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

Previous 20 Messages  
dmoulton
Contributing Member
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canada

Mar 8th, 2008 06:51 AM   Edit   Profile   Print Topic   Search Topic

If you have a 4/8/16 ohm transformer won't it deliver the most at 16 using all the windings on the transformer?? Or doesn't it matter, I'm stll a bit confused about this.


David



ECS-3
Contributing Member
**

USA / Virginia

Mar 8th, 2008 06:58 AM   Edit   Profile   Print Topic   Search Topic

"If you have a 4/8/16 ohm transformer won't it deliver the most at 16 using all the windings on the transformer?? Or doesn't it matter, I'm stll a bit confused about this"

No using 'all the windings' does not give you the maximum power transfer. The maximum power transfer occurs when the impedance tap chosen matches the speaker impedance. If you are using a 4 ohm speaker then yes there will be 'unused' windings. The unused windings just sit there doing nothing.

The maximum power transfer ALWAYS occurs when the impedance of the amplifier matches the impedance of the speaker. The purpose of the output transformer is to take the speaker impedance and 'transform' it up to a high impedance that matches the amp.

Solid state amps don't normally use output transformers because they have a very low impedance. So connecting lower and lower impedance speaker to SS amps draws more power because the lower you go the more closely the impedance matching becomes. There is limit to how much current a SS amp can deliver so that becomes the limiting factor for most SS amps.

Some SS amps do use impedance matching transformers (Mcintosh does) and their pupose is to allow you to lower the impedance of say a 16 ohm speaker to make it more closely match the amp impedance.

(This message was last edited by ECS-3 at 07:02 AM, Mar 8th, 2008)

Leftee
Moderator

NC

Mar 8th, 2008 07:11 AM   Edit   Profile   Print Topic   Search Topic

I'm going to stick this thread to the top. BillM nailed it in post 2 and others added to it. This subject comes up about once a week or so. Let's see if doing this decreases the amount of threads on this subject.

Larrybudd

Nor Cal

We'll get there.
Apr 19th, 2010 01:42 AM   Edit   Profile   Print Topic   Search Topic

Gerald Weber always says the highest impedance tap uses the most of the transformer and sounds best. I don't know personally how true this is, but he has a lot of experience. Of course you'd have to use a 16 ohm speaker to get the most out of a 16 ohm tap.

(This message was last edited by Larrybudd at 01:44 AM, Apr 19th, 2010)

Vibroluxer
Contributing Member
**********
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Land - O - Lux

Pittsburgh: The City of Champions!
Apr 24th, 2010 02:23 PM   Edit   Profile   Print Topic   Search Topic

I have a Dawg Deluxe and it has a 16, 8, and 4 ohm selector on it. I have a Weber P12Q 8 ohm speaker in it. I also have a 4 ohm 2 twelve cabinet. There is also a speaker out jack.

Of course, I can unplug the cable, switch to 4 ohms, and use the cabinet.

Can I use all 3 speakers at the same time without blowing something up?

Thanks!

sixtysixpro

USA

Oct 7th, 2010 12:16 PM   Edit   Profile   Print Topic   Search Topic

I have a tech friend who lets me put my tinkerings up on his bench scope, in order to dial them in properly. I saw one of them range from around 2 watts output to just over 14, just by running different taps into different loads. I'm a pretty tough one to convince that you won't see a difference in output power by properly matching the amp to the correct load.

That said, there's really no way to know what that magical combination is, without putting the amp on a scope and trying different taps and loads. While the maker might very well say to use 4 for 4, 8 for 8, and 16 for 16, the fact is, a given amp MIGHT just be happiest with a mismatch.

The GENERAL rule is to follow manufacturer's guidelines, of course, but that alone doesn't necessarily guarantee the best results for every amp made.

sixtysixpro

USA

Oct 7th, 2010 12:17 PM   Edit   Profile   Print Topic   Search Topic

"Can I use all 3 speakers at the same time without blowing something up?"

Doubt it.

limbe

Sweden

Tibbe
Oct 16th, 2010 10:06 AM   Edit   Profile   Print Topic   Search Topic

Gerald Weber may know a lot about guitar amplifiers
and have a lot of experience but in this case he is mistaken.As long as the transformer tap matches the speakers impedance,4 ohm tap into a 4 ohm speaker or a 16 ohm tap into a 16 ohm speaker you have maximum power transfer.The sound will be identical.

Paul Boudreau
Contributing Member
*******

Washington, D.C.

So' maschio ma non fanatico...
Dec 22nd, 2010 01:32 PM   Edit   Profile   Print Topic   Search Topic

"Rockers from Link Wray to Dave Davies to Eddie Van Halen have professed that they get their best sound from abusing their gear."

I wonder if Leo would have said that any intentional amp distortion was "gear abuse."



ampman91510

USA

Jan 3rd, 2013 01:03 PM   Edit   Profile   Print Topic   Search Topic

The amp "books" are highly overrated. Stuff like Gerald Weber and Aspen Pittman are FULL of misinformation. I've even seen a video of Gerald getting shocked by an amp WHILE he's talking about how not go get shocked by an amp! - grin - :)

Steve Dallman
Contributing Member
******

Merrill, Wisconsin

Age is just a number...mine is big
May 24th, 2013 09:13 AM   Edit   Profile   Print Topic   Search Topic

The 16 ohm tap thing is a myth. The way the field sets up in the transformer, when you are using the amp, matching the speaker load results in the most efficient transfer of power. If you have 4, 8 and 16 ohm taps, and have three identical, same efficiency cabinets of 4, 8 and 16 ohms, each speaker into the matched tap will perform the same.

A few tube amps do suggest mismatching as a tonal tool. Many amps do not handle mismatches well and damage can result in many amps with a mismatch. Fender blackface and silverface amps can handle a 100% mismatch, although you lose a few watts when mismatching. Mesa suggests using mismatches to change tone and response, but even with them, matched is best. One amp brand does not label the taps, really encouraging mismatches.

With SS amps, the amp will produce power depending on the speaker load. The lower the resistance, the more power produced (not true of tube amps.) A SS amp will list the minimum safe load to use. Go lower than the min and the amp will produce more power than the components are rated for and the amp will fail. Going higher will produce fewer watts, but the amp will run cooler.

I have read Weber's explanation for using the 16 ohm tap into a 16 ohm speaker load. It seems to make sense, but a more technical explanation (Kevin O'Connor, author of the "Ultimate Tone" series of books and boutique amp builder has explained it) reveals it is bogus. There are lots of tube amp myths around.

With a blackface or silverface Fender, say a Deluxe Reverb, the most efficient transfer of power will be 8 ohms. Plug in a second 8 ohm cabinet, producing 4 ohms, the amp will run fine, but a little hotter, and will lose a couple watts. The loss of watts will be offset by the increased speaker area, and since no harm will result, use whatever sounds best to you.

If you daisy chain three 8 ohm extension cabinets, for a 2 ohm total load, you are at a 200% mismatch, and you will likely damage the amp.

pdf64

UK

May 31st, 2013 02:44 PM   Edit   Profile   Print Topic   Search Topic

I guess most people understand what is meant by '100% mismatch etc' but I feel that it's not being used in a technically correct manner.
Just to clarify, my understanding is that people mean that (in their opinion) a load between half and double the amp's specified load can be tolerated without damage being likely to occur.
I agree that's likely to be the case with most BF Fenders but I disagree that it's a universal rule. Rather (in my opinion) it's most applicable to a high quality amp running beam tetrode power tubes at a 'reasonable' B+ (eg below say 480V).
Some late 70s / early 80s SF and BF Fenders ran a B+ of over 500V; my view that that they should be run into the specified load. Many of those amps were designed around the STR387 power tube, which was effectively the next evolution of the 6L6GC, and they pushed them very hard. Modern 6L6GC fall well short and to compound their predicament by running into a mismatch could result in short service life
Some Fenders use pentode power tubes, eg BJ; my view is that the default position should be that pentodes be run into the specified load.

The reality is that tubes put out good power over a wide range of loads, which is useful because speaker impedance varies massively with frequency; hence the 'double to half' rule of thumb. However, counter to that, some amps push their tubes up, and beyond, their design limits, such that a load mismatch would make a bad situation worse.
So a generic 'one size fits all' mismatch guideline may not always hold true; the correct thing is to use the specified load on the amp, unless there's very good evidence that a certain deviation from that is tolerable.
In the case of a pair of 6L6 type tubes with a B+ below 480V, my understanding is that current production amps operate them somewhere between 4k and 8k plate to plate impedance.

'If you daisy chain three 8 ohm extension cabinets, for a 2 ohm total load'
Just to clarify, that's 3 x 8ohm extension cabs AND the built-in combo speaker, all in parallel, ie 4 x 8ohm speakers in total.
Pete


Hal G.
Contributing Member
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****

USA

Hi, I'm an ampoholic. Hi Hal!
Oct 4th, 2013 10:51 AM   Edit   Profile   Print Topic   Search Topic

My recently acquired Ampeg BA115HP is supposed to have a 4 ohm speaker 15" and a crossover circuit. A prior owner has been in there and now the crossover circuit is all screwed up and missing parts and there is an 8 ohm 15" speaker. I want to run just the 8 ohm 15". I know it will be down on power but is still pretty darn loud. Will it hurt the amp to run this kind of mismatch? Thanks in advance.

pdf64

UK

Nov 20th, 2013 09:39 AM   Edit   Profile   Print Topic   Search Topic

That doesn't seem to be a tube amp.
Transistor amps are generally fine into higher value loads, the spec value is a minimum, at which the power output is greatest.
However, if you tried to drive it hard into a 2 ohm total load then it may overheat, and possibly go into thermal shutdown or get damaged.

There have been reports of some transistor amps, which may have current or mixed feedback architecture, that don't work well into loads of a higher value than specified.
Pete

Hal G.
Contributing Member
**********
****

USA

Hi, I'm an ampoholic. Hi Hal!
Nov 21st, 2013 03:19 PM   Edit   Profile   Print Topic   Search Topic

Thanks Pete. You confirmed my thoughts on soidstate being ok higher impedance but not lower.

smokestack
Contributing Member

u.k

"Edited for bad taste...."
Apr 4th, 2014 08:56 AM   Edit   Profile   Print Topic   Search Topic

With solid state these issues have seen more discussion and have more relevance in the realm Hi Fi equipment.

The quality of domestic power amplifiers is often partially defined by the stiffness of their power supplies, their ability to swing current and to drive low impedance loads with good control.
Good power amp designs will typically deliver twice as many watts into 4 ohms as they do into 8.

(This message was last edited by smokestack at 10:57 AM, Apr 4th, 2014)

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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