[Elecraft] 120V vs 240V

[Scott Ellington]

Actually, the percentage voltage drop at 120 V is FOUR times that at 240 V. (For the same power, wire gauge and length.)  Since the KPA500, like most tube-type amplifiers, uses an unregulated power supply, the extra drop can significantly reduce output.  For example, suppose the amplifier can put out 500 W with a perfectly regulated input of 120 or 240 V.  Say the line voltage, at 240 V, drops 6 volts at full output.   That's a 2.5 percent drop, which reduces RF output by about 5 percent, to about 475 W.  At 120 V, the drop is 12 volts, 10 percent, which reduces the output by about 20 percent, to about 400 W.  You just lost 75 more Watts, each of which cost you $4.

Keep in mind that 500 W output requires about 1 kW input, that the power factor of the power supply is considerably less than 1, and that the high peak currents make the effects of line voltage drop even worse.  When calculating effective line drop for the KPA500, I would use something like 2 kVA, about 17 A at 120 V. The above example corresponds to a run of 220 feet of AWG 12 (440 feet total), admittedly a very long run.

Bottom line:  If you are adding wiring, use 240 V.  Run the amplifier on 120 V only if you have no choice, or if the run from the distribution panel is under about 50 feet.


Scott  K9MA


On Feb 15, 2011, at 6:25 PM, K4SC wrote:

> If I were going to the trouble to pull the wire, I'd run 220V.  You give
> yourself the option of using a heavier amplifier in the future, and less
> voltage drop (twice as much on 120V).  

Scott Ellington
Madison, Wisconsin
USA