Delta "Gridless Triode" Tube
|This unusual tube was sold
under the Delta name circa 1923. At just over 2 inches in length,
this is one of the smallest tubes of that era, and was very similar to
the Welsh "Peanut" tube.
It is a "gridless" triode, that is, one of those tubes that was created specifically to avoid infringing the DeForest grid patent (at that point controlled by RCA), so it could be sold without worry of lawsuit, or the necessity of paying royalties. By 1923, the Fleming diode patent had expired, so at least that legal impediment had been removed.
The Delta tube used an external control electrode in the form of a copper sleeve that slipped over the narrow portion of the bulb. Inside, was a filament and a collector electrode that would normally be called the "plate". In this case, it was nothing more than a helically wound piece of wire, so the plate term only applies by convention.
Like most tubes of this type, it was only useful as a detector, and didn't work very well for that purpose anyway. Like the others, it disappeared from the market quickly.