The Minimalist Superhet Radio project.

[Geordie McBoyne]

The ECH84 has the sharp cut-off characteristic and was used as a noise cancelled sync separator in 625 line TVs.
The pin out is the same as the vari-mu ECH81. The ECH84 can be used a frequency changer in circuits where the vari-mu characteristic is not required or is not desirable.
No doubt the ECH84 can be used for many other functions. According to the Radiomuseum the valve can be used as a modulator. If a triode section is not required consider the EH90, same pin out as the EK90/6BE6 but not vari-mu.
From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_ech84.html

I don't think there should be any difficulty finding the ECH84, but there is I have a few used examples for disposal.

Geordie McBoyne.