Noise Suppression Diode

11-6660-204-25

charge drain; the plate voltage remains constant,

which eliminates the relatively high current re-

quired to charge the grid-plate capacity under or-

dinary amplification conditions. On the other

hand, the output impedance is low because the

plate, not having any external impedance to limit

its current, will supply any reasonable current de-

mands. (Output A = 1/Gm, nearly. )

2-4. Noise Suppression Diode

(fig. 2-1)

Noise suppression diode V301B is one-half of a

miniature twin-triode tube which has had the

plate and grid pins connected together to form an

anode connection. The purpose of the diode is to

reduce the high noise level in the signal to prevent

false triggering of the univibrator giving false

Figure 2-2. Bias circuit, simplified schematic diagram.

recordings, The diode may be switched in or out

by means of the noise suppression switch (S301).

through R302. Cathode resistor R303 (180,000

When this switch is in the OUT position, the

ohms) is a return path to the negative 175-volt

anode and cathode are shorted together and the

supply, so across the plate-cathode resistance and

signal is bypassed around the diode; the tube is

R303 (in series) there isa tota! potential of 475

inoperative. The explanation that follows (switch

volts. Cathode current in resistor R303 keeps the

at the IN position) applies to actual noise sup-

cathode above the 140-volt potential of the grid, so

pression.

the tube actually has a negative bias. The advan-

tage of this circuitry permits the injection of

a. A small space current is present in the tube

large signal voltages without overloading, and

under no-signal conditions, because the anode is

presents a high impedance to the signal source but

connected to --300 volts through resistors R307,

a low impedance signal source to the load.

R306, and R304; and the cathode is connected to

(1) The higher signal voltage is permitted

--175 volts through resistor R303. Cathode resis-

tor R303 is shared with tube V301A and is the

because the cathode will follow closely any signal

voltage applied to the grid, avoiding any apprecia-

coupling between the two tubes. The portion of

ble change in grid-cathode potential. This action

the current in R303 that is delivered to tube

is probably best explained by an example: ('To

V301B is about 30

or 1/60 of the total current

simplify the figures, only approximate values are

in the resistor.

used).

b. The signal at jack J304 is practically identi-

(a) Assume the no-signal values of circuit

cal in amplitude and shape with that at jack J303

conditions to be--

(para 2-3). This is the signal that appears on the

Plate-cathode potential - - - - - - +151 volts

cathode of the diode. When the signal raises the

Grid-cathode potential - - - - - - 8 volts

cathode potential above that of the anode, the tube

IR drop across R303 -------- 324 volts

will not conduct. When the signal lowers the cath-

Plate (or cathode) current - - 1.8 milliampere

ode potential below that of the anode, the diode is

Transconductance - - - - - - - 2,500 umho.

able to conduct freely and can discharge any accu-

mulated charge in capacitor C302, the hot ter-

(b) This value of transconductance means

minal of which is connected to the anode through

a change in IP of 2.5 microampere (Pa) for each

switch S301.

millivolt change of grid-cathode potential. A posi-

tive grid signal swing of 50 volts would then

c. Under no-signal conditions the two cathodes

create a positive cathode signal swing of 49.9

of tube V301 will be about 150 volts potential

volts; the other tenth of a volt being a grid-cath-

above ground (para 23). Capacitor C302 will

ode change, permitting the increase in lP.

charge through resistors R307, R306, and R304;

the voltage across C302 will rise exponentially to-

(2) The grid input circuit of a cathode-fol-

ward that value determined by the setting of po-

lower stage is high impedance because the small

tentiometer R304. When this positive-going volt-

grid-ground capacity is the only one with any

2-4