FDP Forum / Ohms?? What would draw the most from my amp?/ 24 messages in thread.

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guitarcapo



U.S.A.

Jun 18th, 2014 07:34 AM        

"Impedance" is resistance applied over many frequencies. If you ask a speaker to reproduce a sine wave of a single distinct frequency, it will provide a distinct defined "resistance". But that definite resistance is dependent on which frequency being produced.<br /> <br /> Different frequencies will involve different resistances. "Impedance" is the summation of ALL frequencies in the audible range.<br /> <br /> As such, it's not so much a matter of "maximum power" is it is frequency response efficiency.<br /> Using the correct impedance match will give you the most efficient frequency response in the audible range. Mismatching creates a different response but doesn't necessarily "blow your output transformer" if you are off a little.<br /> <br /> There are lots of amps out there (including Fender) that have an external speaker jack that's hooked up to the main speaker leads in parallel without having a separate secondary tap on the output transformer. Basically when you add that second 8 ohm speaker, the output transformer is dealing with a 4 ohm load instead of 8. The world doesn't come crashing down. Your amp doesn't start smoking.<br /> <br /> Usually when you use a lower impedance speaker than called for, the frequency response shifts brighter. That's about it. Taken to extremes... yea, you will start damaging your OT. <br /> <br /> A extremely low impedance is like tying the two speaker leads together...and a really high impedance is like playing the amp without a speaker connected at all. Neither is good for the amp. But the key word here is "extreme"



pdf64



UK

Oct 24th, 2014 11:43 AM        

'"Impedance" is the summation of ALL frequencies in the audible range.'<br /> <br /> That doesn't seem correct to me;<br /> a/ I think you may mean 'average' rather than summation?<br /> b/ the frequency averaged impedance of a guitar speaker is very likely to be rather higher than its nominal impedance, eg see http://www.eminence.com/pdf/Swamp_Thang.pdf the impedance varies from 8 to 110 ohms.<br /> My understanding is that the nominal impedance is derived from the speaker's impedance over the frequency range of greatest power delivery for the intended application.<br /> <br /> 'it's not so much a matter of "maximum power" is it is frequency response efficiency...Using the correct impedance match will give you the most efficient frequency response in the audible range.'<br /> <br /> I don't understand the distinction being made; please can you explain what is meant by 'the most efficient frequency response' and how it differs from 'maximum power'?<br /> <br /> 'There are lots of amps out there (including Fender) that have an external speaker jack that's hooked up to the main speaker leads in parallel without having a separate secondary tap on the output transformer. Basically when you add that second 8 ohm speaker, the output transformer is dealing with a 4 ohm load instead of 8. The world doesn't come crashing down'<br /> <br /> I agree but interestingly, for the past decade or so Fender seem to advise against this practice, eg http://support.fender.com/manuals/guitar_amplifiers/65_Twin_Reverb_manual.pdf<br /> and the advice is getting more specific and clear as time progresses, eg http://support.fender.com/manuals/guitar_amplifiers/68_Custom_Deluxe_Reverb_Owners_Manual_Rev-B_MULTI.pdf<br /> <br /> 'Usually when you use a lower impedance speaker than called for, the frequency response shifts brighter'<br /> What exactly is meant by this?<br /> I've not noticed it myself or seen any evidence to support it.<br /> <br /> 'when you use a lower impedance speaker than called for.. Taken to extremes... yea, you will start damaging your OT'<br /> <br /> It may be helpful to provide guidance regarding what is meant by extremes here, eg for an amp intended for an 8 ohm load, what is suggested as the minimum load?<br /> <br /> I don't see how a lower than nominal load will damage a guitar amp's OT, other than as a secondary effect, eg resulting from a power tube redplating / shorting?<br /> See linked thoughts on this topic by R G Keen.<br /> <br /> What is the proposed failure mode? What evidence is there to support the theory?<br /> My thinking is that Fender advise against it due the limiting plate dissipation of the power tubes likely being exceeded, rather than the OT somehow being damaged.<br />



amp mad scientist



USA

Jul 16th, 2015 03:46 PM        

The speaker must match the amp, or the amp can be badly damaged.<br /> Only one setting is correct.<br /> Don't try to set it wrong on purpose. That would be very foolish.<br /> <br /> A power increase may take place but:<br /> power does not equal loudness.<br /> <br /> If you need the amp to be louder:<br /> Use a more efficient speaker.<br /> That is the correct method.



amp mad scientist



USA

Oct 17th, 2015 06:35 AM        

You want the amp matched to the speakers.<br /> <br /> This is what happens when the load is incorrect:<br /> A. The tube sockets arc and burn.<br /> B. The power tubes are fried.<br /> C. The output transformer is destroyed.<br /> <br /> In other words: bad idea.



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