How to build a load simulator (is that the name)?

TheFuzzDog

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I have recently acquired several amp heads through a friend’s moving liquidation. I have always had combos before. I know that it is not good for the amp to turn it on without a speaker attached. I assume there must be a way to build a little box that acts like a speaker in case someone switches on the amp, but I don’t know what that’s called and haven’t been able to find anything in my searches so far.

I’d love to know both what they are called and find a circuit to build so I can rest easy.
 

JamesAM

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this might help, but it looks pretty involved:


this might be a fun project but to me I would rather trust it to an attenuator or load box (the weber mass/mini mass are what i have and weber advertises that they can work for the exact application you describe) with a speaker motor in it instead of just resistors.

How powerful are the amps you got? the higher the power the more opportunity there is to blow something up.
 

TheFuzzDog

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this might help, but it looks pretty involved:


this might be a fun project but to me I would rather trust it to an attenuator or load box (the weber mass/mini mass are what i have and weber advertises that they can work for the exact application you describe) with a speaker motor in it instead of just resistors.

How powerful are the amps you got? the higher the power the more opportunity there is to blow something up.
One’s a dual rectifier, the other is some form of Marshall, so both pretty high powered. Knowing that they are called load boxes is a huge help, and knowing that someone makes them already is even more of a help. I just assumed they would need to be diy. I’ll look at the Weber items.

Edit:
Ok I looked at them. Dang they are pricey!
 

Old Verle Miller

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I have recently acquired several amp heads through a friend’s moving liquidation. I have always had combos before. I know that it is not good for the amp to turn it on without a speaker attached. I assume there must be a way to build a little box that acts like a speaker in case someone switches on the amp, but I don’t know what that’s called and haven’t been able to find anything in my searches so far.

I’d love to know both what they are called and find a circuit to build so I can rest easy.
I have to ask ... who would be switching one of them on? I think the solution to that potential problem is to stash the power cords. ;)
 

Peegoo

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There are several attenuators available from different makers. Doc Z and THD make 'em.

One simple thing you can do if all you want to do is periodically idle the amp: get a small inexpensive speaker of sufficient capacity and correct Ohms rating for the amp. Make a pigtail lead for it from lamp cord and a 1/4" plug.

Like this one: CLICK HERE
 

Tom Kamphuys

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Something like this?
IMG_20201008_100114231.jpg

Or this: https://www.tdpri.com/threads/audio-test-set.1063896/post-10531145
 

Paul G.

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Or...now this is just crazy enough to work...unplug amp from the wall when you don't have a speaker plugged in. Cost -- nothing. Effectiveness -- 100%. Used in rehearsal and recording studios since forever.
 

TheFuzzDog

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I have to ask ... who would be switching one of them on? I think the solution to that potential problem is to stash the power cords. ;)
Who? One example would be the furnace repair guy who took it upon himself to plug in my MusicMan RD110 and “play a couple of riffs” because he “always wanted to check out a MusicMan, dude.” That power cable is unremovable.

Another could be the person “helping” at the gig who thinks putting it on standby is enough. There are lots of possibilities and I’d rather wear the belt and the suspenders than have to explain to anyone why I need several hundred dollars from them for a new transformer.
 

TheFuzzDog

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Or...now this is just crazy enough to work...unplug amp from the wall when you don't have a speaker plugged in. Cost -- nothing. Effectiveness -- 100%. Used in rehearsal and recording studios since forever.
Or - and this is just crazy enough to be possible - since I can’t always be by the side of my amp, I can’t predict what will happen to it. See my post above for an example. Crazier still, I’ve been known to make mistakes that I’d like to protect myself from.
 

JamesAM

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One’s a dual rectifier, the other is some form of Marshall, so both pretty high powered. Knowing that they are called load boxes is a huge help, and knowing that someone makes them already is even more of a help. I just assumed they would need to be diy. I’ll look at the Weber items.

Edit:
Ok I looked at them. Dang they are pricey!
Yeah, for high gain amps like the dual recto and any marshalls weber recommends getting one of their attenuators that's rated to twice what the listed wattage is for the amp. So for those that you've got, you'll probably need to buy the most powerful ones - unfortunately, those are actually cheaper than many of the other attenuators on the market. THD makes a less expensive one, I think. To me, an attenuator is a crucial piece of kit anyway - you get the natural gain from your amp at lower volumes - and having the load box capability for the use case you describe is a pretty solid bonus.

Many attenuators are just a bunch of resistors that absorb the current that normally goes to the voice coil of a speaker. Supposedly the Weber ones have an simulated speaker voice coil and magnet in them (this is the 'motor'). All i know is that they sound good with my 5e3 and 18w.

There are definitely some DIY methods for this but for a 100w amp i'm not sure if i'd trust anything I build not to burn up one of those big output transformers. other folks probably have a different risk posture than me, though.
 

Phrygian77

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You should check to see what no-load protection the amp(s) may already have before you go trying to solve what you think may be potential problem.

I posted a reactive load build thread about a year ago, for anyone who's looking for that kind of a solution.
 

Tom Kamphuys

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With no input signal or the volume at 0: not hot at all.

It depends on what you want. Do you only want some safety? Do you want something you can dump 100W in for hours?
 

TheFuzzDog

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With no input signal or the volume at 0: not hot at all.

It depends on what you want. Do you only want some safety? Do you want something you can dump 100W in for hours?
I just want to protect the amp from a stupid mistake. What I’d love is a small thing like you posted that I could just Velcro to the back with a short cable, so if it gets turned on it’s protected.
 

Phrygian77

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I just want to protect the amp from a stupid mistake.

What amps are we talking about here? Like I already said, some amps have some type no-load protection built-in. The biggest danger is infinite resistance. In that case, the output tubes will go into oscillation and wildly swing high voltage through the output transformer. That happens with no signal into the amp at all. There are a couple of ways to protect against that without having to have a full dummy load.

holy crap, @Phrygian77 your build thread is awesome. I wish I had seen this as you were working it.

op, check this out: https://www.tdpri.com/threads/reactive-load-build-aiken-tgp-suhr.1056610/

Thanks. I need to go back and update that thread because I think the Suhr component values are spot on, having compared mine now directly to the Suhr. Physically, it can be built the same as mine was. People have said Suhr copied Aiken, but to me, it's obvious that Suhr put a lot of R&D testing into what they did.
 

TheFuzzDog

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What amps are we talking about here? Like I already said, some amps have some type no-load protection built-in. The biggest danger is infinite resistance. In that case, the output tubes will go into oscillation and wildly swing high voltage through the output transformer. That happens with no signal into the amp at all. There are a couple of ways to protect against that without having to have a full dummy load.



Thanks. I need to go back and update that thread because I think the Suhr component values are spot on, having compared mine now directly to the Suhr. Physically, it can be built the same as mine was. People have said Suhr copied Aiken, but to me, it's obvious that Suhr put a lot of R&D testing into what they did.
As I said above, they are a dual rectifier and “some kind of Marshall” 100 watter. I don’t know the Marshall model because it’s in a box in the garage at the moment. I’d love to know what the other ways to protect them are.
 

chris m.

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As I said above, they are a dual rectifier and “some kind of Marshall” 100 watter. I don’t know the Marshall model because it’s in a box in the garage at the moment. I’d love to know what the other ways to protect them are.
Maybe always physically attach the speaker cable to the power cord with a little velcro strap? That way you have a physical reminder to plug in the speaker cabinet before you plug in the amp head. Because when you grab the power cord to plug it into the wall there's a speaker cable dangling from the end of it.

You could also take a piece of blue masking tape and tape it over the power switch on the amp head, with "SPEAKER!!" written on it in Sharpie. Another reminder to make sure you have a speaker plugged in before you turn on the amp.

I think it takes a while to burn up a transformer. So if you forget to plug in a cabinet and you turn the head on, and no sound comes out, ideally you realize what is happening within a minute or two and turn it off.

As far as turning an amp on standby, there is no need for that. Always just turn it on completely. You can also always just turn it off completely.

If you have a vintage tube amp that is original spec, it may protect the capacitors, not the tubes, to put it on stand by for 30 seconds or so before powering it all the way up. But chances are it has been re-capped with newer electrolytic capacitors that can handle the extra voltage so there is really no need to use the standby switch at all even for a vintage amp since it will have modern electrolytic caps in it. If it hasn't been re-capped, you should re-cap it asap or that could cause a real problem.

 




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