Back in the dim & distant past, in a land far away, shrounded in the mists of time there was a valve................... and what a valve, not like the ones we have today, oh no, it was very different indeed.
At the time, it was very similar to all its contemporaries - not that there were so many - and like them, it was a general purpose triode having a four volt pure tungsten filament which operated at a temperature similar to that of an incandescent light bulb. The electrode cage was mounted an an inverted pedestal sealed into the bottom of the envelope with a length of glass tube being welded onto the pedestal - just the same way in which an incandescent bulb was manufactured. It had a bulbous envelope which had at it's apogee a pip of glass. - yes, it was a BRIGHT EMITTER VALVE.
The bright emitter valve was hampered by two disadvantages. Firstly, the intrinsic characteristics were not best suited to each stage of a radio receiver and compromises in design and performance were the key to garner adequate performance. Secondly, the filament had a whopping current consumption at 1A.
There had to be a better way if this new fangled thermionic electrickery was to catch on.................