A fine article on the "TUNE-A-LITE" appeared In THE VACUUM TUBE column edited by Brother Patrick Dowd in the February 1990 issue of the OTB. It included a simplified circuit diagram of a four-electrode "TUNE-A-LITE" as used by Atwater Kent in the Model 81 receiver.
In the following issue of the OTB (May 1990), Lud Sibley wote "MORE ON THE TUNE-A-LITE", also under Brother Dowd's VACUUM TUBE column.
Atwater Kent promoted the "TUNE-A-LITE" as the "Tonebeam" tube as early as 1932. It appeared in Model 26; Models 228/567; Models 469/189 (ca 1932); Models 228Q/228D/93, and 91/91B/91C. (See www.atwaterkentradio.com/ak_rft.htm for the cover page of the original AK Tonebeam brochure.)
Volume 17 of the AWA Review Volume 12, 1999. "The Atwater Kent Radios" by Ralph O. Williams refers to the Tonebeam on pp 203,4. and describes the circuit of the Model 96 which drove it. (There were three AK Model 96's - the Tonebeam was only used in the third one, and its circuit can be found on page 271 of "Rider's Perpetual Troubleshooting Manual".)
Lud Sibley wrote a full three-page article on "THE TUNE-A-LIGHT" in the February 2004 issue of "The Tube Collector".
Which leaves the AK "TUNE-A-LIGHT" story pretty well told, except for the following contribution based on actual measurements of my own "unobtainium" unit.
Absolute minimum striking voltage - 165 VDC
Current needed for half illumination of the long electrode - 4 mA
Current needed for full illumination of the long electrode - 6.5 mA