# Amplifiers and ground safety



## Church-Audio (Sep 27, 2014)

I had a peavey amp in the shop. In my test setup I use a 100 ohm resistor to the ground side of my dummy load and connect the other side to my scope ground. Why? It's a cheap protection against any ground fault I might have with what I am testing... So I fix this amp up get it working well replace the bias range resistor from 33k to 39k to bring the bias where I want it for the new tubes. Time to put a signal in and see what it looks like before I plug in a guitar and actually play though the amp... That 100ohm resistor starts smoking... I am puzzled initially as to why. I put an ac clamp meter on ground lead going out of amp on the ac side. I see 120mA going to ground!!! Ouch since we know 20mA is enough to kill you I am concerned. I do a further leakage test with my Sencore power rite and its showing me over 500uA to ground. So I'm thinking this circuit has a problem with ac leaking to ground. I disconnect the secondary side if the transformer from the tube cuircuit and I still have the same issue. So now I am thinking it's got to be one of three things. Leaky power transformer? Or leaky power switch I have seen this before with a mesa boogie triple rec. Or there are two caps .01uf 250v one across the hot and neutral ( not likely these) they don't go directly to ground and if it was shorted it would have been blown up. The second went from the nutral to ground this cap was the problem... I removed it the chassis was back to 24uA of leakage way under the maximum of 500 allowed by the Csa or UL. Problem solved called peavey engineering up had a chat with the engineer he confirmed what I suspected these were there for cell phone interferance filters. And they were not necessary unless you place a phone on top of the amp. I cut them out problem solved. I doubt many of us tech check for leakage but from now on every single amp that comes into my shop will be checked. Had an old peavey solid state amp in the same day guess what??? Same dam thing. Although I won't call these death caps they serve no real purpose in a properly designed amp. The one situation where this could lead to a shock? You lose the ac ground going to the amp, and get it from something like a microphone on your lips..... So I say check your amps for current draw on the ac ground there should be none, if there is you have leakage.

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These are the caps in the peavey. You obviously want to keep the mov in the circuit.


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## dtsaudio (Apr 15, 2009)

Actually Chris, I would call that a death cap. They're not even the right kind of cap for that application. Totally unnecessary with a properly grounded chassis.
Does this amp have the "ground" switch?
Notice in the picture those caps are left out of the export version. Not allowed.


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## jb welder (Sep 14, 2010)

The modern safety code requirement for caps used in line to chassis applications is that they must be type "class Y".
You will find them used in certain modern amps, I think I've seen them in some newer Traynor and Peavey stuff.
Some of them even have ground switches.


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## greco (Jul 15, 2007)

@jb welder.....What is this and what is its function? (from the Peavey schems)

Thanks 









Cheers

Dave


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## dtsaudio (Apr 15, 2009)

jb, those definitely weren't class y caps, that's why I consider them death caps.


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## dtsaudio (Apr 15, 2009)

> What is this and what is its function?


Inrush limiter. When you first turn an amp on there is large inrush of current. This can sometimes blow the fuse. The limiter has a high cold resistance, as current flows through it the resistance drops and you get close to full power.


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## greco (Jul 15, 2007)

dtsaudio said:


> Inrush limiter. When you first turn an amp on there is large inrush of current. This can sometimes blow the fuse. The limiter has a high cold resistance, as current flows through it the resistance drops and you get close to full power.


Thanks Dan! 

Very interesting.










I found this pic of one "style" of these being used in an antique radio circuit.

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

> I found this pic of one "style" of these being used in an antique radio circuit.


Looks like the limiter and fuse holders were put in after. Note the ceramic death caps too.


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## Church-Audio (Sep 27, 2014)

lots of amps have these another name for them is a MOV. they are in almost every new amp I work on now. Its always good to have some on hand.


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## jb welder (Sep 14, 2010)

dtsaudio said:


> jb, those definitely weren't class y caps, that's why I consider them death caps.


Yep, I'm with you on that. 
I just wanted to point out that the hazards of "death caps" and ground switches are not about the design, but the components. They can be used safely with the right components and do exist in some modern gear.
A lot of people think ground polarity switches and "line to ground caps" are extinct, and yank them out on sight. In some cases that is incorrect. They are definitely not extinct.


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## dtsaudio (Apr 15, 2009)

Absolutely. I believe Peavey still puts ground switches in on most models. I even think they use the right parts now.


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