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Post subject: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 12:56 pm
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On my 65 twin reissue it says do not turn on without speakers being plugged in. Why is this so, just wondering thanks.

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Post subject: Re: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 1:23 pm
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carsonb10,

The output tranny of the amp should "see" a load. This is because the lack of a load will be be equal to near infinity ohms on the secondary. Heat will build-up over time, due to saturation of the primary windings with this mismatch.

Really, for most output trannies, you are better off with the secondary line shorted out, when there are no speakers connected (no load). This is why most Fender amps have a shorting speaker jack. The resistance (and impedance) goes to zero, when no load is connected to the amp.

This in mind, most Fender amps have over-engineered output trannies. So, a momentary lack of load prolly won't hurt. But don't crank up the amp, if you don't hear anything. Turn the amp off & inspect your connections.


Last edited by BMW2002Ti on Tue Jan 21, 2014 1:28 pm, edited 1 time in total.

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Post subject: Re: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 1:27 pm
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carsonb10 wrote:
On my 65 twin reissue it says do not turn on without speakers being plugged in. Why is this so, just wondering thanks.

Results in an open circuit, or near infinite impedance on the output transformer which will damage it, and in turn other components of the amp.

EDIT: OOps...simultaneous post w/ BMW.

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Post subject: Re: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 1:31 pm
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Plus, if you leave tha amp on under no-load conditions --- you risk a very big flyback voltage when switching the amp off. Big enough to damage the output and power supply sections.

So, if you notice that the amp has been on, without speaker connected... turn all volume controls to zero, plug in the speaker, then turn the amp off.


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Post subject: Re: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 7:22 pm
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BMW2002Ti wrote:
So, if you notice that the amp has been on, without speaker connected... turn all volume controls to zero, plug in the speaker, then turn the amp off.


Would it be safe to grab both leads at the same time as you plug in the speakers, with the amp on? I'm wondering if there would be any current running through the output transformer. I don't think I'm brave enough to try this first hand. :shock:

Oh and one more reason for having the speakers plugged in, your wouldn't be able to hear anything if they weren't. :D

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Post subject: Re: Why must speakers always be plugged in
Posted: Tue Jan 21, 2014 7:49 pm
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socal323 wrote:
Would it be safe to grab both leads at the same time as you plug in the speakers, with the amp on? I'm wondering if there would be any current running through the output transformer. I don't think I'm brave enough to try this first hand. :shock:

It should be perfectly safe. Unless you're seriously sensitive you should need a jolt bigger than what fries a speaker before you take damage.
Now if you use a pacemaker, and your amp always gives a loud crack when turning it on, I would recommend holding both wires with the same hand...

Caveat: I'm not a doctor, Jim. I'm an engineer.


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