standard values of gm for a little-sign vacuum tube are 1 to 10 millisiemens. It is probably the 3 'constants' of a vacuum tube, another two becoming its attain μ and plate resistance Rp or Ra. The Van der Bijl https://blog.globalgoodsguru.com/pu-sealants-for-automotive-applications-enhancing-vehicle-performance-affb2479a44a