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Tube Amp Dummy Load questions
Tube Amp Dummy Load questions
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Old 17th April 2018, 06:57 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by Loudthud View Post
A dummy load puts more stress on a tube amp than a speaker does. It will wear the tubes out faster.
As Printer2 said, this can happen if you attenuate the speaker with a dummy load (to make it quieter), and then turn up the amp more than usual to try and get back the volume you just lost.

This is a fundamentally silly thing to do from the engineering standpoint, but also exactly the sort of thing a frustrated guitarist feeling hamstrung by his reduced volume might do.

There is a good emotional reason to do this as well: quieter sounds have less emotional impact on us (imagine if a lion's roar were no louder than a kitten's miaow: would it be as terrifying?) So, the guitarist, wanting more emotional impact from his / her music, will quite naturally reach for the volume knob.

Though, Thoglette's point notwithstanding, I do know guitarists who normally do not turn up their amps to eleven.

-Gnobuddy
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Old 21st April 2018, 03:56 AM   #22
dlvoots is offline dlvoots  United States
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Location: Dallas, Georgia (USA)
All,

WARNING! Extremely Long Post

Here is my experience in building and using dummy loads, both 100% resistive and 100% reactive. The goal of this post is to explain what is going on from an elecrical engineer's perspective and at the same time, describe what is as going on in the head of every player that ever grabbed a guitar and cranked the amp to 11.

I've lived this story, so I'm hoping that I can explain in an adequate technical manner as to why a resistor doesn't quite cut it from a player's perspective. If I'm successful, then you'll also have an explanation for why players want to turn the amp up louder than they might normally play in order to compensate for the resistor's less-than-perfect performance.

Here we go.

When an electrical engineer tests an amp, the tests fall into two basic families;
  1. Tests of the internal circuitry of the amp where you don't want the outside world's environment to influence the behavior of the amp.
  2. Tests of the amp as an entity attached to the outside world, where you want to verify that the amp will survive and function well, no matter what it is connected to or what kind of trash the environment throws at it.
For all of the Group A tests (amp internals), a non-inductive resistor that can handle the entire output of the amp at the proper resistance and power handling level works perfectly. Why? Because even though an amplifier generates an alternating current signal, both current and voltage in the signal are in phase with each other when the amp amplifies the signal. The current and voltage swings also stay in phase as the amp heats up the load resistor. The resistive dummy load simplifies the diagnosing of things in the event the amp goes south internally.

If the engineer has done his/her design homework properly, you'll find you have an amp where it's DC / resting characteristics into a resistive load (i.e. no signal) are 100% in sync with it's AC / operating characteristics into a resistive load (i.e. amplifying a signal). But alas, congratulations are NOT in order if things are successful up to this point. Not yet.

In reality, amps don't get hooked up to resistive loads very often (which brings us to the purpose of the Group B tests). Instead, most amps get connected to these horrible electro-mechanical monsters called loudspeakers. In turning the amp's electrical energy into mechanical energy (sound), speakers attempt to "pay the amp back" by fighting the amp electrically every step of the way. Turns out that this is a good Real World 101 test for an amp as it interacts with its environment. Passing Group B tests succcessfully demonstrates that an amp is stable at the same time it is amplifying signal and causing a speaker make sound with its output.

As compared to a resistive dummy load, speakers change everything. Internally, a speaker consists of a permanent magnet, a coil of wire and usually, a paper cone to which you permanently attach the coil of wire via an assembly known as a Voice Coil (the cone can be made of other materials, but most guitar speakers have paper cones). If you orient the voice coil so it physically sits at rest in the middle of the magnet's magnetic field and send an AC or DC voltage into the voice coil, the voltage will generate an electromagnetic field in the voice coil that will either move the voice coil in towards the back of the magnet or out towards the front, depending on the polarity of the electromagnetic field.

The problem is that in constructing a speaker in this way, we have also created a nasty conundrum related to its use; time. In a Perfect Speaker, we want our Perfect Speaker Cone to start moving at exactly the same instant as the voltage from the amp reaches the voice coil. In reality, generating that electromagnetic field via the voice coil takes a finite amount of time to build up. Moving the voice coil physically forwards or backwards adds even more time. Bottom line is that speaker movement always lags behind the AC voltage swings that came from amp.

Additionally, since the cone is physical and is attached to the voice coil, the mass of the cone is going to physically fight tooth and nail against any change the in direction the voice coil tries to move it. The physical mass of the cone adds its mechanical resistance to change to the electrical resistance to change that we already have.

It can't get any worse than that, right? Wrong. Most AC signals cross back and forth across a "zero voltage" line, meaning that the positive half of the wave form causes the cone to move physically out of the speaker, and the negative half of the wave form causes the cone to move physically towards the back of the speaker. When that transition occurs, the electromagnetic field surrounding the voice coil collapses and reforms in opposite polarity. That energy doesn't just disappear. It now turns into voltage that ALSO opposes the movement of the cone. In extreme high decibel playing, the opposition voltage may "flow backwards" into the amp and cause mischief for a half-cycle or two.

To make matters worse, the amp usually wants everything to move in the opposite direction for the second half of the wave form. That electromagnetic field around the voice coil will fight any change in polarity the amp sends it.

From an engineer's perspective, that electrical opposition to AC current flow has a name -- Reactance. The electrical engineer knows that the only way you have resistance is with a resistor. Resistors oppose the flow of current by turning it into heat, but here we have that same opposition to current flow....without any resistor at all. That's the purpose of both Inductive and Capacitive Reactance -- to quantify the opposition to current flow in an AC circuit or simulation just like a resistor does with DC. And a voice coil qualifies as a genuine official card carrying inductor by any definition.

Capacitors and Inductors "mess with" an AC signal and oppose its passage by throwing the voltage and current in the signal out of phase with each other by a predictable amount. The old electrical school memory aid -- ELI the ICE man -- is an attempt to help students understand that phase relationship. ELI tells you that the voltage (E) leads or comes before current (I) as it passes through an inductor (L). ICE does the opposite as electricity passes through a capacitor.

So what does the player care about any of this? Very little to nothing.

What makes the player happy is that as an entity, the speaker fights the amp tooth and nail to keep from having to move the voice coil / speaker cone at all. The side effect of that fight is the generation of a delightful layer of distortion in the signal going to the speaker that is separate from any distortion voice the amp itself may have. And THAT is what makes the player happy.

It ain't over yet. In response, the amp tries to fight back at the speaker, and in so doing, it tries to force the speaker move when the amp wants it to. To the electrical engineer, that back and forth fight has a name - Damping. It is a measurement of the amp's ability to win that fight and control the movement of the speaker as it tries to fight back.

It depends on the player and style of playing, but most players love the sound a poorly damped amplifier makes (to a Hi-Fi Listener, poor damping is a disaster). The reason is that lack of good damping adds another layer of distortion on top of the layer of distortion already caused by the speaker constantly fighting the amp. My subjective description is that poor damping gives a "jangly" sound to a complex guitar passage, almost as if the amp forgets to stop playing when the guitar stops.

To summarize the situation described above, the engineer-type can define several types of distortion that have conspired together to make the signal different than what was input to the amp. From input jack to the speaker cone, they are:
  1. The "designed in" amplitude and intermodulation distortions that are characteristic side effects of the amp itself, including clipping if the amp is over-driven. This distortion will show up in a resistive dummy load and also when the amp is connected to a speaker.
  2. Distortion caused by the Inductive and Capacitve reactance of the speaker voice coil as its electromagnetic field builds, collapses, anf reforms based on changes dictated by the amp. This kind of distortion will not exist if the load is 100% resistive. It only shows up if the amp is connected to a speaker.
  3. Distortion caused by the physical resistance of the speaker cone as it fights movement (damping). It translates into additional inductive and capacitive reactance (all coils have both capactive and inductive reactance, but normally the inductive component overshadows everything. Cranking the amp up to "11"? Well..........). This kind of distortion will not exists if the load is 100% resistive. It only shows up if the amp is connected to a speaker.
  4. Distortion that originates as "back current" from the speaker. If the cone is moving in the opposite direction as the current from the amp wants it to move, the speaker will momentarily turn into a generator and create electricity from the movement. It may cancel electricity arriving from the amp in any given microsecond, or it could just as randomly add itself to the electricity coming from the amp. This kind of distortion will not exists if the load is 100% resistive. It only shows up if the amp is connected to a speaker.

You could make a pretty convincing argument that 3 and 4 are different descriptions of the same phenomenon and I would probably agree with you. The take away is that they contribute in a fundamental way to the amp's overall distortion voice, and that distortion voice is what delights a player.

As you may have already noticed, the engineer-type can effectively remove three out of four distortion layers in the amp's sound by the simple act of substituting a load resistor for the speaker. The engineer is quite happy with the results of that swap. The player? Not so much.

Enough of that boring-but-fascinating theory......

I suspect the anecdotes that other posters have written about players turning the amp higher when playing into a resistive dummy load are attempts to compensate for the three (or four or more) missing distortion layers. The player hooks into a resistive dummy load, cranks the amp and plays, and then says "It Just Don't Seem To Sound Right". Aaaand, in response, the player cranks the amp even more, maybe even causing it to clip in an effort to make it sound better. And if so, turning the amp up louder than "normal" will absolutely stress the amp above and beyond the level it would have been normally played at.

Subjectively, the speaker-caused distortion is rich in audible components, but it is just as hard to describe objectively as it is fantastic sounding. It's one of those things where you just know it when you hear it. One thing it is definitely NOT is the sound of an amplifier circuit hitting the power supply rails and clipping the signal before it hits the speaker.

If the above scenario has any truth to it, it would explain why the manufacturer would refuse warranty work on an amp that is routinely played into a resistive dummy load. The amp is truly stressed more, but it is not caused by using the resistive load itself. To the contrary, a 100% resistive load is actually easier on the amp, because voltage and current are nicely in phase with each other.

Unfortunately for the player, that electrical nicety just don't cut it, so the amp gets cranked higher than it normally would. My betting money is on the poor amp electronically clipping the signal, resulting in more stress on the electronics even as the clipped sound the poor amp makes causes even more player-undesirable sounds.

The bottom line of this whole discussion is that in order to make a dummy load sound like the amp is connected to a speaker, you have to connect the amp to a speaker.

Huh?

At first read, the statement above might sound like nonsense. At the very least, it immediately strikes you as counter-intuitive. But trust me, there is a method in the madness (and to this story)

To achieve that speaker-amp interaction and get those additional layers of distortion, you don't have to have a specific speaker. ANY speaker will do. Big or little, fat or thin, even round versus square it doesn't matter. As long as the amp can handle the speaker impedance and the speaker can handle the power, you're good to go. And if you have several speakers -- say like four speakers to mimic a half-stack -- you'll find your're getting close to Tone Heaven.

The trick is to connect the speakers to the amp in the normal fashion, but you DON'T put them in a speaker cabinet. Instead, you put them into a sealed wooden box all nestled comfortably amongst multiple layers of carpet foam. Each layer of foam gets round holes cut into it to hold the speakers still, but they simply sit inside the sealed box and make a lot of noise. Thankfully, nobody outside the box can hear them unless you put your ears up next to the box.

In my case, I bought three 5" 50 watt "Hot Spot" speakers because they were cheap and could handle 150 watts of amplifier power in total without burning up. I wired them in series / parallel just like you would normally find in any garden variety half-stack, but in place of the fourth speaker, I substituted a 100 Watt 8 Ohm Fader / L Pad Attenuator and connected it to an output jack. Then I hooked any old speaker cab to the jack. Even your normal gig cab will work just fine.

In operation, the three Hot Spot speakers fight the amp tooth and nail, but nobody hears them. However, because they are legitimate card carrying speakers, they all contribute to the distortion voice of the amp-as-a-system in the best possible way.

The fader? Yeah, it's resistive, but its only purpose is to control how much power goes to the cab that I'm listening to. The cab that is connected doesn't contribute anything to the amp's distortion voice either, but it doesn't need to -- the three Hot Spots and the amp itself have already done that for you.

The primary benefit of making an "Active Dummy Load" like this is that a player doesn't have to turn the amp up any higher than normal to get That Sound. It's already there!

I built one of these active dummy loads several years ago. Even put a line out jack for DI recording and a volume control on it to boot. Over the next couple of days, I'll take it apart, take some "gut shot" pictures and start a new topic on how to build one.

Dave

Last edited by dlvoots; 21st April 2018 at 04:14 AM.
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Old 25th April 2018, 09:12 AM   #23
Brettn56 is offline Brettn56  Australia
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Use a mains voltage light bulb. Speakers are generally quite inductive (due to the voice coil), so a light bulb is not much different. Obviously the cold resistance of a light bulb is very low. I generally series a string of 3 240v bulbs, each being about 60w rating.
Cheap, easy and readily available.
I've also used a single 60w light bulb on my large high power amplifier, not once have i had a problem.
Give it a try, you might be surprised.
As for taking low level audio from the load, use an audio transformer maybe? Obviously you will need to match voltages and impedance...
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Old 25th April 2018, 09:44 PM   #24
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Brettn56
Speakers are generally quite inductive (due to the voice coil), so a light bulb is not much different.
Light bulbs aren't inductive, you know. They're resistive. So I don't understand why you say a voice coil and a light bulb are not much different.

Electrically, the only big difference between a regular power resistor and a light bulb is the fact that the resistance of the light bulb changes dramatically as it heats up. The resistance also varies wildly with the applied voltage - more resistance when you apply more voltage (until you apply so much voltage that the bulb burns out.)

If light bulbs work for you in a speaker attenuator, ordinary power resistors will work just as well. Better, actually because they won't present the amp with a load that varies erratically with power output.

I also wouldn't say speakers are mostly inductive. Look at the impedance plots of a few guitar speakers, they are usually mostly resistive around 300 Hz, which is where the meat of a guitar signal lies. There is certainly some inductive rise above that, but keep in mind there isn't much guitar output above 4 or 5 kHz.

-Gnobuddy
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Old 3rd May 2018, 09:14 AM   #25
JohnDH is offline JohnDH  Australia
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Ive been experimenting with resistive loads and attenuators recently, to bring my 50W tube amp (Marshall VM2266c) down to home-friendly volume. As a simple L-pad gets down to attenuation below say -6 or -8db, you can definitely hear the tone get duller and less lively. I believe the biggest culprit is the added damping of the speaker. A -12db Lpad calculated for an 8Ohm speaker gives an output resistance of about 2 Ohms, whereas a tube guitar amp has effectively much higher output Z, dependent on NFB.

So Im working out my attenuator designs as a star arrangement, keeping output Z higher. Around 8 ohms (happens to equal input Z), as seen by the speaker is sounding very nice. It lets the natural inductance and low resonance of the speaker restore the treble and bass, at high attenuation. With this, testing attenuated (-12db) and non-attenuated miced recordings at say 6 on the volume pot, I actually cant hear a difference in tone once recordings are normalised though small differences are visible on response traces.
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