Tube Amp Dummy Load questions

[exnomine]

I know there are a million forum posts regarding dummy loads, but my question has not been addresses (at least to my knowledge.)
I know that a purely resistive dummy load will affect your tone in a terrible way. However most posts I have seen are asking about a dummy load with a line out.
I like running my amp's speaker out into a DI Box (Behringer G10, which lets me either use strickly a DI signal, or color it with a 412 sim, which I actually like). The Di box has a line level out which goes to my mixer and a speaker out which usually goes to my cabinet. I just moved into an apartment and can no longer record this way, so I was wondering if a simple resistor based load box in place of the speaker would work well. In other words would it "suck" the tone from my DI box which comes in line before it. Also my amp is rated at 100 watts RMS, It is a very high gain amp. I would run the amp at half volume, maybe a little less, so would a 150 ohm resistor do the trick?

 

[Nigel Goodwin]

You need a high power resistor, equal to the speaker impedance - so 8 ohm if your speaker is 8 ohm. For a 100W resistor I would suggest a 150W resistor (or more likely stack of resistors).

You can buy (fairly cheaply), 1 ohm 17W wirewound resistors, and 8 of those in series would do the job. There's a lot of crap about using wirewound resistors (because of their inductance - which is only low anyway), but it should be fine, and relatively low cost.

The resistors will get HOT!!! - so need mounting in free air, and well apart - and don't touch them!.

 

[exnomine]

That sounds reasonable. Would this be acceptable: 8 Ohm 200W Non-Inductive Dummy Load Resistor 019-030
Also, I was wondering if I could use something like this Aura AST-2B-4 Pro Bass Shaker 299-028 as a reactive load (like the weber speaker motors) and just put it in a box of pillows?

 

[Printer2]

Sounds real odd but an old 1200W hair dryer or electric heater can dissipate more than any amp most have.

 

[Enzo]

Yes, that Parts Express item ought to work fine.


I don't think a pure resistive dummy load can fairly be called terrible. It may not be optimal tonally, but a bit of EQ tweaking usually takes care of that. The "cab emulation" that some products like your DI have is just a little EQ.

 

[sreten]

I know that a purely resistive dummy load will affect your tone in a terrible way.

Hi,

That is pure myth, it simply doesn't. It loads your amplifier consistently
and creates consistent distortion. A real speaker load is barely different,
if it has an effect its less distortion around the speakers Fs and towards
the top end of the range, hardly a difference that is terrible / preferable.

rgds, sreten.

A cheap dummy load is a 1.5KW / 110VAC kettle or heating element = 8 ohms.

 

[Loudthud]

You didn't say if your amp is tube or solid state. If it has a tube output stage, you will likely hear a difference in the DI output. Behringer doesn't like to disclose their schematics, so what's in there is a mystery at this point.

Some 100W resistors require a heatsink and/or a fan. A 100W guitar amp will put out close to 200W when it's making a rail to rail square wave, but you probably won't be doing that on a continuous basis. If the amp is solid state, it will probably work fine without a load at all.

 

[Enzo]

I don't see a G10, I see a GI100. Can you verify the model number?

Now that I look, the GI100 has a rather complex emulator, four op amp stages with bunches of tone shaping, and a transformer out, which circuit can run off battery or phantom.

 

[exnomine]

Thanks for all the advice. Sorry, I wasn't specific. My amp is a tube amp, a Peavey JSX. The DI is the Behringer GI100. As for the resistive dummy load sounding "terrible" I am just going by what I hear. Then again, opinions are just that, opinions. I am thinking the Parts Express 200 watt 8 ohm dummy load resistor, with a jack hooked to the leads may work fine. Just gotta figure out how to mount it.

 

[Nigel Goodwin]

It sounds absolutely perfect - it's made for the job.

 

[exnomine]

So, I got that big resistor and jack, soldered it a heavy duty 1/4" jack and put it to the test. I ran my amp at about half volume, which is normally extremely loud. I ran a speaker cable to my GI100, then plugged the dummy load in where the speaker would go, and of course ran the direct out w/4x12 emulation into my mixer, into my recording interface. Over all I am pleased. I can record screaming, fat tube amp sounds that sound WAY better (IMHO) than the amp modeling plugins I have. The only concerns I have are: The amp responds slightly different to my playing. This was expected, and its just a subtle difference, not that big of a deal. My only real concern is that the back panel of my amp, where the speaker jacks are, gets so hot you really don't want to touch it. The transformer felt reasonably cool to the touch, but the power tubes were getting HOT (but then again, they normally do anyways.) Also, the big box with the resistor in it was very hot as well. I think it'll be fine for recording sessions, as long as I take breaks and let my amp cool off. That back panel is my main concern.

 

[Loudthud]

A dummy load puts more stress on a tube amp than a speaker does. It will wear the tubes out faster. If there are no live microphones in your recording setup, a fan would be a good idea. Some load boxes use a slightly higher resistance than the nominal load. Perhaps 10 or 12 ohms for an 8 ohm speaker tap.

 

[exnomine]

I have a selectable speaker out jack. So, should I set it at 4 ohms for the 8 ohm resistor?

 

[Nigel Goodwin]

A dummy load puts more stress on a tube amp than a speaker does. It will wear the tubes out faster.

Sorry, but that's utter rubbish - a correctly designed dummy load will put slightly less load if anything on the amp (as it's a condstant correct value), and certainly won't 'wear the valves out' any differently

 

[Loudthud]

Tell that to all the Marshall owners who had their warranty claims denyed.

 

[Nigel Goodwin]

Sorry, still utter rubbish - and why would you even tell the service agent what you and been doing with the amp?.

An 8 ohm dummy load on an 8 ohm output is an easier load for the amp than an actual speaker.

If you're deliberately mismatching the output impedance, then yes you could well damage the amp - but why would you do something so stupid?.

 

[Printer2]

What I have heard what happens is when on a power soak or other load the guitar player turns up the output of the amp until it screams, far beyond than what they would do through a speaker. Whether this is true or not I do not know but it does make sense.

 

[sarkipinty]

I am planning to make a dummy load with AUX output, for recording and having a headphone output for night-time playing

the question is how long is it "safe" to play on an amp with a dummy load
I'm not talking playing for hours, but is a daily half an hour practice session OK with a non-inductive dummy load?

Thanks!

 

[thoglette]

...beyond than what they would do through a speaker. Whether this is true or not I do not know but it does make sense.

You need to get out more. "Everything at eleven" might be a cliche but it's been how certain genres have played for at least four decades.

And I have the hearing damage that results (we'd never heard of ear plugs)

 

[Gnobuddy]

I know that a purely resistive dummy load will affect your tone in a terrible way.

I know you've already heard people expressing strong opinions on both sides of this particular issue.

Simple engineering theory says a dummy load resistor should be perfect, better than a reactive load. My ears said no, the dummy resistor made my amp sound a little flatter and less-good.

A fellow on another electronics forum had a wonderful signature line: "If theory and practice don't agree, you haven't used enough theory."

The point being that magic and superstition are never necessary to explain something that you are actually seeing / hearing / measuring in a consistent and reproducible way. If you're seeing a real phenomenon (and not just imagining it), then there is a sound theoretical explanation, you just have to find it.

I was hearing something different when I used a dummy resistive load, so I needed a bit more theory to understand what it was that I was hearing.

It took a while to find the proper answer, but only because of an unconscious bias that I didn't even know I had: I grew up, literally from childhood, building and learning about solid-state amps. These have very low output resistance, and they behave perfectly with resistive dummy loads. They behave almost exactly the same with a real speaker load, too.

It turns out that tube amps with pentode output stages and little or no negative feedback - the sort of amp we use for electric guitar - don't behave the same way. These types of tube guitar amps have a high output resistance, and that interacts with the guitar speaker impedance, in such a way as to produce a bit of bass boost, and a bit of treble boost.

When you replace the guitar amp loudspeaker with an 8 ohm load resistor, you get a flat frequency response instead. You loose a bit of brightness, and a bit of punch, and usually the sound becomes more ordinary, more blah, less enjoyable.

So the bad news is that yes, using a proper resistive dummy load on a tube guitar amp usually will change the sound, and usually not in a good way.

But the (very) good news is that the change is merely a change in EQ. The dummy load took away a bit of bass and treble, so all you have to do is put that back with EQ. You can do this with a bit of bass and treble boost in your mixer or DAW software. Or you can run your line-out signal into a graphic EQ (guitar pedal or rack-mount) before it gets to your mixer / DAW.

I've had good results with a simple dummy load, a pot across that to divide down the signal, and a Danelectro Fish-n-Chips graphic EQ pedal to restore the lost bass and treble, before the signal gets to the mixer.

Most of the time, I find I only need to add a gentle treble boost starting around 1 kHz, and rising smoothly and evenly up to maybe 5 kHz. I usually don't add bass boost, it just makes the guitar clash with the bass guitar, drums, and/or keyboard.

From my exerience, the stuff about magic reactive dummy loads, eye of newt, and toe of frog, is nonsense, and you can safely ignore it. A dummy resistive load plus EQ is all you need.

-Gnobuddy