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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 11:00 am
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GilgaFrank wrote:
An 8 ohm load is an 8 ohm load no matter who manufactured it. In fact a resistive load is a far better match for your amp than any speaker. Speaker impedance varies wildly with frequency, a load box presents a far more linear load.


Indeed.

A loudspeaker (or loudspeaker system) represents an RL or RCL load, a combination of resistance and inductance, plus capacitance if a crossover network is present. As the frequency of the input increases, so follows the measurable impedance of the total load. OTOH, a dummy load is usually a large wire-wound resistor or series of resistors whose DCR never varies despite changes in frequency.

Quiz on Monday.

:wink:

Arjay

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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 2:12 pm
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Cool stuff.

The isolation box will work. But if your only concern is for the protection of your amp, I'm sure the manufacture could recommend a dummy load for it if you shoot them an email. I have been emailing and calling a lot of companies lately before I do business with them and it's amazing what they will do extra for customers. Maybe I have had a run of luck.

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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 2:42 pm
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Depending on where the direct out comes from, you could just pull the PI tube, and that would eliminate sound from the speaker connected to the amp.

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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 3:06 pm
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shimmilou wrote:
Depending on where the direct out comes from, you could just pull the PI tube, and that would eliminate sound from the speaker connected to the amp.


Sorry but that is incorrect. The output tubes need a resistive load on the output transformer whenever they are powered up.

It is not safe to operate a tube output stage in that way with no speaker connected.

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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 3:18 pm
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I'm getting worried by some of the poor advice offered in this thread. So I'm going to type in big red letters.

Do not ever, under any circumstances, operate a tube amp without a speaker or dummy load connected.

Even with no audio signal through the amp - for example with a preamp tube or phase inverter removed - there will almost inevitably be some AC in the output transformer, whether from stray signals or hum or PSU ripple. Remember the tubes will never be perfectly matched, there will always be a difference in current between the tubes.

Without a speaker on the output transformer secondary, the OT becomes a pure inductor that resists current fluctuations by generating "back EMF" or "flyback voltage". These voltages can be high enough to cause arcs across the transformer.

So I'm going to say it again. In more big red letters.

Do not ever, under any circumstances, operate a tube amp without a speaker or dummy load connected.

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Post subject: Re: Speaker load question.
Posted: Sun Jul 20, 2014 6:54 pm
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GilgaFrank wrote:
shimmilou wrote:
Depending on where the direct out comes from, you could just pull the PI tube, and that would eliminate sound from the speaker connected to the amp.


Sorry but that is incorrect. The output tubes need a resistive load on the output transformer whenever they are powered up.

It is not safe to operate a tube output stage in that way with no speaker connected.


Your comprehension skills need some work. As I very plainly stated, pulling the PI tube will eliminate the sound from the speaker connected to the amp. :roll:

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