The 6AK5 looks interesting for low voltage work too

It has low heater current.


The suppressor grid is not internaly connected to the cathode. Later addition: wrong but it should still work at low voltage. After all most electrons should collected by the screen grid at low plate voltage operation since it is the nearest to the grid. See experiments on EF183 on the Pleiades breadboard experimental jig.


So all grids can be connected to the anode except the control grid which can be pull up biased.


This idea came from this article:


http://www.rfcafe.com/references/radio-news/low-plate-potential-tubes-january-1957-radio-television-news.htm


There are other interesting electron tubes mentioned too in Atkins brilliant article.


Datasheet from Frank's:


https://frank.pocnet.net/sheets/049/6/6AK5.pdf