Gas Test Circuit

d. The partial schematic diagram (fig. 10)

through the circuit. The decreasing

current will flow from the cathode to

shows the various switch sections used and the

Contacts made to test a type 6SK7 electron tube.

the plate of the 6SK7 and through

Refer to the simplified circuit (fig. 11) for the

R103 to the cathode of V101. Current

operation of the circuit (e below).

will flow from the cathode of V101 to

pin 2, and through secondary No. 1 of

e. When pushbutton switch S110-3 is de-

T101 to the junction of R118 and

pressed, a voltage of approximately +150 volts

R115 and R137 in parallel. Current

dc is applied to the plate of the tube under

will divide at this point. Some cur-

rent will flow through R115 and R137

( 1 ) When the polarity of the voltage

to the cathode of the tube under test;

across secondary No. 2 of T101 is such

the remainder of the current will flow

that pin 3 of V101 is positive with

through R118, M101 and C103 in

respect to the cathode of V101, the

parallel, R111, and R113 and R135 in

signal voltage applied between the con-

parallel, to the cathode of the tube

trol grid and the cathode of the tube

under test.

under test will swing in a positive

( 3 ) With unbalanced currents flowing

direction, thereby decreasing the bias

through M101 on adjacent half cycles,

a n d increasing the current flow

and unequal forces applied to the

through the circuit. The increasing

meter pointer, the deflection of the

current will flow from the cathode to

pointer will be proportional to the

the plate of the 6SK7, through R103,

difference between the two currents.

a neutralizing resistor, to the cathode

T h e heavier current flow through

of V101. Current will flow from the

M101 occurs when pin 3 of V101 is

cathode of V101 to pin 3 and through

positive with respect to the cathode

secondary No. 2 of T101 to the junc-

of V101, and results in an essentially

tion of R111 and R135 in parallel. Cur-

steady, upscale reading on the meter,

rent will divide at this point. Some

current will flow through R113 and

11. Gas Test Circuit

R135 to the cathode of the tube under

(fig. 12)

test; the remainder of the current will

a. The plate voltage for the tube under test

flow through R111, M101 and C103

is supplied by secondary No. 2, terminals 11

in parallel, R118, and R115 and R137

and 13, of T101. Screen and bias rectifier tube

in parallel, to the cathode of the tube

V102 supplies the bias voltage. Since there is

under test. The screen grid will draw

no screen grid in the tube under test in this

some current since it is positive with

instance, no connection is made to the screen

respect to the cathode. The screen

grid voltage supply.

grid current will flow through R102,

b. When pushbutton switch S110-4 (4 --

which is used to suppress oscillations

GAS 1) is depressed, a definite value of plate

that may be produced by the tube un-

voltage and bias voltage is applied to the tube

der test, to V102, and back to the

under test and causes a definite value of plate

cathode of the tube. Resistor R104,

current to flow. Meter M101 is in the plate-to-

in the control grid circuit, is also used

cathode circuit of the tube under test and will

to suppress oscillations that may be

be actuated by the plate current.

produced by the tube under test.

c. When pushbutton switch S110-5 (5 --

( 2 ) When the polarity of the voltage

GAS 2) is depressed, grid resistor R128, which

across secondary No. 1 of T101 is

was shorted by part of switch S1105, is in-

such that pin 2 of V101 is positive

s e r t e d into the control grid circuit. If grid

with respect to the cathode of V101,

current flows from the bias voltage source

the signal voltage applied to the tube

through the grid circuit to the cathode as

under test will swing in a negative

shown by the arrows, due to gas in the tube,

direction, thereby increasing the bias

the grid current will develop a voltage drop

and decreasing t h e current flow

a c r o s s R128 in the direction indicated. A