[turretslug]

Apparently, one of the strictures behind the design of the ECH81 was to act as an effective 1st IF amp in VHF/FM receivers with simple, minimal-budget and low-gain VHF tuners- that would have emphasised good noise performance and it's likely that all the tricks of 20 years of multi-grid mixer development would have been deployed, such as grid alignment for minimal screen-grid current and hence noise contribution. The compact and precise construction also helps to minimise self-inductance and transit-time, helping with extending its frequency capability. The US 6BA7 was also a last-fling, deploy-all-the-lessons multi-grid frequency changer but using the US-favoured topology of the self-oscillating heptode (as opposed to externally-fed oscillator grid). Don't overlook the common-place ECH42- it's also a highly-developed device and doesn't give much away to the ECH81, the latter was an incremental evolution of what came before, rather than a revolution.