Grounded Power Cord

Hi

I've been doing some repairs on my ST-70 and I was thinking about changing the power cords on the ST-70 and my PAS-3 to the grounded type. Will this cause any problems? They will be plugged into the same grounded outlet in the house so I don't think I would have any issues with ground loops.

Chris

I am nearly done rebuilding my ST 70. I am replacing the original line cord with a IEC socket and 3 wire cord. My question is should the neutral go to the fuse or the switch? Common sense tells me to put the neutral to the switch (the part I will be handling most often) but I just want to make sure.

"My question is should the neutral go to the fuse or the switch?"

Fuse the Hot lead immediately out of the IEC jack.  The other side of the fuse goes to the switch.  Neutral connects directly transformer.  Theory being in N. America if your house is wired properly Neutral is near ground potential since it is grounded at power entry to the building or main fuse box.  This is the way UL likes to see chassis wired for best safety concerns.  

Here's a schematic that illustrates the wiring (lower right).  "L" is the Hot, Live, or Line depending up where you live.  "E" is Chassis GND or Earth.
https://www.keepandshare.com/doc/1782400/jtm50-schematic-custom-spec-a15-1-76k?da=y&dnad=y

You are right. For safety, FUSE THE HOT SIDE, switch the neutral.

Some people will point to manufacturers having fused the neutral, but for the most part that was back in the day prior to three-wire power cords, and many were GUITAR AMPLIFIERS with nothing on the outside that would be touched by a user.

Also you may find hum from ground loop problems, once you replace the two-wire power cord on a Dynaco with a three-wire cord grounding the chassis.

So, what you're saying is, if I'm not getting tossed across the room whenever I touch the metal chassis, I got it right?   

Still a little bummed I let yall talk me out of the "floating" ground plate I was gonna add to my ST120 build. Now I'll never know if it would have worked.



On the bright side, it does make a nice back scratcher ...

GP49 wrote:You are right.  For safety, FUSE THE HOT SIDE, switch the neutral.

Some people will point to manufacturers having fused the neutral, but for the most part that was back in the day prior to three-wire power cords, and many were GUITAR AMPLIFIERS with nothing on the outside that would be touched by a user.

Also you may find hum from ground loop problems, once you replace the two-wire power cord on a Dynaco with a three-wire cord grounding the chassis.

You seem to be saying the OP should not switch the live and to switch the neutral instead.
I would never ever use anything in a metal chassis without a safety earth and I would  switch both live and neutral with the live passing through a fuse first.

Typical fault scenarios come to mind - neutral and live being reversed in the house wiring or some extension lead. Live touching a chassis part in the amplifier - you become the earth when you touch it.

I would urge you to follow the advice of arledgsc but consider a double pole switch for live and neutral.

"Typical fault scenarios come to mind - neutral and live being reversed in the house wiring or some extension lead"

That is a very good point.  My garage was wired reversed until I corrected the wall sockets.  With Neutral only switched or a reversed line even when the amp is turn off you still have voltage on the power transformer input winding.  Any leakage at all may pose a shock hazard.  So switch both if unsure of your wiring or for maximum safety.

An ungrounded chassis is actually "grounded" in use by the audio input cabling.   So any potential difference between two chassis causes current to flow in the ground leads of the audio cable.  I burned up an oscilloscope ground lead once working on an ungrounded chassis.  The AC grounded scope and unit under test were on different phases of the AC line and perhaps an air conditioner kicked on and shifted the ground potential of one of the phases.  Poof!  So if you use an ungrounded chassis keep all equipment on the same wall plug.  I never have any noise issues with my grounded chassis ST-120 and I have a noisy AC environment - CFLs, light dimmers, Insteon automation, wireless Ethernet, solar panel inverter, etc.

.

I used an IEC well with built-in fuses, one on each lead. This saves panel space, i could
mill the hole for the IEC right over the existing fuse.

As regards to hum , it is more quiet when all stuff that has a ground inlet is grounded. My thorens
do not, i don't care.

" I wonder if removing the 10-ohm resistor in the St-70 would help, as I would like to keep the safety ground in use all the time..."

That was common practice 15-20 years ago to isolate the chassis ground from the signal/ power grounds by a small value resistor.  Sometimes a ferrite bead was used in place of the resistor.  This causes a lot of EMI headaches and the practice nearly abandoned for a central or star ground scheme.   The ST-120 does this nicely with all the signal/power grounds tied together near the quad cap.  The 10 ohm resistor could be lifting the chassis potential so when three 3-prong plug is used you have a voltage difference between the amp and preamp.  And when there is a voltage difference you have current (on the interconnects).  I never liked the resistor isolation method as if it ever fried and failed you lost your ground protection.

.

sKiZo wrote:So, what you're saying is, if I'm not getting tossed across the room whenever I touch the metal chassis, I got it right?   

Still a little bummed I let yall talk me out of the "floating" ground plate I was gonna add to my ST120 build. Now I'll never know if it would have worked.



On the bright side, it does make a nice back scratcher ...

hm that looks interesting though - what made you give up on it sKiZo ?!

In some circumstances, altering an item away from the OEM UL-listed design will lead to a whole raft of unfortunate consequences. They are few, but significant. Pay special attention to the exclusions, conditions and requirements in your homeowners policy.

We are going through testing the market, and why this came to my attention.

Ah.. Yes  there is that..
The  difference in  wording though does seem to cause some confusion, least for me lol..
What is  commonly referred to as  'grounding' here is actually 'earthing'
Anything that would lead to the chassis never becoming hot is   something that would have my interest.

Roy D wrote:Ah.. Yes  there is that..
The  difference in  wording though does seem to cause some confusion, least for me lol..
What is  commonly referred to as  'grounding' here  is actually 'earthing'
Anything that would lead to the chassis never becoming hot is   something that would have my interest.

Oh MY!!

Don't (really, don't!) get me started on how chassis could get 'hot'. I worked my way through school as an electrician, and somewhat beyond as well. And my many vices - err... hobbies, includes restoring vintage radios.

In the immortal words of Stan Lee: 'nuff said.

Lol he keeps saying that over n over though eh ^^
I will stick to the script when I do the 120 kit, and drop by my EE neighbour couple doors down the street for a good onceover and perhaps he has some pointers in this area of earthing.. When I showed him the tube amp last week he had 5 mins worth of gallows humor to spend on the subject XD

..so looking at an IEC socket, its just ground to chassis hookup and hope for the best ( no hum )
If there is hum, it may be due to the other component in the chain..
Other components here would be class CEII with no earthing so I suppose it will be ok.

An IEC connector fits very well into the VTA-70 / vta-120 chassies. Just don't behind the
transformer but where the fuse is located. Fuse is built-into this IEC connector.

See http://n.manet.nu/vta70/index.html , scroll down to second picture.

Thanks Bob, Peter!
This month some car fixes, planting a new tree or two and fixing our shower - next month fingers crossed for my wife's ok and..

https://www.conrad.nl/nl/schaffner-fn-9260-4-06-netfilter-met-iec-connector-met-zekering-250-vac-4-a-1-mh-1-stuks-398692.html would something like this with a built in mains filter serve much of a useful purpose, with the huge main powersupply transformer of an st-120..?

Roy wrote:https://www.conrad.nl/nl/schaffner-fn-9260-4-06-netfilter-met-iec-connector-met-zekering-250-vac-4-a-1-mh-1-stuks-398692.html would something like this with a built in mains filter serve much of a useful purpose, with the huge main powersupply transformer of an st-120..?

If it fits ...
You will have to measure the space and see if it fits.

So   this would be a good thing to put in over just a bare IEC with fuse holder  íf it fits.. Mind you this supplier has a pretty wide variety of these things with various dimensions within the same spec.
From  what little I have managed to learn on the subject, the pretty massive input transformer of the ST-120 would  already be a pretty decent filter would it not though Peter?

Roy wrote:So   this would be a good thing to put in over just a bare IEC with fuse holder  íf it fits.. Mind you this supplier has a pretty wide variety of these things with various dimensions within the same spec.
From  what little I have managed to learn on the subject, the pretty massive input transformer of the ST-120 would  already be a pretty decent filter would it not though Peter?

Mains filter won't hurt. But it might do any difference either. The cruical part is if it
fits inside, vta120 has an additional choke ( i think) that might interfere.
And, a mains filter would do more if it filters preamp, sources AND poweramp.

A barebones IEC with a fuse ( as the one i showed ) won't be longer then the fuse it replaces,
the hard part is to make the hole, i had a friend with a milling machine do it.