[Amps] fil inrush

[John T. M. Lyles]

In large tubes, that require a hoist to pull them from the socket, we 
either us DC on the filament or use a specially shunted transformer 
as has been discussed before. The manufacturers have been keen to 
advise us about mechanical vibration of the filament which can break 
it. Use of SCR or Triac phase-controlled AC is not advisable. Ever 
listen to a lightbulb which is contolled by a dimmer?

Since a special transformer must be wound anyhow, it is simple to ask 
the supplier to set it up to be current limited to 2 x the normal fil 
current. This takes care of the cold filament inrush current, and 
actually makes the transformer sloppy.

Perhaps the use of phase controlled AC is ok for smaller tubes with 
and without handles?