How to replace a power cord on a tube amp. - diyAudio
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Old 30th September 2012, 07:18 AM   #1
ianhs is offline ianhs  United States
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Default How to replace a power cord on a tube amp.

My old tube guitar amp has a 2 prong (USA) unpolarized plug. When I plug it in one way all is well. When I plug it in the other way I get zapped. I want to replace the power cord with a 3 prong (USA) cord. I have read elsewhere that it doesn't matter which side of the power transformer is connected to hot and neutral. Is this correct? I have also read that the capacitor running from one side of the transformer to chassis ground can be eliminated. Is this correct? All inputs and outputs are connected to chassis ground. I assume that as long as the ground wire from the new power cord is connected to chassis ground it won't matter which side of the transformer is connected to hot and neutral. Please advise or send the newbie to the appropriate thread. Thanks!
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Old 30th September 2012, 07:26 AM   #2
Mooly is offline Mooly  United Kingdom
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If by "getting zapped" you mean you get a shock then it sounds like you may have a dangerous fault and this should be checked by someone qualified and experienced.

A "mains live" to chassis fault would give that effect and with a two prong plug there is no safety earth to protect you.

That has to be stated on safety grounds.

If the amp was originally designed for a three cord (earthed) lead then you must use that. There are other conditions that can give rise to a "shock" that would show using a two core rather than a three and that don't necessarily indicate a problem.

But you must be sure for your safety and that of others
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Old 30th September 2012, 07:36 AM   #3
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Thanks for the quick response! The amp was built in the 60's with a two prong unpolarized plug. It is similar to amps that have a mains polarity switch. Here is a quote from someone restoring an old amp: "On the back of the chassis is a switch which shunts either the hot or neutral side of the AC to ground via a 0.047 uF capacitor. After converting to a grounded AC power cord, I just removed this capacitor and left the switch intact." I guess I am trying to determine what this capacitor does and whether it can be removed when going to a grounded power cord. And also whether the mains really can be connected to either side of the transformer.
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Old 30th September 2012, 07:40 AM   #4
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By the way, if the experts here say take it to a technician, that's what I'll do. I have wired entire houses and built turntables for 7 years so I have some experience with wiring and safety. I was just hoping for clarification on the capacitor to ground and the phasing of the transformer.
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Old 30th September 2012, 07:51 AM   #5
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Hi,
If you do not have polarized AC plug since you do not know which it is the AC common you may be connecting the AC high side to the chassis. This will shock you. By reversing the AC plug then you may be connected the AC common to ground you will not get a shock.
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Old 30th September 2012, 08:07 AM   #6
Mooly is offline Mooly  United Kingdom
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Transformers themselves can be connected either way round for polarity. Where there would be an issue is if one side were also connected to something a user could touch. That should never be of course.

This 0.047uf cap. That will pass enough current at 60hz to give a shock... so that could be what you are experiencing if it connects to chassis (which is floating). The function of the cap is probably switch suppresion ? and also to remove any noise and hash on the AC supply.
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Old 30th September 2012, 02:32 PM   #7
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Does the amp have a mains transformer? I understand some early guitar amps did not. If it is one of these then you should only use it via an isolating transformer, unless you want to join the great gig in the sky.
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Old 30th September 2012, 04:06 PM   #8
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Thanks to all. Yes it has a transformer. My plan is to put the hot to the switch then the fuse then the transformer. The neutral will go to the other side of the transformer with the capacitor going to chassis common ground. The ground wire will also go to chassis common ground. This should preserve whatever the function of the cap to ground serves, but with no possibility of the chassis becoming hot. I think I've got it but let me know if you see any flaws in my logic. And thanks again.
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Old 30th September 2012, 04:24 PM   #9
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Any capacitors from either side of mains (hot or neutral) to chassis should not exceed about 5nF (0.005uF) and must be suitable for mains connection (X or Y). Much better to put a suitable cap from hot to neutral on the transformer side of the switch.

The original cap was probably an attempt to connect a poor RF ground (i.e. neutral) to chassis, in the absence of a proper ground connection. Adding a ground lead means that it is no longer necessary. You are better off without it.
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Old 30th September 2012, 04:40 PM   #10
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That's what I thought. I will eliminate it and come back to the thread if there is any increase in noise or hum. Thank you very much.
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