Method of manufacturing a coil heater of an indirectly-heated type cathode electrode of electronic tubes

A method of manufacturing a coil heater of an indirectly-heated type cathode electrode of an electronic tube of the type wherein said coil heater comprises a main portion and leg portions connected to said main portion and is adapted to be contained in a cathode sleeve, said method comprising the steps of winding a length of heater wire about a core in a given direction over a first distance to form a single-layer first coil;

• winding said heater wire about said first coil in the opposite direction over a second distance to form a double-layer second coil;
• winding said heater wire about said second coil and then about said core over a third distance and in said given direction to form a triple-layer third coil and additional single layer coil;
• winding said heater wire about said additional single-layer core over a fourth distance and in said opposite direction to form an additional double-layer coil;
• winding said heater wire about said additional double-layer coil over said fourth distance in said given direction to form an additional triple-layer coil;
• continuously baking the resulting coils;
• severing said coils at the centers of said triple-layer coils;
• shaping said additional single-layer coil into said main portion;
• applying an insulating coating onto said main portion and a part contiguous thereto of said triple-layer third and additional triple-layer coils;
• firing said insulating coating;
• and
• removing said core from said coils.