MIL-DTL-24251F
3.3.2 Pure tin: The use of pure tin, as an underplate or final finish, is prohibited both internally and externally. Tin
content of electron tube components and solder shall not exceed 97 percent, by mass. Tin shall be alloyed with a
minimum of 3 percent lead, by mass (see 6.3).
3.3.3 Liners. Shields shall be constructed in such a way that the liners are securely retained in position within the
shield (see 3.5.3). Liners with circumferential corrugations or other configurations that could allow tube breakage and
that do not permit spring action within the shield to allow for maximum tube diameter variations are not acceptable.
3.3.4 Installation and removal. Design of the shield shall be such that installation and removal of the shield from a
seated electron tube shall not scratch or damage the electron tube envelope when tested as specified in 4.5.3.4.
Unless otherwise specified, no tool shall be required for installation or removal of the shield from the tube or retainer
(base) or adapter.
3.3.5 Spring retention. When used (see 3.1), the coil spring shall be securely retained within the shield. The free
end of the spring, or the loop that makes contact with the electron tube envelope shall be at least seven-eighths of a
complete turn and the spring tip shall not touch the tube envelope.
3.3.6 Retainer (base) and adapter. The retainer (base) and adapter design shall be such that it rigidly retains the
tube shield during all tests and examination of this specification.
3.4 Permeability. All parts shall be made from materials that are classed as nonmagnetic. The permeability of the
tube shield assembly shall be less than 2 (air = 1.0) (see 4.5.2).
3.5 Installation, locking and unlocking torque or load, and removal force.
3.5.1 Initial force. The installation, locking and unlocking torque or load, and removal forces necessary to install
and remove the shield over the applicable slug (see figure 1) and retainer (base), and adapter shall be not greater
than specified on the applicable specification sheet (see 3.1), when tested in accordance with 4.5.3.1.
3.5.2 Force at temperature extremes. The installation, locking and unlocking torque or load, and removal force
shall be not greater than 150 percent nor less than 50 percent of the initial force when tested at 85°C ± 5°C and
-55°C ± 5°C, in accordance with 4.5.3.2.
3.5.3 Fatigue. When tested in accordance with 4.5.3.3, the installation or removal force shall be not greater than
125 percent nor less than 50 percent of the initial force and the liner shall remain in position within the shield.
3.6 Vibration. When tested in accordance with 4.5.4, the shield shall not loosen from the retainer (base) or
adapter. There shall be no evidence of damage to the shield, or retainer (base), and adapter, nor the glass envelope
of the electron tube, during installation, during the test, or upon removal after the test.
3.7 Shock. When tested in accordance with 4.5.5, the shield shall not loosen from the retainer (base) or adapter
as applicable, and there shall be no evidence of damage to the shield, tube socket, shield retainer (base), or the
glass envelope of the electron tube.
3.8 Heat resistance. When shields, retainers (bases) and adapters are tested in accordance with 4.5.6, there shall
be no peeling, blistering, chipping, or pitting of the finish.
3.9 Heat dissipation. The average reduction of a minimum of two surface temperature readings of the
instrumented tube shall be not less than the value specified (see 3.1), when tested in accordance with 4.5.7.1.
3.10 Salt atmosphere (corrosion). When shields, retainers (bases) and adapters are tested as specified in 4.5.8,
there shall be no evidence of corrosion and the finish shall not peel, blister, chip, or pit.
3.11 Electrical contact. The dc resistance from the shield to the chassis shall be not greater than 1 ohm when
tested in accordance with 4.5.9.
3.12 Marking. Each tube shield, retainer (base), or adapter shall be permanently and legibly marked in
accordance with MIL-STD-130 with the following:
a.
Military Part or Identifying Number (PIN).
Date and source code in accordance with MIL-STD-1285.
b.
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