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FDP Forum / Amp Mods, Repairs, and Projects / Ohms?? What would draw the most from my amp?

Previous 20 Messages  
guitarcapo

U.S.A.

Jun 18th, 2014 07:34 AM   Edit   Profile   Print Topic   Search Topic

"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.

Different frequencies will involve different resistances. "Impedance" is the summation of ALL frequencies in the audible range.

As such, 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. Mismatching creates a different response but doesn't necessarily "blow your output transformer" if you are off a little.

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.

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.

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"

(This message was last edited by guitarcapo at 09:36 AM, Jun 18th, 2014)

pdf64

UK

Oct 24th, 2014 11:43 AM   Edit   Profile   Print Topic   Search Topic

'"Impedance" is the summation of ALL frequencies in the audible range.'

That doesn't seem correct to me;
a/ I think you may mean 'average' rather than summation?
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.
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.

'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.'

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'?

'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'

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
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

'Usually when you use a lower impedance speaker than called for, the frequency response shifts brighter'
What exactly is meant by this?
I've not noticed it myself or seen any evidence to support it.

'when you use a lower impedance speaker than called for.. Taken to extremes... yea, you will start damaging your OT'

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?

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?
See linked thoughts on this topic by R G Keen.

What is the proposed failure mode? What evidence is there to support the theory?
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.

R G Keen

(This message was last edited by pdf64 at 12:46 PM, Oct 25th, 2014)

amp mad scientist

USA

Jul 16th, 2015 03:46 PM   Edit   Profile   Print Topic   Search Topic

The speaker must match the amp, or the amp can be badly damaged.
Only one setting is correct.
Don't try to set it wrong on purpose. That would be very foolish.

A power increase may take place but:
power does not equal loudness.

If you need the amp to be louder:
Use a more efficient speaker.
That is the correct method.

amp mad scientist

USA

Oct 17th, 2015 06:35 AM   Edit   Profile   Print Topic   Search Topic

You want the amp matched to the speakers.

This is what happens when the load is incorrect:
A. The tube sockets arc and burn.
B. The power tubes are fried.
C. The output transformer is destroyed.

In other words: bad idea.

Previous 20 Messages  

FDP Forum / Amp Mods, Repairs, and Projects / Ohms?? What would draw the most from my amp?




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