Hot chassis circuit made safe?

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Hello people.

I like old stuff.
Specifically, I like old tube amps of all kinds. I plug my guitar into them and make noise.

A lot of the old stuff I like weren't meant to be used with a guitar. So they are dangerous.
I kinda like living, so I'd like to avoid dying.

So, what I did when I found out old tube amps weren't necessarily safem was to change my pants and then install an isolation transformer in one of the old radios.
I did this very quick and dirty;
Cut the old wire, wire it up to the secondary. Wire the primary to my plug, plug it in.

Now this probably isn't completely safe. I should put in a fuse and a switch to turn off power as well.

And finally, here comes the question.
If I do those steps, will it be as safe as a store bought guitar amp?
I have this amp:
http://4.bp.blogspot.com/-ElQP_uSdAdI/T6rV5PvRQEI/AAAAAAAAAnc/r3KI_BS4vSk/s1600/GA5+stock.JPG

I don't need to make it anymore safe than that amp.
Mind you, although circuit ground is wired to socket ground, it is not connected to ground because I'm not using a 3 prong plug (here in europe, that is actually real ground, I have a 2 meter iron rod under my house that's connected to ground in my sockets)
 
Please bend it in neon for me.
What do you mean by "earth or ground" the chassis?

In my store bought amp, the ground points are connected to the 3rd prong on the input socket. But my power cable doesn't have 3 prongs when going into the wall. That is not considered safe then?
 
What do you mean by "earth or ground" the chassis?...But my power cable doesn't have 3 prongs when going into the wall. That is not considered safe then?

If the cable doesn't have 3 prong it should be replaced, then the metal case should be connected to the ground wire (usually green). The reason for doing this, is because this offers some protection in the event something inside breaks, in such a way that the "hot" wire gets connected to the metal case.
 
the four conditions of : whether or not a ground exists, incoming phase of the AC, presence of ac line bypass cap ( also know as the death cap and on some old models, switchable) and a grounded vocal mic are the common protagonists in the shock scenario.

i would urge background reading on old school service methods for identifying a hot chassis and insulation resistance tests.

it only takes 30 to 40 milliamps across your heart to cause fibrillation so proceed with caution.
 
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Unless it is a museum piece, ie not to be plugged in(!) every older amp getting serviced should have the grounding checked thoroughly. One of my guitar amps from 1973 still was originally built with non-polarized 2 prong plug, a polarity switch and the infamous death cap. So you basically had a 50% chance of getting a shock if you touched a PA mic that was on opposite polarity. Not fun.. Installing a 3 prong polarized plug, with earth prong wired directly to the chassis and removal of said cap and polarity switch was the very first thing I did when restoring it.
 
I've not seen the documentary but I've heard this story a number of times (anyone know the original source?)

Chris Keth a decade ago said:
... in some documentary where David Gilmour was telling someone about their method of checking if the mic or strings were hot when Pink Floyd played all over in the 70s and 80s. He said when you're all plugged in you put your hands behind your back and touch the mic stand with the strings of the instrument. He also mentioned that one time in the 70s Roger Waters (PF's bassist) did that and it arced and popped two of his strings. That's some current to burn through a couple of electric bass strings!

There's also some quotes from Charlie Watkins (founder of WEM amps) here on SOS noting that people used plug the power plug in the wrong way round, resulting in fatalities.
 
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anyone know the original source?
Some years ago I bought a Pink Floyd documentary / live performance DVD, "Remember That Night". Not long after, my wife bought me a second Pink Floyd DVD, "Live in Gdańsk" for a birthday present.

The story you quoted - the live mic stand that arced and burned entirely through one of Roger Water's bass guitar strings - was narrated by Mr. Pink Floyd himself (David Gilmour) in the extra material on one of those DVDs. I don't recall which one, but I suspect it was the "Gdańsk" one.

I don't know how much current you need to burn right through a bass guitar string, but its seems like a fair guess that there wasn't even any sort of reasonable fuse in that AC line. Perhaps that's unavoidable if you're trying to power 5000 watts of P.A. system?


-Gnobuddy
 
I'm very eager to not die.
We will all lose that battle eventually, but I understand what you mean, and I share the sentiment.

For additional safety, I have wondered about installing a blank-face GFCI ( https://www.homedepot.ca/product/decora-20-amp-blank-face-gfci/1000166706 ) on my DIY guitar amps, and running the AC wiring through it before it gets to the power transformer.

Some blank-face GFCIs seem to be rated for use as on-off switches, so it could double as the amp's power switch.

However I'm not an electrician, and would welcome pros and cons from someone who knows more about GFCIs and shock hazards than I do.


-Gnobuddy
 

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PRR

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...Some blank-face GFCIs seem to be rated for use as on-off switches, so it could double as the amp's power switch....

The recessed test/reset buttons are clearly not a general-use power switch. They are rated for the 'required" monthly test but that does not imply everyday switching. (Related topic in fusebox circuit breakers: can you turn store lights out every day with the breaker? Recent rulings tend to require "switch rated" breakers, not ordinary breakers.)

However there *seems* to be GFIs plus a switch. Listed here and there but I have not found any in-stock. As this is a $20 GFI and a $1 switch in one $36 unit, the advantage is dim.

I know this is not what you were saying, but the FIRST steps are good isolation transformer and good 3rd-pin grounding. After that, a GFI does seem wise, but essentially never done.

A construction-grade GFI Outlet Strip, on stage, for the entire band to use (assuming no 2KW bass amp) might be a good alternative.
 

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For additional safety, I have wondered about installing a blank-face GFCI ( https://www.homedepot.ca/product/decora-20-amp-blank-face-gfci/1000166706 ) on my DIY guitar amps, and running the AC wiring through it before it gets to the power transformer.
I'm not an electrician but here's what I think.

The GFCI outlets are designed for wet areas which is where we see them most, so would it be an issue in damp places, or when playing an outside gig on a covered deck after it had been raining, say.. I'm afraid they would trip needlessly.

If it's true that old tube amps could have power transformers that leak small amounts of current to ground, surely not enough to trip a circuit breaker but would it be enough to trip a GFCI?

Looking at this on a DIY point of view, I think it could possibly offer one some more protection, as long as the amp remains in the hands of its owner, or by making sure to warn the futur owner about this option inside the amplifier.

Maybe some other GFCI models are more adapted for tube amps.. I'm also curious to hear someone who knows more about this.
 
The "Ground Cap" on 2-pin amps should not leak 5mA (GFI trip); but a common failure mode is dead-short. A GFI would alert you.
Hi PRR, I didn't see you message while posting mine.. What you call the "Ground Cap", is what some are calling "capacitor of death", right?

So would it be a good practice to have such GFCI outlet on the bench while testing amps or when powered for the first time, say.. could it be equivalent to the lightbulb test?
 
The GFCI outlets are designed for wet areas...so would it be an issue in damp places, or when playing an outside gig on a covered deck after it had been raining...
The thing is, damp / wet areas are exactly where lethal shocks are most likely to happen. I think having a GFCI to protect musicians playing in such locations would be a very good idea, so much so, that it really should be mandatory.

(GFCIs are already mandatory in hot-tub / Jacuzzi wiring for the same reason.)
Looking at this on a DIY point of view...
DIY is what I was thinking too. We DIY types might think it's worth shelling out an extra $20 or $30 to avoid electrocution. But Big Musical Corp. Inc would loose far too much annual profit if they increase the raw cost of each amplifier by ten bucks, so they would never consider it unless it became law and they were forced to comply.
...making sure to warn the future owner about this option inside the amplifier.
I was thinking the GFCI would go on the rear outside panel of the amplifier, somewhere near the IEC inlet. That would make it pretty obvious to the owner.

I'm not sure how common it is for GFCIs to trip by mistake. My wife uses her hair-dryer in the washroom, plugged into a GFCI outlet by the sink. I've never heard her complain that the GFCI shut off, even once in all the years we've been married.


-Gnobuddy
 
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I don't think I would put one inside an amp <snip>
How about on the back panel, possibly also connected to an auxiliary AC outlet on the amp back panel, to plug your pedal board power supply into?

As an aside, I'm stunned that not a single guitar amp manufacturer so far seems to offer a built-in 9V DC power jack for your FX pedals. It would cost pennies, and be a really nice convenience for guitarists who only use one or two pedals.


-Gnobuddy
 
We will all lose that battle eventually, but I understand what you mean, and I share the sentiment.

For additional safety, I have wondered about installing a blank-face GFCI ( https://www.homedepot.ca/product/decora-20-amp-blank-face-gfci/1000166706 ) on my DIY guitar amps, and running the AC wiring through it before it gets to the power transformer.

Some blank-face GFCIs seem to be rated for use as on-off switches, so it could double as the amp's power switch.

However I'm not an electrician, and would welcome pros and cons from someone who knows more about GFCIs and shock hazards than I do.


-Gnobuddy
Those are mandatory in all Danish houses. At least I think it's one of those. They are called HPFI in Denmark. My entire house is protected by one of those.
 
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