made from portions of the resistance element of a regular 6-ohm rheostat. Each portion was 2" in length and utilized the resistance wire salvaged from the rheostats.
A much neater job can be made however if f " insulating rod cut into 2" strips is fitted with two metal end pieces and wound with the resistance wire, both ends of which are soldered to the end pieces. The rod should be threaded so the wire will not slip and short circuit adjacent turns. Small thumb tacks or similiar devices have been employed as connecting end terminals.
It is necessary to know or determine the resistance per unit length of wire in order that the proper amount may be employed to offer the correct resistance.
The following table shows the value of resistance for use with different A battery potentials on various tubes in order to restrict the current to a point slightly below the normal consumption rate.
TUBE
BATT. VOLTAGE
RESISTANCE
CURRENT
(Representing i, 2,
and 3 storage cells
and i, 2, 3, and 4
dry cells.)
UV-20I-A
6.0
6.0
•23
UV-20I-A
4-5
0.*
.22
UV-20I-A
4.0
0.*
.20
WD-I2
1-5
2.0
•23
WD-I2
2.0
4.1
.23
UV-I99
3.0
0.*
.06
UV-199
4.0
18.0
.06
UV-I99
4-5
30.0
.06
UV-199
6.0
55.0
.06
UV-20I
6.0
1.5
.92
*Direct short. May be made like other resistance strips, but has only a copper wire joining the end terminals.