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Ampeg B25B Tube amp louder with 8ohm or 16ohm cab?


SurroundedByManatees
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I've got a Ampeg B25B tube amp. The speaker out is for a 16ohm cabinet, but apparently it can be switced to 8ohm by putting a dummy plug in the output, and connect the 8ohm cab to the ext. Speaker output.

 

Will there be a difference in volume between 16ohm and 8ohm, given the cabs are the same apart from impedance (glockenklang Quattro)?

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Just to be clear. A dummy plug is a standard ¼” plug that is left open. When the plug is inserted into the speaker jack, it reroutes the amp’s output from the 16 ohm to the 8 ohm transformer tap. This allows you to plug in an 8 ohm speaker cab and the impedance between the amp and the speaker cab will be matched. A dummy load is something else.

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A tube amp is designed to produce the same power output on each output transformer tap with a cab that matches the impedance. So a 16 ohm cab on a 16 ohm tap will produce full output power, an 8 ohm cab on an 8 ohm tap should produce the same output power. 

 

You might hear differences with 8 and 16 ohm cabs. The speakers are different, the cab designs are also different. One cab may be louder than the other due to sensitivity differences. Even the same speakers can have different specs due to tolerance difference coming off the assembly line.  

 

Looking up the Quattro, they say that it can be either 4 or 8 ohms. The same cabinet, with the same speakers, just wired differently internally should sound the same.

 

 

 

 

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13 hours ago, beans-on-toast said:

Looking up the Quattro, they say that it can be either 4 or 8 ohms. The same cabinet, with the same speakers, just wired differently internally should sound the same.

Eh? It's not possible to rewire 8 to 4 or vice versa!

 

Maybe they meant to say it's available for purchase as 4 ohm OR 8 ohm?

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Well, i did A/B the cabs.

 

I didn't expect them to differ so much in sound as they do. Volume wise they are pretty much similar with the Ampeg B25B. 

 

The 4/16 Ohm cab seems more mid oriented where the 8 Ohm cab has definitely deeper lows in the same neutral EQ settings. Also the the highs seem a bit warmer and more clear on this one

 

Both cabs sound really good, and maybe the differences narrow at higher volumes. Now I have to decide which one is the best choice for me... I am leaning towards the 8ohm, though the more mid focused 16ohm cab can of course be great as well. Let's not forget that there's is an amp on top with some eq on it, and i also have a preamp pedal I can use. It's a tough one for me.

 

Anyone has some suggestions to come to a wise decision?

Edited by SurroundedByManatees
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When you play live how does the audience hear the bass guitar? Direct from your rig or via the PA?

 

If it's the first use whichever cab sounds best in the band mix. However make this decision based on what can be heard out in the audience and not from the stage.

 

If it's the second use whichever cab allows your to hear yourself most clearly at the lowest volume when on stage with your band.

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On 03/11/2021 at 09:54, beans-on-toast said:

Just to be clear. A dummy plug is a standard ¼” plug that is left open. When the plug is inserted into the speaker jack, it reroutes the amp’s output from the 16 ohm to the 8 ohm transformer tap. This allows you to plug in an 8 ohm speaker cab and the impedance between the amp and the speaker cab will be matched. A dummy load is something else.

Ah - I misread that as Dummy Load.

 

That makes sense!

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On 01/11/2021 at 21:25, SurroundedByManatees said:

Thanks for the answer. Both cabs concerned are Glockenklang Quattro, so pretty much the same I guess.

 

 

On 04/11/2021 at 11:37, SurroundedByManatees said:

Well, i did A/B the cabs.

 

I didn't expect them to differ so much in sound as they do. Volume wise they are pretty much similar with the Ampeg B25B. 

 

 

Anyone has some suggestions to come to a wise decision?

I did wonder about saying something, it isn't that surprising as to change the impedance you have to change the voice coil at the very least. That could change the excursion levels and a few of the electrical characteristics even if as usual the maker keeps the same magnet and cone. Effectively it becomes a different speaker. That might mean having to tweak the cab to match what are now different drivers or making other minor tweaks to restore the 'sound' of the cab. The differences aren't necessarily that big but in A to B tests our ears are really sensitive. I'm not sure how much tweaking goes on in practice.

 

Sorry can't help with wisdom :)

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