872A中文資料ETC數(shù)據(jù)手冊(cè)PDF規(guī)格書(shū)

APPLICATION

MECHANICAL

MOUNTING —The 872-A/872 must be mounted vertically, base down.

COOLING—Provision should be made for adequate air circulation

around the tube, because cooling is accomplished by convection. The

temperature of the condensed-mercury in the 872-A/872 should be kept

within the ranges given under MAXIMUM RATINGS. This temperature

should be maintained at 40 degrees plus or minus 5 degrees C for most

satisfactory operation of the tube. To measure the condensed-mercury

temperature a thermocouple or small thermometer may be attached to

the bulb in the area designated on the outline drawing, using a very

small amount of putty. A condensed-mercury temperature lower than

the recommended value raises the voltage at which the tube becomes

conducting and fends fo reduce the life of the filament. A femperature

higher than recommended lowers the voltage at which the tube becomes

conducting and fends fo increase the life of the filament, but reduces

the peak inverse voltage rating of the tube. When if is necessary fo

use a shield around the 872-A/872, care must be taken fo insure ade-

quate ventilation and maintenance’ of normal condensed-mercury tem.

perature.

ELECTRICAL

FILAMENT VOLTAGE—For maximum tube life, the filament voltage as

measured directly at the filament pins, should be held at the rated

value of 5.0 volts. Unavoidable variations in filament voltage must be

kept within the range of 4.75 fo 5.25 volts. A filament voltage less than

the ‘minimum recommended value may cause a high tube voltage drop,

with consequent bombardment of the filament and eventual loss of

emission. A filament voltage higher than the recommended maximum

value will also decrease the life of the filament.

CAUTION SHOULD BE OBSERVED IN MEASURING THE FILAMENT

VOLTAGE, AS THE FILAMENT CIRCUIT MAY BE AT A HIGH D-C

POTENTIAL.

The plate-circuit return of each tube should preferably be connected

to the center tap of the transformer winding supplying the filament

voltage; if this cannot be done, the return should be connected to that

side of the filament to which the cathode shields are connected (pin

No. 2). When the filaments of two or more tubes are connected in

parallel, the filament terminals fo which the cathode shields are con-

nected should be joined. These precautions are recommended to insure

uniform starting voltage for each fube when several are used in a given

circuit.

The filament of the 872-A/872 should be allowed fo reach operating

temperature before the plate voltage is applied. Under normal condi-

tions, a delay of approximately 30 seconds will be required. The delay

time should be increased if there is any evidence of arc-back within the

tube. In ‘radio transmitter applications the filament should be kept af

its rated voltage during “standby periods fo avoid delay due to

warm-up. It is desirable fo use a protective relay in the plate circuit

to prevent the application of plate voltage before the filament has

reached operating temperature. This relay should have a fime delay

adjustable up fo a maximum of one minute.

When an 872-A/872 is first installed, the filament should be oper-

ated at normal voltage for approximately ten minutes with no plate

voltage applied, in order that the mercury may be properly distributed.

1t will not be necessary to repeat this procedure unless the mercury is

spattered on the filament and plate during subsequent handling.

SHIELDING —Electromagnetic and electrostatic fields tend to cause the

mercury vapor fo break down, are detrimental to tube life and make

proper filtering difficult. Consequently, the 872-A/872 should be

Isolated from such fields as exist around a transmitter or other similar

squipment. When the tubes are located in the region of such fields

shielding with adequate ventilation should be used around the tubes.

Ref filtering should also be employed when the tubes are affected by

+f voltages.

FILTERING—A “choke input” filter will allow the greatest usable d-c

output current to the load. When using a section of filter between rec-

fifier and load, to prevent exceeding the maximum peak current of §

amperes, a suitable maximum value for the first capacitor should be

determined. Determination of this capacitance should be made under

conditions simulating those to be used in service.

The relationship of voltage input, inductance, and capacitance is

one in which a higher operating voltage requires’ greater input induct

ance, and less following capacitance to keep the peak STARTING

current from exceeding 5 amperes. This is for the usual case where the

supply is controlled by an on-off switch.

Where the rectifier plate voltage is started by a control which gra:

dually raises the voltage from zero or a small amount fo the desired

operating value, starting current need not ordinarily be considered,

and the characteristics of the filter may be based on preventing ex:

cessive peak current under normal operating conditions.

In the single phase circuits (I and 3), if the current drawn by the

load is reduced or varies under operafing conditions to values less

than approximately 70 of the indicated maximum d-c load current, a

swinging (input) choke will supply the necessary additional inductance

to prevent the voltage from rising, and will afford proper filtering

over a much greater current range.

Where a larger value of inductance is desirable to obtain additional

filtering, the subsequent capacitance may be proportionately increased

to aid in still further filtering without excessive peak starting and oper:

ating current. Still lower ripple may of course be obtained by added

sections of filter.

When “condenser input” filter is used, the peak current will be rela-

tively high in respect fo the usable load current, and the voltage across

the capacitor may be as high as 1.4 times the nominal RMS voltage of

the transformer.

For parallel operation of 872-A/872 rectifiers, suitable resistors or

small inductors may be used in series with each plate lead to permit

equal loading and ‘starting characteristics. The inductors aid in reduc-

ing the peak current, and are more desirable due fo their low d-c re-

sistance. An approximate value for suitable resistors is 50 ohms, and

for inductors, approximately one-third henry each.