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This 866Jr has a broken filament and the filament helix can be seen in the lower left of the bulb.
Type 866Jr. where Jr. really does stand for 'junior'. The valve is a scaled-down version of type 866/A. Both types are half-wave mercury-vapour HT rectifiers intended for supplying HT to Class B and Single Side-band (SSB) transmitters which draw widely-varying HT currents in proportion to the instantaneous modulation level. In normal high-vacuum rectifiers the voltage drop across the rectifier can vary from a few volts up to a couple of hundred, depending on the instantaneous current demand. This means that the HT can 'surge' by a couple of hundred volts every time the person at the microphone stops talking for a second or two. With mercury rectifiers the voltage drop is essentially constant (around 15 V) at all current levels within the valve rating, so HT surges due to this cause are eliminated.
Unfortunately, mercury rectifiers have disadvantages of their own, including the need for pre-heating and delayed HT switching, the need for a (heavy, but efficient) inductor-input smoothing network, susceptibility to flashover and generation of HF interference. Type 866/A was the post-war industry standard rated at 1.5 kW (3000 VDC @ 0.5 A), too much and too bothersome for most amateurs. As far as we know, type 866 Jr. was introduced specifically with the amateur market in mind.
The peak inverse voltage is 3,500 Volts and peak current is 250 mA. A pair of these rectifiers will therefore supply up to 250 mA at around 1,000 V. This voltage is on the high side for a single-ended mercury rectifier (most types have anode top caps).
The Taylor company specialised in supplying transmitting and rectifying valves for the amateur market after WWII.
This is another 866JR with a complete filament.
This is a cloe-up of the filament and although the glass distorts the image the surface really is rough. In fact the filament is a mesh tape.
The balloon envelope is 50 mm in diameter and, excluding the UX4 base pins, is 117 mm tall.
References: Data-sheet & private communication.


Absolute Maximum Operating Conditions
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated March 21, 2014.
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