I should add this as well...
The B+ switch on and off is provided to allow filament warm up of New old stock or used 45 tubes from many years ago.
This allows filament conditioning of 45 tubes that may be up to 70 years old, (without applying B+), this allows the tube to come to life slowly.
Of course with new production 45 tubes it is not be necessary to have the B+ switch.
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