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

Next 20 Messages  


Fender's are all I need!
Mar 6th, 2008 01:35 PM   Edit   Profile  

I do have a 4/8/16ohm switch. I'm currently running a 8ohm speaker (switch on 8). Would a 4 draw more watts? With the switch on 4? I have a Trinity 18 watter. Thanks

Contributing Member

New Jersey, USA

Hey! What exit?
Mar 6th, 2008 02:03 PM   Edit   Profile  

It's a tube amp. You don't get more power when you go to lower ohms, just an impedance mismatch that prevents the output transformer from transferring power as efficiently as it should. If you have the option, always use the impedance setting that matches your speaker(s).

Contributing Member

USA / Virginia

Mar 6th, 2008 04:08 PM   Edit   Profile  

Exactly. Amplifiers transfer the most power to a speaker when the speaker impedance matches the impedance of the amp.

In the case of solid state amps the impedance is typically extremely low so going to lower and lower speaker ohms improves the power transfer.....until the amp starts to current limit, or possibly blow up.....

Contributing Member

Austin TX

notes from the edge of the Bell curve
Mar 6th, 2008 04:18 PM   Edit   Profile  

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

Granted, many have pushed boundaries and captured signature tones.

The question is, unless you're an electronics whiz, how many amps can you afford to fry in the pursuit of "tone"?

If the amp wants to see 8 ohms, show it 8 ohms.

Contributing Member


Mar 8th, 2008 06:51 AM   Edit   Profile  

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.


Contributing Member

USA / Virginia

Mar 8th, 2008 06:58 AM   Edit   Profile  

"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)



Mar 8th, 2008 07:11 AM   Edit   Profile  

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.


Nor Cal

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

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)

Contributing Member

Land - O - Lux

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

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?




Oct 7th, 2010 12:16 PM   Edit   Profile  

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.



Oct 7th, 2010 12:17 PM   Edit   Profile  

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

Doubt it.



Oct 16th, 2010 10:06 AM   Edit   Profile  

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  

"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."



Jan 3rd, 2013 01:03 PM   Edit   Profile  

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  

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.



May 31st, 2013 02:44 PM   Edit   Profile  

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.

Hal G.
Contributing Member


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

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.



Nov 20th, 2013 09:39 AM   Edit   Profile  

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.

Hal G.
Contributing Member


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

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

Contributing Member


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

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)

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

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