Variable dummy load resister.

[Maude]

I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher powered amps this is mainly irrelevant these days it got me thinking.
With the old valve amps I believe you could plug a dummy resister in to simulate having a speaker plugged in so as not to damage the amp if powering up with no cab connected. Could a variable resister to used in cabs to allow you to alter the ohms rating of a cab, allowing for greater versatility and mixing and optimal matching of cabs?
Like I say, probably redundant these days with the more efficient equipment easily available but I was just wondering.
Waddya think

[Bill Fitzmaurice]

The power handling required of said resistor makes it impractical. That's why you don't see it done. Besides, you should never need more than two cabs, and if they're both 8 ohms there's no problem running one or the other or both. OTOH buying a 4 ohm cab is usually a mistake, unless it's large enough that you'd never need a second cab.

[Count Bassy]

[quote name='Maude' timestamp='1506117372' post='3376642']
I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher powered amps this is mainly irrelevant these days it got me thinking.
With the old valve amps I believe you could plug a dummy resister in to simulate having a speaker plugged in so as not to damage the amp if powering up with no cab connected. Could a variable resister to used in cabs to allow you to alter the ohms rating of a cab, allowing for greater versatility and mixing and optimal matching of cabs?
Like I say, probably redundant these days with the more efficient equipment easily available but I was just wondering.
Waddya think
[/quote]

Are you thinking of the Power dump resistors you used to get, so you could run your valve amp flat out to get the sound, but at a much reduced volume level? I think Marshall used to do them.

Just checked - Marshall did one called a Power Brake, but it seems there were also many others available.

[Guest bassman7755]

[quote name='Maude' timestamp='1506117372' post='3376642']
I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher Could a variable resister to used in cabs to allow you to alter the ohms rating of a cab, allowing for greater versatility and mixing and optimal matching of cabs?
[/quote]

In addition to bills points ... it would entirely self defeating because such a resistor is essentially a speaker that makes no sound yet still consumer power so I cant think of any possible scenario where this would increase efficiency.

[Maude]

I didn't think it a viable option or it would have been done and marketed in such a way that it would've sold.
It was just my mind wandering following the post in this section where someone wanted a single speaker with a 4ohm load but could only find 8ohm ones.
Wondered about the option of an 8ohm single speaker cab that you could flick a switch on that would put an 8ohm resister in giving a 4ohm load to the amp to get the most from the amp.
I know it's pointless, but many pointless things have been made and sold.

You can all ignore my idle ramblings now

[Bill Fitzmaurice]

[quote name='Maude' timestamp='1506154448' post='3376734']
Wondered about the option of an 8ohm single speaker cab that you could flick a switch on that would put an 8ohm resister in giving a 4ohm load to the amp to get the most from the amp.[/quote]You could, and half of what the amp produced would be burned off in the resistor. It's as logical as driving with your brakes always on so you can get the most power out of your engine.
[quote]I know it's pointless, but many pointless things have been made and sold.
[/quote]And elected...

[Merton]

[quote name='Bill Fitzmaurice' timestamp='1506170763' post='3376845']
And elected...
[/quote]

No idea who you're talking about Bill

(There's at least three on this side of the pond as well.....)

Ahem, sorry, strayed into an off topic subject we're not allowed to discuss!

[aitkenaudio]

[quote name='Maude' timestamp='1506117372' post='3376642']
I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher powered amps this is mainly irrelevant these days it got me thinking.
With the old valve amps I believe you could plug a dummy resister in to simulate having a speaker plugged in so as not to damage the amp if powering up with no cab connected. Could a variable resister to used in cabs to allow you to alter the ohms rating of a cab, allowing for greater versatility and mixing and optimal matching of cabs?
Like I say, probably redundant these days with the more efficient equipment easily available but I was just wondering.
Waddya think
[/quote]
[quote name='Maude' timestamp='1506117372' post='3376642']
I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher powered amps this is mainly irrelevant these days it got me thinking.
With the old valve amps I believe you could plug a dummy resister in to simulate having a speaker plugged in so as not to damage the amp if powering up with no cab connected. Could a variable resister to used in cabs to allow you to alter the ohms rating of a cab, allowing for greater versatility and mixing and optimal matching of cabs?
Like I say, probably redundant these days with the more efficient equipment easily available but I was just wondering.
Waddya think
[/quote]

thats why they have different impedance taps

[Yardicus]

Would something like this be useful for the likes of an Ampeg SVT CL head?
The head is 4 ohm or 2 ohm only, faced with an 8ohm cab I could drop in line an 8ohm resistor and run my head at 4ohms.

[Bill Fitzmaurice]

Thus sending 150w to the cab and 150w to the resistor.

[Yardicus]

[quote name='Bill Fitzmaurice' timestamp='1506271897' post='3377496']
Thus sending 150w to the cab and 150w to the resistor.
[/quote]

Wouldn’t that offer the head some protection tho?
8ohms is outside the design spec of the SVT CL head, 150w is better than damaging an output transformer.

[Norris]

A resistor would convert the power into heat. If you recall how hot an old 100W filament bulb used to be...

Edit: Actually I wonder what the resistance of one of those would be. It wouldn't do your amp much good when the bulb eventually blows though!

Edited September 24, 2017 by Norris

[Raslee]

There is a switch on the back of the Orange Terror Bass 500 watt amplifier that’s switches between 4 & 8 ohm. Orange claim the speaker will get 500 watts in either setting. I wonder how that works ?

[Yardicus]

[quote name='Norris' timestamp='1506275469' post='3377533']
A resistor would convert the power into heat. If you recall how hot an old 100W filament bulb used to be...

Edit: Actually I wonder what the resistance of one of those would be. It wouldn't do your amp much good when the bulb eventually blows though!
[/quote]

You wouldn’t have to use a bulb maybe something built for that specific purpose, how about an 8ohm cab and something like an 8ohm THD Hotplate set to load, would that achieve 4ohms, thus protecting the output transformer?

[Count Bassy]

[quote name='Raslee' timestamp='1506275702' post='3377534']
There is a switch on the back of the Orange Terror Bass 500 watt amplifier that's switches between 4 & 8 ohm. Orange claim the speaker will get 500 watts in either setting. I wonder how that works ?
[/quote]

I guess that it would change the tapping on the output transformer between 4 and 8 ohm tappings.

[Raslee]

[quote name='Count Bassy' timestamp='1506280754' post='3377591']
I guess that it would change the tapping on the output transformer between 4 and 8 ohm tappings.
[/quote]
Ahh thanks, that's how the voodoo is done then. It's a good little quirk, surprised more manufactures don't do this.

[Bill Fitzmaurice]

[quote name='Raslee' timestamp='1506275702' post='3377534']Orange claim the speaker will get 500 watts in either setting. I wonder how that works ?[/quote]With an SS amp the power is delivered by the output transistors to the speakers. The lower the speaker impedance the higher the current draw and therefore the higher the power. Valves deliver their power into the output transformer. Since it presents a constant impedance load current, and therefore power, is also constant.

[Tech21NYC]

What the OP is talking about does exist but more for the guitar amp market not for bass amps. Weber makes a device to change and match impedances but it is only rated for 100 watts. You could hook up a dummy load that that would be able to absorb the power of an amp but resistance based loads don't represent the reactive load of a speaker so the sound can be a bit flat. Most guitar attenuators on the market are designed for 100 watt guitar amps not 300 watt bass amps. We have load resistors for test purposes here and they handle about 400 watts.

Tube or valve amps have output transformers which is why they will deliver the same amount of power into different impedance loads. SS amps by contrast don't have output transformers so their wattage will change when hooked up to 2,4 or 8 ohm loads.

I wasn't quite sure what the OP was looking to accomplish.

[Count Bassy]

[quote name='Bill Fitzmaurice' timestamp='1506288085' post='3377698']
With an SS amp the power is delivered by the output transistors to the speakers. The lower the speaker impedance the higher the current draw and therefore the higher the power. Valves deliver their power into the output transformer. Since it presents a constant impedance load current, and therefore power, is also constant.
[/quote]

For the primary side of output transformer I agree, but the power transfer efficiency from the secondary to the speaker is best when the two impendences are the same, and falls off if mismatched either way, so the right impedance match is still going to give you more vaolume than a poor match.

(There was a time when I used to be able to do the sums to prove this - and the same sort of sums for an exponential horn - but that was a long time ago!)

[BigRedX]

I've got the Marshall Power Brake which I use with my guitar amp. It is big (twice the size of your average class D mini amp), heavy (almost 5kg) and has a rather noisy fan that kicks in when you stick anything extreme through it. Also it's only rated for 150W maximum. It does it's intended job (allowing you to run guitar power amp valves flat out without having to endure ear-bleeding sound levels) brilliantly. As an impedance matching solution for a modern bass amp it's completely pointless.

[Bill Fitzmaurice]

[quote name='Count Bassy' timestamp='1506521715' post='3379319']
the power transfer efficiency from the secondary to the speaker is best when the two impendences are the same, and falls off if mismatched either way
[/quote]That's why some amps have multiple taps. Nonetheless you won't get more power from a tube amp by using a lower impedance load the way you do with SS.

[Count Bassy]

[quote name='Bill Fitzmaurice' timestamp='1506524596' post='3379343']
That's why some amps have multiple taps. Nonetheless you won't get more power from a tube amp by using a lower impedance load the way you do with SS.
[/quote]
Yes I see what you mean (I think). The maximum power output from the tubes remains the same, but getting the right impedance match means you lose less of it in the transformer/speaker interface.

Edited September 27, 2017 by Count Bassy

[obbm]

[quote name='Count Bassy' timestamp='1506280754' post='3377591']
I guess that it would change the tapping on the output transformer between 4 and 8 ohm tappings.
[/quote]

Orange Terror Bass 500 is Class D Solid State and does not have an output transformer. The 4/8-ohm switch changes the settings of a limiter circuit at the front end of the power amp.

[bazzbass]

[quote name='Maude' timestamp='1506117372' post='3376642']
I keep seeing posts about the concerns with speaker cab ohms ratings, and although with more efficient speakers and higher powered amps [b]this is mainly irrelevant these days [/b]it got me thinking.

[/quote]

what makes you think speaker Ohms ratings are irrelevant?

Hook up two 4 Ohm cabs to a 4 Ohm amp and tell me what happens

[Maude]

[quote name='bazztard' timestamp='1506838005' post='3381368']


what makes you think speaker Ohms ratings are irrelevant?

Hook up two 4 Ohm cabs to a 4 Ohm amp and tell me what happens
[/quote]

I was meaning folks worrying about showing a 4ohm output amp a 4ohm load rather than an 8ohm load to get the most out of it. With the amount of amp power cheaply available and the more efficient speakers available there is little point worrying about wringing the last drop of power out of an amp these days, that was all.
Obviously nobody is going to plug two 4ohm speakers into a 4ohm amp unless they really don't know what they're doing, and again with the abundance of information at hand these days that's unlikely as well.