the 6BZ6 RF amplifier tube and  6CB6 premixer tube can be easily replaced by the russian 6J1P heptal pentode. 
  all you have to do is to cut the 7th pin of the replacement 6J1P-s with a dremel tool (avoid to damage the glass of the tube) , and plug the tubes in without any other modification. Take care to plug in the tubes with the pin 1 to the original pin 1 in the sockets.
  You HAVE to change both tubes (rf preamplifier and the VFO premixer) because the 6J1P has only 0,175A heater current, and those two tubes have the heaters connected in series. 
  If you want to change only the premixer tube, please connect a 47 ohm 5 watt resistor parallel to the heater of the new tube, to equalise the heating currents.
   YO5OFT, Tibi.