Thus tube was produced in at least two different versions, with 2 or 3 ribs in the plate sheet. The curves are similar, with about 10-15% different gm. The grid current is quite different, so it is really meaningful to provide a construction-specific spice model.
3-ribs version This version shows a bit Gm-variation when the plate gets hot. In hot condition, Gm is reduced by about -4%.
The 6S4A version with 2 rippes showes a more or less doubled grid current, a larger Gm (+13% or so) and a bit higher Mu (21.3 instead of 19.1) compared to the 3-ribs version. The curves of the 2-rips version are quite stable, regardless of the plate losses.