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Mailbag: Is it OK to Run an Amplifier on 120 Volts AC?

Question: Is it OK to run an amplifier on 120 volts AC?

This is a question that many new Amateurs ask themselves when they’re buying their first amplifier. There are many reasons to ponder this, and one of them is: “I’ve already got a 120 volt outlet in the shack, and I don’t want to spend a fortune putting in a 240 volt line.  Electricians are expensive!”

“What difference could it make? I’m going to be running about a kilowatt output regardless of the line voltage. I know that a typical 1 kW output amplifier will draw a maximum of about 15 amps on 120 Vac and 7.5 amps on 240 Vac. In either case, that’s about 1,800 watts input (120V x 15A or 240V x 7.5A) and since Ohm’s law tells us that volts x amps = watts, there should be no difference – right?”

Wrong! That ignores voltage drop.

There are many reasons why running your amplifier on 240 Vac is a lot better:

  • Line voltage regulation is better.
    1. Typical voltage drop of drawing 15A on a 120 Vac 20 Ampere circuit (12 gauge wire) is about 3 percent. This means that every time you transmit, your 120 Vac line is going to sag to about 116 volts.
    2. This doesn’t seem like much of a difference, but in a tube amplifier, your high voltage is going to also be sagging by that same 3 percent! If your amplifier uses 3,000 Vdc, it will be dropping 3 percent to about 2900 Vdc (and all your other power supply voltages will drop by 3 percent, too!)
    3. This will cause the line voltage that your other appliances receive to drop whenever you transmit. This is enough to cause noticeable dimming or blinking of typical household lighting. This doesn’t go over well with spouses and kids!
  • On a 240 Vac 20 amp circuit, you will draw half as much current.
    1. By the above analysis, you’d think that this would result in your 240 line voltage dropping by only half as much as we saw on 120, or 1.5% – but, you’d be wrong.
    2. The benefit of twice the line voltage results in a whopping 4:1 improvement in line voltage regulation and your 240 volt line will only vary by 0.75% to 238.2 Vac when you transmit. Even on a 240 Vac 15 amp circuit (with smaller 14 gauge wire) you’d only have a 1.2% drop to 237.12 Vac.
    3. On 240 volts, your high voltage will only vary 0.75% instead of 3 percent. That 3,000 Vdc will only drop 22.5 Vdc when you transmit to 2977.5 Vdc – way better than dropping to 2900 Vdc! All of the other power supply voltages will remain more stable and only vary by 0.75%!

You’re making a significant investment in your amplifier. Give it the 240 Vac power it deserves so it can give you the best performance!

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