I have two amps. Heathkit SB-220 (two 3-500z) Hammond HL-2000A (two 3-500z) The SB-220 requires 55-65W (depending on band) to get 600W out in "CW mode" (2500 V resting plate voltage). The HL-2000A requires 80-95W (depending on band) to get 600W out in "CW mode" (2300 V resting plate voltage). I swapped the tubes, and the HL-2000A behaved the same (I haven't operated the SB-220 after swapping tubes). Does this make sense? Should I be concerned about the additional drive required by the HL-2000A? Does it have something to do with the input circuit?