Hey-hey!!!,
One could try the 6C19Pi in place of the 12B4. Mu is about 2.5, and R_p is ~500R. So try the 6C45>6C19 and use a 3-4k resistor on the 6C19. Either way, the limit is still a quarter of B+ for swing. We just get a bit closer to that limit with the lower plate Z of the 6C19.
cheers,
Douglas