Safety considering output of amplifier

[TNb]

Hi! Few months ago I built a attenuator  for my tube guitar amp(100W).
It works well, everything is fine, but when I was designing it I omitted few things because I don't really have knowledge about it.
1) So, at first, since I have a stuff that possibly can have 125 volts on input it is pretty scare to think of touching the wires inside.
The problem is that I don't know how to do a beautiful metal case for this without having this case touching wires and kill myself.
Other attenuators on the market have metal cases, so there is a way to have it safe.
How to do that? I obviously can't get access to ground through mono jack... have no ideas.
For now I just made a wooden case, eh... 
here's example of commercial attenuator - as you see it's metal.


2) The second problem is that I want to protect the output transformer of amplifier from burning in case of high current, so I put the fuse on the output.... but what output should I fuse? I mean it's two wires coming out, which do I need to put the fuse on? I suppose it makes not much difference, but there must be an explanation how to do better. For now I have both wires with fuses, which is probably stupid, but anyway...

Here's some photos

[xwarp]

Just curious, wouldn't an L-pad work for you?

[katzohki]

If you have 125 Volts coming in I assume that's mains voltage. You would connect GROUND to your metal case, that way if any exposed wires touch the metal case they are shorted to ground. It will cause a short and probably blow a fuse / throw a breaker, but it will fail SAFE.

Also, don't let any wires touch the metal case! 

[TNb]

If you have 125 Volts coming in I assume that's mains voltage. You would connect GROUND to your metal case, that way if any exposed wires touch the metal case they are shorted to ground. It will cause a short and probably blow a fuse / throw a breaker, but it will fail SAFE.

Also, don't let any wires touch the metal case! 

Nooo, I'm in Europe, mains are 220V here 125 V is the output of amplifier on the full volume and attenuation turned to maximum(I have a potentiometer there). Sorry, didn't make it clear...

Just curious, wouldn't an L-pad work for you?

Eh... It was quite time ago, I don't remember exactly why I decided not use just simple L-pad, I think it was because in this case I needed a super-powerful resistor(pot) and it will heat like hell on the full power going in. Now I have a L-pad resistor and  one big-ass high power resistor like that on the image(not that one, just for pic) and the thing remains relatively cold.

[homebrew]

Some questions and suggestions:

1) Where does the 125V input come from? If I understood you correctly your device is a box that goes between the output of the amp and the speaker. You talked about an output transformer at the output of your tube amp. If this thing is correctly built (one can never be sure ...) the output of the amp might not be ground referenced at all...

As with all other devices having a metal case there are basically two options: Either earth the whole case (including every touchable part), or apply double insulation. The first one might not be option for you as you don't have ground (PE) available. Putting on double insulation seems to be a good idea. Hence extra pvc flexible tubes goes over the wires and extra heat shrinking tube goes over every contact. In essence every component should be insulated twice so that if one layer of insulation fails, the other one is still there. Also pay attention to fixate the wires that they can not touch the case if a solder joint breaks.

2) When in unattenuated mode (R1=4R and R2=8R) you are dropping serious amounts of power on those resistors. So you should pay attention to cooling. Mounting them in a wooden case might not be an ideal solution!

3) Why are you using Mono-Jacks at all? I know that some vendors do so but this is a very bad connector for that purpose i my opinion. Use Speakon-Connectors instead. Insulated, Locked, rated for high currents, can't be confused with microphone cables and very robust on stage.

[TNb]

1) Where does the 125V input come from? If I understood you correctly your device is a box that goes between the output of the amp and the speaker. You talked about an output transformer at the output of your tube amp. If this thing is correctly built (one can never be sure ...) the output of the amp might not be ground referenced at all...

125V is from the output of amplifier on the maximum volume and on maximum attenuation(adjusted by pot). So on the input of the pox I have signal to the speakers that is approximately 125V.

2) When in unattenuated mode (R1=4R and R2=8R) you are dropping serious amounts of power on those resistors. So you should pay attention to cooling. Mounting them in a wooden case might not be an ideal solution!

yeah, I calculated the power and I use high-power resistors that are enough to handle that. Also since it's 2 resistors and not one L-pad thing it is distributed and doesn't heat so much.

3) Why are you using Mono-Jacks at all? I know that some vendors do so but this is a very bad connector for that purpose i my opinion. Use Speakon-Connectors instead. Insulated, Locked, rated for high currents, can't be confused with microphone cables and very robust on stage.

Mono-jack is standard for all guitar amplifiers.

As with all other devices having a metal case there are basically two options: Either earth the whole case (including every touchable part), or apply double insulation. The first one might not be option for you as you don't have ground (PE) available. Putting on double insulation seems to be a good idea. Hence extra pvc flexible tubes goes over the wires and extra heat shrinking tube goes over every contact. In essence every component should be insulated twice so that if one layer of insulation fails, the other one is still there. Also pay attention to fixate the wires that they can not touch the case if a solder joint breaks.

eh.... this is the good idea, though I was hoping there is some other solution. Double-insulating everything is pain in the ass
anyway, thanks for a great answer

[IanB]

1. Voltage will only kill you if it makes a circuit through your body.

2. The output from an amplifier, especially if it comes from an output transformer, is likely floating.

3. If the output is floating, it won't hurt you unless you touch both speaker terminals at the same time while powered. (Don't do that.)

4. If you put something in a metal case, make sure no wires touch the inside of the case. It's not hard. An internal case short could easily damage some circuit components.

5. 100 watts into 4 ohms is about 20 volts RMS. Where does this 125 V come from?

[TNb]

IanB, thanks for clearing out

5. 100 watts into 4 ohms is about 20 volts RMS. Where does this 125 V come from?

On full volume and the R2 pot on max the impedance of R1(4ohm) goes to 155 ohms and overall impedance is equal to 157 ohms. Hence V=sqrt(100*157)=125 volts. Isn't it like this in AC circuit? Or do I say something dumb?

[IanB]

For correct analysis you need to ignore the attenuator for the moment and look at the amplifier specs. What is the specified output of the amplifier into what loads?

The amplifier cannot do more than it is designed to do.

If the amplifier is designed to deliver 100 W into a 4 ohm load, it cannot deliver 100 W into a 150 ohm load. The output voltage of the amplifier is more or less limited to what it needs to be for the design.

For instance, if you gave it a 150 ohm load, the maximum output power to a first approximation would be about 3 W (= 4/150 x 100).

[TNb]

oh... damn, you are right, it's not 100W in this case... don't know how I missed that  thanks! So, it's pretty much ~20-25 volts on output... maybe I need to check it with oscilloscope
aaa, yeah - what about the fuses? or maybe I can forget about them if it's not 125V as turns out...

2) The second problem is that I want to protect the output transformer of amplifier from burning in case of high current, so I put the fuse on the output.... but what output should I fuse? I mean it's two wires coming out, which do I need to put the fuse on? I suppose it makes not much difference, but there must be an explanation how to do better. For now I have both wires with fuses, which is probably stupid, but anyway...

[IanB]

The need for a fuse doesn't much depend on the voltage, it depends on how easy it is to damage the amp if you accidentally short the output. Some amplifiers have overload protection to protect themselves if this happens. If yours doesn't, you could try putting a 5 A slow blow fuse in one of the output wires.

[TNb]

The need for a fuse doesn't much depend on the voltage, it depends on how easy it is to damage the amp if you accidentally short the output. Some amplifiers have overload protection to protect themselves if this happens. If yours doesn't, you could try putting a 5 A slow blow fuse in one of the output wires.

Thanks

[IvoS]

Can someone explain to me what kind of advantage this load has compared to potentiometer attenuation on amplifers input?

[homebrew]

Can someone explain to me what kind of advantage this load has compared to potentiometer attenuation on amplifers input?

Because you want the amp to be working at its maximum to get the distortion sound typical for guitars...

[TNb]

Can someone explain to me what kind of advantage this load has compared to potentiometer attenuation on amplifers input?

It is mostly for tube amps.
Tube amps tend to sound better on max power because tubes get hotter and somehow the sound gets better(debatable).
But on max volume amp is freaking loud, that's why you need attenuator to play quite and with good sound.
Though attenuator doesn't solve all problem concerning "ideal" guitar sound since on low volume the 12" speaker behaves differently than on loud volume.
well... that's debatable, some people tend to say it doesn't make any difference, but for me sound is actually slightly better

[IvoS]

Can someone explain to me what kind of advantage this load has compared to potentiometer attenuation on amplifers input?

Because you want the amp to be working at its maximum to get the distortion sound typical for guitars...

That kind of doesn't make a sense to me, because you can easily overdrive the input stage and achieve the desirable distortion that way.

[Richard Crowley]

That kind of doesn't make a sense to me, because you can easily overdrive the input stage and achieve the desirable distortion that way.

Yeah, on one level, deliberately causing distortion doesn't make sense to many of us.
OTOH, overloading the front end gives a different effect than over-driving the output stage.
Over-driving various stages has become an important "performance effect" for electric instruments.
You can't argue with "art"! 

125V into 8 ohm implies a 2000 watt amplifier.  WOW!  Even those guys with the "wall of sound" probably werent' using 2KW guitar amps!

[TNb]

125V into 8 ohm implies a 2000 watt amplifier.  WOW!  Even those guys with the "wall of sound" probably werent' using 2KW guitar amps!

yaeh, we discussed above that I miscalculated this 125V, it's 25V, sorry