04 Tube Operation.pdf - Kambing UI

presented in sec. 6.7.3.
Proper choice of materials for interconnecting the tube and circuit is
also important where long life and good performance is concerned.
Copper is the preferred material for connection straps although
brass and aluminum are also suitable in most cases; surface
plating is optional in low power applications and not generally
necessary, except for cosmetic reasons or corrosion prevention.
Silver plating is preferred in components used for VHF and UHF,
especially where high power (over approx. 10 KW) is anticipated;
oxidized or tarnished silver does not exhibit significantly more
loss than its untarnished state, due to the fact that it is still a good
conductor. Nickel plating is often employed where cost may be a
factor; its greater resistance to chemicals is another advantage.
Components used with water–cooled tubes or tubes designed to
be immersed in insulating oil or gas are often nickel plated. Gold
plating is preferred in microwave cavities to reduce losses due to
skin effect and because gold does not oxidize. Steel and stainless
steel alloys should be avoided in rf power amplifiers because of the
high rf losses associated with those materials in the presence of
rf fields. If steel hardware is necessary for mechanical strength, it
can be copper plated; at HF and above, rf current will flow primarily
through the plating. At LF iron or steel should be avoided due to
increased skin depth and losses associated with its ferromagnetic
properties.
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3.5 LEAD LENGTHS
Most of the connecting leads should be designed with extremely
low inductance to minimize the possible VHF parasitic circuits.
Filament and screen bypass leads, suppressor bypass leads, leads
to the grid and the anode tuning capacitors, and interconnections
between tubes in both push-pull and parallel circuits are in this
category. An exception is a parasitic suppressor (if used), which
must have an inductance selected according to the operating
frequency. For a lead to have low inductance, it must have a
relatively large surface area and be short in length, as in a strap
or ribbon shape. This consideration also applies to that portion of
a lead inside a bypass capacitor or connecting from the tube to
a bypass capacitor. If two or more tubes are used, they should
be placed reasonably close together to help provide short lead
lengths. At HF the length of leads connecting rf tuned circuits to
the tube(s) can usually be much longer than bypass capacitor lead
lengths. The dc supply, keying, modulating and control circuit wires
can be quite long if properly filtered or shielded and arranged out
of any rf fields.