Possibly stupid question about dummy load

[Zork]

Maybe this is dumb but I want to discuss this anyway: I've got an old Peavey Mark IV head and I like it a lot. However, there's a problem: it only runs at its full power rating of 320 watts if it has a total load of 2 ohms which means I need a second cab if I want to keep the 4 ohms cab I already have and like.

Now, at 4 ohms, it "only" has about 150 watts which means, that at high volumes the genius "DDT compression circuit" gets triggered all the time. This is a very sqishy and brutal limiter/compressor that you can't turn off, as it's designed to protect the power amp and it's super annoying because it switches in and out as it likes. Sometimes there's even a pop audible through the speakers.

So I made up my mind. I don't want to buy new cabs. I don't want to rewire my cab because of possible damage and I want to use it with other amps as well. So I had the idea to build a dummy load of 4 ohms and connect it to the second speaker output. Voila, total load of 2 ohms.

Three of these bad boys wired in parallel with a heat sink in a 19" enclosure would make a 4 ohms dummy load that can handle up to 450 watts, right?

So, people use dummy loads like this to measure tube amps but apparently it would alter the sound somehow because they don't interact with the amp like a speaker. In my case it doesn't seem to matter, though, as the Peavey is a solid state amp anyway, I actually have a speaker on one of the 2 parallel outputs and I only use the dummy load to run the power amp at full power. Am I right or is this just one of these dumb late night quarantine ideas?

[somanytoys]

I’m glad you posted this, I’d be interested in the answer.

I would tend to agree with you, but I’m not a wizard about all of this. Wish I’d learned more about it earlier in life, instead of trying to grapple with all of it now.

Watts, volts, ohms, amps/milliamps, farads (I think), it just gets overwhelming (and especially hard to remember it all) if you don’t have a good knowledge base. And some mistakes can be expensive, or even deadly.

[Danny W.]

Any extra power you get will go into the dummy load--it just makes your amp work harder and get hotter. Your acoustic output will not increase.

Danny W.

[MT]

I’ve seen comments from amp builders saying that you shouldn’t connect SS amps to dummy loads (can’t remember if it was only reactive loads that were an issue) because the amp sees the reactive portion of the load box as a short which damages the amp. Most of the new reactive load boxes aren’t supported for SS amps.

Might be worth a look.

[Zork]

Any extra power you get will go into the dummy load--it just makes your amp work harder and get hotter. Your acoustic output will not increase.

Danny W.

It's not about getting louder, it's about giving the amp more headroom before that limiter kicks in. I know there are now 210 watts pushing the speakers. With the dummy load there will be only 150 watts left for the speaker, so it will be less loud. But the poweramp runs then on 300 watts which means more headroom, doesn't it?

[Zork]

I’ve seen comments from amp builders saying that you shouldn’t connect SS amps to dummy loads (can’t remember if it was only reactive loads that were an issue) because the amp sees the reactive portion of the load box as a short which damages the amp. Most of the new reactive load boxes aren’t supported for SS amps.

Might be worth a look.

Might be! I will check that out, thank you.

[Danny W.]

Any extra power you get will go into the dummy load--it just makes your amp work harder and get hotter. Your acoustic output will not increase.

Danny W.

It's not about getting louder, it's about giving the amp more headroom before that limiter kicks in. I know there are now 210 watts pushing the speakers. With the dummy load there will be only 150 watts left for the speaker, so it will be less loud. But the poweramp runs then on 300 watts which means more headroom, doesn't it?

NO! The extra "headroom" just goes into the dummy load, heating the room.

Danny W.

[DeathJag]

https://www.tedweber.com/z-matcher



I don’t think this uses resistors, and is expensive, but it works! I don’t know about loss or gain of headroom.

[andy_tchp]

It's not about getting louder, it's about giving the amp more headroom before that limiter kicks in. I know there are now 210 watts pushing the speakers. With the dummy load there will be only 150 watts left for the speaker, so it will be less loud. But the poweramp runs then on 300 watts which means more headroom, doesn't it?

NO! The extra "headroom" just goes into the dummy load, heating the room.

Yep.

In fact you may find the limiter kicks in more aggressively/at lower volumes to protect the output section with the dummy load (which won't react even remotely similar to the way a speaker's voice coil does when it's producing sound, given the thing you linked to is literally just a resistor with a high power rating) connected.

Rewire the cab/find a 2 Ohm cab/find more efficient speakers/turn the volume down so the DDT circuit doesn't activate to prevent nasty clipping/blowing up the amp.

You're at (and well beyond when there's an audible 'pop') the limits of sound output/"headroom" with the current setup.

https://www.tedweber.com/z-matcher

Limited to 100 Watts input.

[Zork]

Well, thank you all a lot for all the helpful advice. I spent some time with different guitars, different cabs and amps and could solve the initial problems of my post by just not cranking the preamp into overdrive (wich really sounds supercool, even though solid state and stuff...) but using a Rat instead. I still was too curious and got a cheap Harley Benton attenuator that you can use as a dummy load as well. It's only designed for 100 watts, but I thought for just a quick test - what the heck. Surprise: It doesn't make a difference. Like, not at all. It doesn't even change the volume. So the conclusio is: my idea was a dumb idea. I still don't know how my experience adapts to other amps or other setups, though.

[somanytoys]

That seems weird.

I have a couple of 5w deLisle attenuators for my AC4 TV stacks, and they do a VERY noticeable job, but only get down to a point. I have them mostly because there is only a volume knob, not separate gain & volume, and the attenuators built into the Vox TVs are pretty meh.

[andy_tchp]

No surprise: that was the suggested outcome...

That seems weird.

I have a couple of 5w deLisle attenuators for my AC4 TV stacks, and they do a VERY noticeable job, but only get down to a point. I have them mostly because there is only a volume knob, not separate gain & volume, and the attenuators built into the Vox TVs are pretty meh.

Not weird at all. You're using them as they were intended to be used, not loading up the second (parallel and previously unplugged) output of a solid state amp attempting to trick a self-protection circuit into not engaging when the amp is being pushed too hard.