Hi Robert, The heater of the 6K6 draws 0.4 A, while the heaters of the other 6 V tubes in the set draw 0.3 A. So, it's NOT plug-and-play - you actually need to open the bottom and put a shunt resistor across the heater pins of the 6SR7 - say, 67 Ohm in E12 values, or better, 62 Ohm [E24 values]. 73 de Brian, VK2GCE.