MK -
Yes, that set sounds like it has an instant-on feature. If you open it up, look for a silicon diode (usually across the on-off switch). This diode is electrically in opposition to the power supply rectifier, and supplies the filament string with half-wave rectified DC, keeping the tubes warm. When you turn the power switch on, you short out the diode, applying full voltage to the filament string and allowing the power supply to produce B+. This feature can be disabled by either snipping or desoldering one lead of the diode or by removing the diode altogether. The first method allows you to return the set to "original" status if so desired.
By keeping the filament string at a reduced voltage (87 or so if I remember right), the tubes were able to produce emission almost instantly on power-on. However, the tubes were also producing heat (and burning cathode material) all the time. Probably not a problem when tubes were available for a buck or so at the corner drugstore, but they're a little more difficult to source now. Another concern is heat - most of these portable sets use a PC board chassis which has been baked by the heat of the tubes; by heating the board only when the set is in use, the detrimental effect of the heat is reduced.
I have two Admiral B/W portables which had this feature - one of which I discovered in a manner similar to yours; in the second, the diode has failed shorted, so the set is "always on" instead of "instant on". Haven't gotten around to opening that one up yet, though.
-Jim
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