EL34 tubes can be used ONLY if both sockets have pin 1 connected to pin 8 to connect the supressor grid to the cathode - this connection is made internally in a 6550, but not with an EL34.
In addition, The bias will have to be reduced by about 10 volts - the pot might be able to make this adjustment, but if not, a resistor or two might need to be changed to place the pot within the appropriate adjustment range.
This is not a plug-in swap.
