MIL-PRF-1/1619D
TABLE I. Testing and inspection - Continued.
1/
In all electrical tests involving application of filament voltage, forced-air cooling is allowable as follows: base-to-anode airflow,
67 cfm maximum; anode-to-base airflow, 84 cfm maximum through the anode cooler, plus 3 cfm maximum directed into the
filament stem structure. These figures apply for air at 40C maximum and at sea level. At higher air temperature or altitude,
suitable correction of the airflow is allowable. Cooling air should be supplied before or simultaneously with application of
electrode voltages (including the filament) and may be removed simultaneously with them.
2/
The 8251 type may be made by attaching the base flying leads to an 8161 which has met all the requirements listed herein.
After attachment of the flying leads and any other mechanical processing, including plating, the 8251 shall meet the
requirements of the holding period and the total grid current test.
3/
For the cooling test, the tube shall be mounted in an infinite baffle system, or equivalent, so that all air passing in a base-to-
anode direction shall pass through the anode cooler of the tube. With cooling air of not less than 20C nor more than 40C,
both base and anode shall be cooled by applying an airflow of 67 cfm maximum at sea level. At altitudes above sea level, the
equivalent mass flow may be used. An appropriate value of anode voltage and current shall be used to produce the specified
anode dissipation. Temperatures shall be measured by means of thermocouples, located as follows:
Anode core:
The thermocouple shall be embedded in the base of the anode core, at the base of the cooling fins, by means
of drilling a small hole shallow enough so that the tube vacuum shall not be lost, placing the welded
thermocouple junction therein, and then peening the edges of the hole to hold the thermocouple firmly in place.
Filament seal: The thermocouple shall be attached using a suitable cement to either the inner or the outer filament conductor
and as close as possible to where the two conductors join the stem dielectric material.
Good electrical continuity between the thermocouple and the metal surface to which it is attached shall be demonstrated before
the cooling test is performed. The size and material of the thermocouples, their installation, and the measuring instrument
used to determine temperature shall be in accordance with good engineering practice.
4/
The pressure drop across the anode cooler shall be measured while the tube is operating as specified during the cooling test.
If test is performed at altitudes significantly above sea level and the airflow is adjusted for an equivalent mass-flow rate, the
allowable pressure drop may also be recomputed on the same basis.
5/
This test to be performed first at the conclusion of the holding period.
6/ This specification sheet uses accept on zero defect sampling plan in accordance with MIL-PRF-1, table III.
7/
Reclaimed materials shall be utilized to the maximum extent possible.
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