Question:

Guitar amp 3 ohm and 15 ohm output?

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I have an old valve guitar amp and the two outputs are for 3ohm or 15 ohm speakers the speakers that this used to run were old celestions but they are discontinued can i hook up either 4 ohm or 16 ohm speakers i know nothing about ohms law so any help would be great

Thanks Marc

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  1. The impedance rating for speakers (in ohms) is basically the amount of resistance that the speakers have electronically to alternating current (an audio signal is an alternating current signal). Some speakers are low-impedance (around 4 ohms) and others are high impedance (about 16 ohms), while others are somewhere in between (8 ohms).

    When an amplifier is hooked up to a speaker, the speaker becomes part of the amplifier circuit. And, the amplifier is designed to operate with a certain impedance on the output for optimum performance.

    If the impedance is too low, the current output from the amplifier is too high and it goes into saturation too easily. This causes the amplifier to go into distortion at a relatively low volume and the excessive current can damage the amplifier components.

    If the impedance is too high, not enough current goes to the speaker and it does not put out enough volume for a given amplifier setting.

    By selecting the output impedance on the amplifier that matches the speaker, you get the greatest amount of volume and the cleanest sound out of the speaker. By having an impedance mismatch between the amplifier and speaker, you will have less than optimal sound, and you could damage the amplifier and/or the speaker.


  2. It should be OK.

    When the impedance is matched, you have the best power transfer. If they are slightly mismatched, there will be some additional loss in power but this should not be too much of an issue for you.

  3. I think it is okay. The 3ohm or 15 ohm are just for speaker impedance matching, it is measure in AC ohms. If you hook it up with 3 ohm and 15 ohm speakers, you get the maximum power transfer to your speakers. If not so, you probably don't get the loudest sound output. Your best bet for starting to use an antique guitar amplifier is to buy the best contact cleaner (spray type) by taking out the tube out one by one, spray on the pins of the valve and  the socket then let it dry for a while. You must do it one at a time to avoid any mistake you may make. You will get a much better sound effect and best performance of grand old guitar amp.

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