two prong to three prong

[ilyaa]

im fixing up an acoustic 150b head

works great - just had some caps that had desoldered themselves and other ones that seem leaky - just doing a basic clean up and recap

this amp actually sounds awesome! a real growler

anyway, the power cord is kind of deteriorated and also only 2 prong

just wondering: what is standard procedure for retrofitting an older SS amp with a three prong cable?

im particularly confused because the cable it has now is polarity indifferent (equal sized prongs) -

guidance please!

[DrGonz78]

As I was always told don't go by looking at things like prongs or color coding, but by function. Credit that one to Enzo foremost as I can visualize his post in my mind every time this type of question comes up. Pictures are great for reference... Look at the hot wire going through a fused line and negative as the other one. Look for the typical "death cap", but I call it the simulated grounding cap. So post some pics too and we can all look at it too. I always ask for pic huh? I like to see amp guts what can I say?

I forgot to say don't just look for the death cap but remove it.

[Enzo]

particularly confused because the cable it has now is polarity indifferent (equal sized prongs)

Actually it is not polarized at all.   That is what the ground polarity switch was all about.



In its old two wire glory days, the amp worked.   SO the basic change is to add a earth ground wire to the power cord.  So your new three wire cord should have the same hot and neutral plus an added earth wire.   That earth wire should be connected to the chassis by itself.  It is supposed to get its own screw on the chassis.  DO not connect it to all your other grounds for the circuits.


In a USA power cord, the earth will be green, black is hot, and white is neutral.   In cords from other parts of the world, you ,may see green/yellow, brown, and blue for those.

But in those old days, they didn't care about polarity, the hot side today could be the cold side tomorrow.   But we do care now.  Then they would take the two wires and solder them to convenient points.  So hot might go to the power switch, and neutral went to the fuse holder.   Now we want the hot going to the fuse holder, then a wire from there to the power switch, and the transformer wired on the other side of the switch.  The other end of the transformer primary then is wired direct to the neutral.


The "death cap" is the cap the old polarity switched to either side of the power line.  I just remove those.

[ilyaa]

k rewired it seems all good - thanks guys!

two follow up questions:

1) does it matter where the switch is? because of the way things were set up in there, it was easier to wire the switch on the neutral side of the transformer primary, not the hot side. that doesnt really matter, right?

2) the amp has a ground switch that was wired to the death caps. can someone give me a run down of why this switch is there and whether or not, with a probably wired 3 prong cord, it could serve any useful purpose? i know plenty of 3 pronged gear still has one for eliminating ground loops and that kind of thing. i disabled it completely, though, because im not sure how i would wire it in a useful way. insight?

and a side note: when i was using the amp before, with the two pronger, the chassis was hot! meaning, it was shocking me when i touched the amp! and even with the ground switch flipped and the amp not shocking, there was still a differential somewhere because then the mic hooked up to my PA would shock me if I held the bass. thank goodness the new wiring setup fixed that.

[R.G.]

1) does it matter where the switch is? because of the way things were set up in there, it was easier to wire the switch on the neutral side of the transformer primary, not the hot side. that doesnt really matter, right?

Only if you don't mind being a little dead.
The main power switch **must** interrupt the Line, not Neutral side of the incoming power line. So must the fuse.

The incoming power mains Line must go to (1) the fuse holder terminal on the end away from the panel, so that pulling the fuse holder out clears the Line connection first and you can't get shocked from an intact fuse (2) from the middle ring on the fuse holder to the mains power switch so the power switch opening makes the entire chassis not live (4) from the switched side of the mains switch to the power transformer primary. The neutral can go directly to the power transformer primary, but it's better safety practice to use a double pole mains power switch and break both Line and Neutral in case you have a backwards-wired wall socket.

The safety ground must go to a bolt/stud on the metal chassis that is not used for other purposes than grounding. The stud/bolt should have (1) a toothed lock washer (2) the ring terminal you have so thoughtfully crimped onto the ground wire, (3) another toothed lock washer and (4) the nut holding this down. The ground wire should be long enough that a pull strong enough to tear the strain relief (and you must use a strain relief on the line cord) out of the chassis will tear apart the line and neutral wires before the ground is broken.

2) the amp has a ground switch that was wired to the death caps. can someone give me a run down of why this switch is there and whether or not, with a probably wired 3 prong cord, it could serve any useful purpose? i know plenty of 3 pronged gear still has one for eliminating ground loops and that kind of thing. i disabled it completely, though, because im not sure how i would wire it in a useful way. insight?

The death cap switch is useless for AC power line functions. It was used to "ground" one side or the other of the incoming power line to the metal chassis for RF shielding purposes. Hum and RF were least when the mains line that was really connected to earth ground was connected through the cap to the chassis. But flipping the switch the wrong way let AC from the line mains wire leak directly onto the chassis, and if it leaked too much, shock - or kill - you. It is useless in a properly set up three wire system. Using it to do an ill-conceived "ground lift" is positively dangerous.

and a side note: when i was using the amp before, with the two pronger, the chassis was hot! meaning, it was shocking me when i touched the amp! and even with the ground switch flipped and the amp not shocking, there was still a differential somewhere because then the mic hooked up to my PA would shock me if I held the bass. thank goodness the new wiring setup fixed that.

And that's what three-wire is for. 

[Roly]

Excellent post R.G.   :dbtu:

Your comments about the ground-chassis connection may seem excessive, but they are based on painful experience and should be taken very seriously by guitarists whose guitar strings are connected directly to the amp chassis - if it goes live, so do you.

I'll just add w.r.t. the "ground switch"/death cap; this came about because the old US two-wire mains connectors were not polarised, so it was 50/50 which side would actually be active and neutral at the amp.  The (misnamed) "ground" switch was the alternative to pulling out the plug and flipping it over.

In Australia, like most 240V countries, the connectors are polarised and can't be inserted backwards, and provided that the outlets are wired as they should be there is no risk of active-neutral transposition, however a lot of wiring around venues/stages is (ahem) "informal" and it is not unknown to come across outlets that have the active and neutral transposed, or the safety earth missing altogether.

After a couple of nasty experiences I have long used a homebrew power board that has two neon indicators between active and earth, and neutral and earth and is clearly marked that only the one on the active-earth side should light.  If the other one lights you have a transposition, and if they both light you have no safety earth.

The most deadly combination is a transposed outlet and a homebrew extension lead with a neutral-earth transposition.  Individually these will still work fine, but in combination they put the active on the chassis and have killed at least one person that I know of for sure.

You can get away with a lot around electrics/electronics, but on the mains side you've really got to take care of business every time - the person you kill may be you.