Unless the adapters have some sort of voltage or current limiting (apparently they do) , the ST70 plate voltages will probably launch thosee EL84s out of the YJ sockets, like miliature Polaris missiles ;-) But then, I don't se how limited V and I will rsult in clean sound...
Seriously, the B+ in the ST70 is almost 100VDC higher than the EL84s can safely withstand, and the plate curent is about 50% too high, which is a rather deadly combination for these smaller tubes.. (There are ways to run them of much higher B+ voltages, at lower currents, but the ST70 circuitry is not compatible with these unusual approaches)
Probably want to put the fire department number or 911 on your speed-dialer before you try this dangerous experiment in electro-pyrotechnics
In a nutshell: DON'T try this at home, or anywhere else, except perhaps at the Nevada Nuclear Test Site (Area 51, Groom Lake, etc). These yellowjackets will deliver a very nasty sting
/ed B