Uhh the el84 is a small audio penthode for 4 watts/ 250 V anode voltage or so , 9 pin noval , less than a 6v6. The EL509/519 is a line end tube 35 watts anode dissipation, much bigger. In a lineair for 27MHz it does 100 watts or so @ 800 V . Not quite the same. More a 6KD6.
Anton
Sorry, my mistake. I switched the tube letters around. The EL509 is equivalent to the 6KG6 not the 6GK6. However, the EL84 is equivalent to the 6BQ5 and under "typical operating conditions" the specs seem to correspond fairly close to the 6GK6 except for tube socket wiring. I still think it's probably less aggravating to just go buy one somewhere.