A mad man's approach to attenuation

[DUG]

Nichrome wire from discarded toasters...wind it non-inductively. (400ppm/C)

Light bulbs: you will need a few to get down to 4R but they would handle the power and give you visual feedback of output level.

But their cold resistance is lower than their hot resistance. (4500ppm/C for tungsten)

🙂

400ppm/deg C means that for every 1 degree Centigrade change of the temperature of the resistor element the resistance will go up by 400 parts per million of its' value at 25C

Or 0.04% / C

 

[chrish]

What is it you are trying to achieve? You don't really state it clearly in your posts. The heading of the thread implies you are after attenuation? Are you trying to achieve an overdriven sound from a 300 watt amp, but want that sound with lower volume? So, you are trying to shunt some of the power going to the speaker, while driving the output of the amp into overdriven territory for the sound?

If this is the case, there may be better solutions than shunting the power with a resistor. What amp is it? Is there a schematic for it? For 300 watts I suspect there are several pairs of tubes running push-pull. You may be able to remove all but one pair and/or change tube type to get a similar sound at lower volume.

 

[Hilmar53]

He wants not only tube-overdrive but also Output-transformer-saturation. This gives the sound a guitarist wants to go for.

Guitarski:
Try this link:Widerstände
Scroll down the page to the resistor: KWD 82-10. Get 40 pieces of them in parallel and you will get 2,05 OHMs at 400Watts.
For 10 pieces you pay 2,30 Euros so you have a total of 9,20 Euros.
Thats not so expensive isn´t it?🙂

Rock on,
Hilmar

 

[funk1980]

So with a dummy impedance is not an issue because no sound is produced? And so for dummy purposes resistance = impedance?

Not an issue indeed. It's both 4 ohm, although a speaker will vary around this value and a resistor stays (roughly) a fixed value.

It's tricky. This resistor (made for short intervals, like motor starting, breaking) can withstand 400 C. Its temperature coefficient is 400 ppm/C, whatever that means, so the ohmage may be off at high temps. All in all not something I can try and see if my amp doesn't explode.

Thanks/regards
Guitarski

'Warm' is a broad term. Hot to the touch could be around 40C, which is WAY below the max rating. It can be safely ran at much higher temperatures, even until you cant touch it anymore.

PPM/C is the amount of resistance it will vary (max) with for each one degree celsius increase or decrease. So if the temperature rises from 28C to 29C, the resistance will vary a maximum of 0,0004 ohm. It has nothing to do with the max temperature.
Edit: DUG beat me to it I see 🙂

 

[jan.didden]

Actually 300W isn't much for bass. I would have settled for 1W, but my amp was the least expensive amp with a truly driven tube sound.Guitarski

Maybe I'm missing something, but you can easily throttle down your 300W amp to deliver 1 W in a reasonable resistor (volume control?), then take the signal from the resistor to your recorder. I mean, 300W in 4 ohms will be 35V or something, you'll also fry the recorder!

Jan

 

[funk1980]

Maybe I'm missing something, but you can easily throttle down your 300W amp to deliver 1 W in a reasonable resistor (volume control?), then take the signal from the resistor to your recorder. I mean, 300W in 4 ohms will be 35V or something, you'll also fry the recorder!

Jan

Yes, but he wants to capture the characteristics of the close-to-overdriven power stage. But you're right, some severe attenuation afterwards has to be done.

 

[FlaCharlie]

Maybe I read it wrong, but I was under the impression that his amp has a DI circuit that will go to the mixing board / recorder. The dummy load would replace the speaker(s) so there would be no sound coming from the speaker and, therefore, no output transformer saturation involved.

One solution would be to just wire in a preamp out jack and an amp in jack and then to not have them connected during recording. A simple switch could also be used, or insert a master volume control between the pre and amp sections. This would eliminate the need for a dummy load entirely. No input to the amp section = no sound coming from the speaker(s).

Obviously, some info on the amp and a schematic would help.

. . . Charlie

 

[Tarzan]

Go to a elctric apparatus junk yard and recover some resistances from a washing machine.
They are 1000, 2500 W for 220V.
That's about 4.5 and 9 ohms.
Keep them wel ventilated as they are going to become very hot.
Or dip them in a bucket of water and make sure the wires are not emerged in the water.
This is the cheapest sollution ever to test an amp.

Make a voltage divider to make that signal suitable for your recorde.
By using a bunch of these resistors you can make have any value you want with very high wattage...
I used it in my younger days...

 

[jan.didden]

... or a bunch of car headlight lamps...

 

[Merlinb]

He wants not only tube-overdrive but also Output-transformer-saturation. This gives the sound a guitarist wants to go for.

Removing pairs of output tubes doesn't affect the amps ability to saturate (or not) the output transformer. This still seems like a sensible approach to me...

 

[Guitarski]

Hi folks,

This http://avalondesign.com/pdfs/u5speakerin.pdf may explain what I'm after.

Obviously, some info on the amp and a schematic would help.

Amp info: BUGERA: BVV3000-INFINIUM

Try this link:Widerstände
Scroll down the page to the resistor: KWD 82-10. Get 40 pieces of them in parallel and you will get 2,05 OHMs at 400Watts.
For 10 pieces you pay 2,30 Euros so you have a total of 9,20 Euros.
Thats not so expensive isn´t it?

Compared to this https://www.distrelec.nl/elfa3~nl_nl/elfa/init.do?item=60-543-10&toc=20085, at 2.84 euro (20 resistors -> 4.1 ohm), it is expensive, actually 😉. But the idea is yours, thanks.

Or, for some redundancy, 25 of these https://www.distrelec.nl/elfa3~nl_nl/elfa/init.do?item=60-543-11&toc=20085, for 375 W.

Where do you put all these resistors? On something like this 8000-45-LF Twin Industries | Mouser ?

 

[funk1980]

Ah, that changes things. The U5 doesn't present a load to the amp! So no 400W resistors or lamps or whatever. It can not be used to load down e.g. a tube amp when no speaker is connected. I've never seen a schematic, but I recon it's basically a buffer/attenuator that needs to be paralleled with an existing speaker in the case of a tube amp (SS amps don't care about load/no load).

So, just a high impedance attenuator (resistor + pot for volume), op-amp as buffer and balanced output driver and you're done. Costs maybe, I don't know, 3 euro's?? Maybe 4. Could even be battery operated.

 

[Guitarski]

So, just a high impedance attenuator (resistor + pot for volume), op-amp as buffer and balanced output driver and you're done.

Doing what? Are you trying to reverse engineer the U5?
Is this the Hammond box you referred to? Nice indeed.

BTW, Hilger was right. His Widerstände page has the best value, I misread mine.

BTBTW, it's weird, that I, an electronics nitwit, have to make my own dummy load. Why isn't such a circuit included in the amp, or at least sold by third parties without the bells and whistles, like cabinet simulators and headphone connections.

 

[funk1980]

Doing what? Are you trying to reverse engineer the U5?
Is this the Hammond box you referred to? Nice indeed.

BTW, Hilger was right. His Widerstände page has the best value, I misread mine.

BTBTW, it's weird, that I, an electronics nitwit, have to make my own dummy load. Why isn't such a circuit included in the amp, or at least sold by third parties without the bells and whistles, like cabinet simulators and headphone connections.

Yes,... I mean no!!! 😀
If I need a high impedance load (ie no load), with a balanced output and the ability to drive a long cable, that's the way I'd do it. Simple and effective, and you can bet I'm not far off from the U5. Simple doesn't mean bad, quite te contrary.

No the Hammond box is just a black powder-coated metal box. Some drilling and tooling and you end up with a nice dummy load.

There are different speaker simulators. Some good, some bad. But you don't really want/need a speaker simulator, since you're going to connect your own speaker as well. At least that's what the U5 leaflet shows us. The amp is going to react to the 'real' speaker. A simulator will have different characteristics and that'll miss the point. You want a linear high impedance attenuated buffer that transparently puts out the signal according to the existing amp/cab interaction.

Dummy loads (resistive) are usually used for testing/repair, so it's not that odd they mainly reside with techs.

The reason why speaker sims are not seen that much, is that 99% of the time, a DI out or mic is all you need. Second, all speakers are different in the impedance area. An amp will react different to a speaker sim than to your cab, so although you're using the poweramp's output, the sound will be different. Third, the cab itself will contribute to the overall sound too. So if you really want to capture the entire soundstage, the cab needs to be included i.o.w. you need to mic.

 

[Guitarski]

In the U5 diagram I will replace the cabinet with a dummy load.
Do I understand you correctly that the amp will sound different through the U5 with a dummy load connected, than it would with a cab connected?
That would explain the cab simulators as opposed to bare dummy loads.
Holy Mother of God. I'm in for more big spending.

Thanks/regards
Guitarski

 

[Guitarski]

Yeah, reading your message again, that's what you're saying. You know your stuff. I christen Groningen Funky Town.
So dummies are out, cabs are in, for additional bedlam.

Thanks/regards
Guitarski

 

[funk1980]

In the U5 diagram I will replace the cabinet with a dummy load.
Do I understand you correctly that the amp will sound different through the U5 with a dummy load connected, than it would with a cab connected?
That would explain the cab simulators as opposed to bare dummy loads.
Holy Mother of God. I'm in for more big spending.

Thanks/regards
Guitarski

Yeah, reading your message again, that's what you're saying. You know your stuff. I christen Groningen Funky Town.
So dummies are out, cabs are in, for additional bedlam.

Thanks/regards
Guitarski

I toast to that!
The output tubes 'see' the connected load. And since this load varies with frequency, so does the way the power tubes react.

 

[kuroguy]

...and the speaker also adds distortion in the way the cone moves, so omitting the speaker would certainly alter the sound.

 

[Guitarski]

Well, I wired my 4x12 16 ohm guitar speakers in parallel, just for the load, so I can use the Bugera's DI output.
Then I have some time to figure out what else I'm going to do, buy a cab or not.

As I told you, I'm an electronics nobody, did some metering and got 4.1 ohm right at the speakers, but 8 ohm at the end of the speaker cable (when I solder, blobs fly).
Is this going to fry anything?

The amp sounds marvelous, really impressive tube sound, even through a road worn 70s guitar cabinet (London City, if that rings a bell in Groningen), with a torn speaker cone, you hear that on the e-string.

 

[funk1980]

How long is the speaker cable?? 4 ohm seems a lot. If you short-circuit the cable, you can measure the resistance of just the cable in order to check if it's a possible wiring error at the speaker side of things.

Yes, the London City 4x12 cab. Our former guitarist owned one of those. Great cab. I had to battle sometimes with him for the low-mid spectrum 😀