Typical values of gm for a small-sign vacuum tube are one to ten millisiemens. It is amongst the 3 'constants' of a vacuum tube, one other two being its gain μ and plate resistance Rp or Ra. The Van der Bijl equation https://bookmarksoflife.com/story3708898/the-single-best-strategy-to-use-for-boron-carbide-ceramic-plates