Float a scope

[Richard Crowley]

Can anyone explain WHY guitar amps and other dodgy consumer gadgets are NOT properly grounded?
Other than the fact that they are so cheap, they don't want to spend the extra pocket-change it takes to do it right?

Instead of spending hours repairing a guitar amp and then kludging a dangerous setup to test it,
why not simply install the proper grounded power mains cord and bring it into the 20th century?

I just don't get it.   

[helius]

For certain types of audio gear "ground lift" switches are common, either from the factory or bodged on by users. The reason they are there is as one way to break ground loops. Ideally the ground current should be minimized so as not to cause audible interference, but in setups that are slapped together quickly and need to work right away (familiar to musicians) this is not always possible. It's much safer to break ground loops using transformers but that is an additional cost and device that needs to be carried.

Intro to ground loops: http://www.bestaudio.com/s/generic_grounding_seminar.pdf

[LukeW]

There's not necessarily anything wrong with the DUT equipment design, it may have the mains earth connected to the chassis for safety where it should be, but the OP just wants to use the scope to measure a voltage at some point relative to some other reference other than mains earth - which means the scope ground can't be connected directly there without shorting it to mains earth.

[Richard Crowley]

For certain types of audio gear "ground lift" switches are common, either from the factory or bodged on by users. The reason they are there is as one way to break ground loops. Ideally the ground current should be minimized so as not to cause audible interference, but in setups that are slapped together quickly and need to work right away (familiar to musicians) this is not always possible. It's much safer to break ground loops using transformers but that is an additional cost and device that needs to be carried.

Intro to ground loops: http://www.bestaudio.com/s/generic_grounding_seminar.pdf

Yes, audio is my primary engineering field. I am well acquainted with ground loops. I also know that decent modern audio gear is either equipped with a proper grounded mains cord, or else is completely floating (as gear powered from a wall-wart, etc.)  The dodgy design of guitar amplifiers (in particular) with their suicide capacitors has always baffled me.

[Bassman59]

Can anyone explain WHY guitar amps and other dodgy consumer gadgets are NOT properly grounded?
Other than the fact that they are so cheap, they don't want to spend the extra pocket-change it takes to do it right?

Because back in the 1950s and 1960s, there was no real notion of grounded, or even polarized, outlets, and mains wiring was as good as the training of the electrician who did the work.

So Leo Fender designed his amps with a "ground" switch, which connected one or the other leg of the amp's mains transformer to chassis via a capacitor (called "death caps" in the trade). This mitigated noise but wasn't safe.

Once proper standards were introduced and safety agencies started to require proper grounding of all equipment, designs changed to meet the requirement. You won't find an amp built from the 70s on with a two-prong power cord.

Instead of spending hours repairing a guitar amp and then kludging a dangerous setup to test it,
why not simply install the proper grounded power mains cord and bring it into the 20th century?

ALL competent amp techs will suggest to their clients that their vintage amps be properly grounded, and it's a simple fix. Or they'll suggest that the amps be retired to a glass case, unused. All of mine (including a '62 Bandmaster, a '62 Deluxe, a '64 Bandmaster, and a '66 Bassman) all shipped with ungrounded mains cords, and all have been fixed.

Back when I was a regular club sound guy, bands would roll through with amps in all states of disrepair. A lot of guys would have ground lifts ("death cubes") on their amps, and I would ALWAYS just take them away. "Why did you do that?" "I don't like people dying on my stage." The worst were the guys who purposely cut off the ground pin on the mains cord.

There is a whole other sordid tale about audio gear installed in studios where the install techs would regularly cut off mains ground pins in a futile attempt to solve buzz and hum problems caused by poorly-designed gear ("pin 1 problems") ...

[jlmoon]

Back when I was a regular club sound guy, bands would roll through with amps in all states of disrepair. A lot of guys would have ground lifts ("death cubes") on their amps, and I would ALWAYS just take them away. "Why did you do that?" "I don't like people dying on my stage." The worst were the guys who purposely cut off the ground pin on the mains cord.

You should never underestimate the power of an Electron!   

[nbritton]

It's really not that hard to stay safe if you understand how split phase power works. Electricity always takes the path of least resistance. Figuratively speaking, if your in the electrician business long enough you learn to probe live circuits with one hand in your pocket, because you never want to accidentally touch something else that causes you to become part of the circuit. Currents as low as 30 milliamps can trigger fibrillation in the heart; a shock entering from one hand and exiting through the other hand can cause current to go directly through the heart, very deadly. In every workshop I've had I've gone out of my way to make sure every outlet is GFCI protected, in fact in my last shop I installed a sub panel that was supplied by a 50 amp GFCI breaker in the main panel. These things really do save lives and equipment.

[knks]

What experts think about inserting a fuse into the probe ground lead?

Sure it will create some inductance, but this is all about audio frequencies

[nbritton]

What experts think about inserting a fuse into the probe ground lead?

Sure it will create some inductance, but this is all about audio frequencies

I'm no expert, but I don't see a major problem using an inline fuse on the ground lead. You would not want to fuse the ground coming out of the oscilloscope chassis through the power cord though, that would be a safety hazard. The internal resistance in a scope is usually 1 Mohm and then you have a 9 Mohm resister inside a 10X probe. Assuming you have 120V mains you would use 120V and ohms law to get the current rating of the fuse.

1X Probe: 120V @ 1 Mohm = .12 mA
10X Probe: 120V @ 10 Mohm = .012 mA

However, if you were working with a 12V circuit you would need to use that voltage in your ohms law calculations because power changes with voltage. I think in practice you would still blow up your device under test or constantly blow the fuse because you'll have to deal with varying voltages.

I think (haven't tested this) a better approach is to put all of your equipment (scope + device under test) on a single GFCI receptacle. These devices have circuitry inside of them that constantly monitors how the current is distributed on the hot and natural wires, it compares the current going through each wire and if they are not equal it trips the breaker. Class A GFCI trips when ground fault current exceeds 5 milliamps and Class B GFCI trips when ground fault current exceeds 20 milliamps. 5 mA @ 120V = 0.6W