The 12HG7 is quite an obscure valve originally designed for use as the luminance output stage for colour television receivers.
B&O of Denmark imported them from America for use in their early colour sets as they (supposedly) give better performance than the otherwise standard PL802 in terms of voltage swing and bandwidth. Thanks to Tim Jarman for this detailed information.
Henning Hertz adds that the 12HG7 was used in the Bang & Olufsen late 1960's Beovision 2600/3100 series for luminance (video) amplification.
The 12HG7 has a 12.6 volt centre tapped heater. The maximum dissipation of the anode is 10 Watts with a maximum anode voltage of 400 Volts. It is s sharp cut-off pentode and is of frame grid construction with a high value of mutual conductance.
Our other 12HG7 fits into the standard tube width for B9A valves, whereas this is fitted into a 28 mm diameter tube and thus will keep the glass cooler for a given anode dissipation.
The makers name in paint the other side of the etched Type designation. The anode closes in to the grids at the working faces and away to a larger surface area outside the central zone.
The control grid has a heat sink at the top.
The thin glass tube envelope is 28 mm in diameter and, excluding the B9A base pins, is 63 mm tall.
References: Datasheet, Tim Jarman & Henning Hertz. Type 12HG7 was first introduced in 1965.