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Discussion Starter #1 (Edited)
My friend gave me his little KEIL tube amp (? 3 -5 watts) to try out. Nice amp.

Someone had put a new 3 prong AC cord on it, but left the green ground wire hanging (about 1" long) and "blowin in the wind" outside the chassis.

The amp hummed a bit, until I touched the jack on my guitar or the unpainted parts on the front of the control panel/ amp chassis. Then it was quiet. This must be a grounding issue, thinks myself (I'm reading about electronics and very slowly building a tube amp.....so I THINK that I know about SOME of these things...RIGHT)

WRONG (damn*t)

I touched the cut end of the green ground wire while also toching the front of the chassis and......:oops:



ZAP


...a nice little tingle in the fingle.
My hair stood on end...which ain't easy cause I'm bald :eek:

I then got scientific, got out my DMM and measured the AC voltage between said ground wire and the chassis....a mere 120 volts.

My friend is coming over and we are going to pull the chassis (power off, of course) and see where this short is coming from and attach the green AC ground wire. Yes, I will drain the caps first.

The tone pot acts as the on/off switch. The shaft on the tone pot is also quite loose, so the pot/switch might need replacing. I'm curious to see if it is obvious as to where this short is. That is what I'm hoping.

Go ahead and have a laugh on me.......

OK......now that you are done....


any (non safety oriented) comments are welcome.

Thanks

Dave
 

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Discussion Starter #4
I'm waiting for Wild Bill's humerous and helpful comments.

Have a laugh on me Wild Bill.

Dave
 

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I'm waiting for Wild Bill's humerous and helpful comments.

Have a laugh on me Wild Bill.

Dave
Hey Dave, I've been zapped too many times over the years to find it humorous, even when it happens to someone else! :smile:

As for why it's happening, there could be a number of reasons. If the amp is very simple and doesn't use a power supply transformer then that could explain a hot chassis. Obviously there was a reason the green wire was left dangling!

If there's no trannie then 3-wire cord can't work, period.

Even with no trannie the amp should not be grounded to the chassis anyway. At least not directly. There should be a big cap in series with the circuit ground and the chassis. It's probably shorted, causing the problem.

Even with a power transformer there's likely a bypass cap to ground that's shorted. Like the famous "death cap" used with the "ground reverse" switch.

With so many possibilities it's like diagnosing a car that won't start over the telephone!:smile:

Someone's got to crawl inside with a meter and look around...

:food-smiley-004:
 

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Discussion Starter #6 (Edited)
Thanks for the comments Wild Bill.

I was only trying to have folks understand that the "zap" was very minor, thankfully, and that I was a dummy to have learned the hard way.
Maybe "have a laugh on me" would be better said with "learn from my mistake" (not for you obviously, but for the enthusiastic and unexperienced)

If it isn't extremely obvious, than I won't be doing anything to the amp.

It will be fun to have a look inside, if nothing else.

Dave
 
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