ST-70 power cord

We started working on my my kids ST-70 tonight.
Early progress is going smoothly.
The input jacks went in tonight and the stereo-mono switch came out.

My son raised the question about the line cord. Should we replace it with a new polarized line cord? And if so should we put a 3 prong grounded ? Or will that potentially cause a ground loop?

Adding additional grounding points is asking for hum. Not grounding your amplifier is potentially dangerous. On my MkIIIs I replaced the pathetic Dynaco power cords with AWG14 cords and three-prong plugs. Of course I subsequently had to float the new grounds to eliminate hum after this "upgrade".

ejc wrote:We started working on my my kids ST-70 tonight.
Early progress is going smoothly.
The input jacks went in tonight and the stereo-mono switch came out.

My son raised the question about the line cord. Should we replace it with a new polarized line cord? And if so should we put a 3 prong grounded ? Or will that potentially cause a ground loop?

Yes
Or even better, install an IEC input.
Grounding will enhance security, and if it ever gives problem that cannot be solved ( unlikely)
the lifting the ground will cure it.

Since all plugs are polarized today, and you can't "flip" a 3 prong plug, can you help me figure out which side the white and black wires would go to?
Would it be black to fuse, and white to switch?

I just edited the question, reversing the leads

ejc wrote:Since all plugs are polarized today, and you can't "flip" a 3 prong plug,  can you help me figure out which side the white and black wires would go to?
Would it be black to fuse, and white to switch?

I just edited the question, reversing the leads

OK - I will be the voice of dissent here. If you have an OEM 70, with good transformers and no discernible leakage (I measure any current-to-ground from the chassis of any equipment that passes my bench), stay with the two-lead cord. If you choose to use a polarized plug, the large blade will be the neutral (by code) and the smaller blade will be the hot. So, safety strongly suggests that the black wire goes to the switch. Making the entire chassis "cold" when turned off.

I have a Scott LK150 that had been modified by its previous owner with a 3-lead cord. Hummed like a sub-station. Converted it back to two-lead, and it became much better behaved. So, clearly, adding miscellaneous grounds is risky for no real gain.

Thanks, I figured out my mistake with the neutral and black.
Not sure if I'll connect the ground, but I guess if it causes hum it can:
1- Be diconnected
Or
2- use a 2 Prong adaptor on the plug

Peter W. wrote:
OK - I will be the voice of dissent here. If you have an OEM 70, with good transformers and no discernible leakage (I measure any current-to-ground from the chassis of any equipment that passes my bench), stay with the two-lead cord. If you choose to use a polarized plug, the large blade will be the neutral (by code) and the smaller blade will be the hot. So, safety strongly suggests that the black wire goes to the switch. Making the entire chassis "cold" when turned off.

I have a Scott LK150 that had been modified by its previous owner with a 3-lead cord. Hummed like a sub-station. Converted it back to two-lead, and it became much better behaved. So, clearly, adding miscellaneous grounds is risky for no real gain.

I've seen hum caused by a mains ground, too...more than once.

As for polarity of the two prongs in the wall outlet, I did some electrical work at the house where Peter picked up the tubes from me earlier this month. I checked the outlets, every single one. Four were wired backwards which I fixed (in addition to the whole house having aluminum wire, which I consider a big problem but I didn't fix it!

Any hum caused by connecting ground is caused not by connection of ground, but by a improper
signal grounding scheme.
Connect ground ( it's a protective thing!) and fix the signal grounding !

It seems weird to me but when I built my ST120 using a custom layout and star grounding I got a bunch of hum. I had the hot side going to the fuse and switch and initially did not connect the box to ground. As soon as I connected the box to ground the hum vanished. I felt better about it too.

corndog71 wrote:It seems weird to me but when I built my ST120 using a custom layout and star grounding I got a bunch of hum.  I had the hot side going to the fuse and switch and initially did not connect the box to ground.  As soon as I connected the box to ground the hum vanished.  I felt better about it too.

And here is where I agree with a systems (third wire) ground. The key words being "I built my" and "custom layout".

However, when dealing with a OEM unit of the type under discussion, adding the ground becomes much more problematic as the OEM designer did not design for it. I also agree that all sorts of configurations and adaptations may be created if one *must* add a ground to a vintage amp - but it will require a great deal more care and may not be actually necessary or add much, if any safety. This, assuming that there are no stray leaks from the chassis, transformer or any other part, all of which may be checked.

It is also possible to run a spade-lug from any screw on outside of the chassis to a suitable ground (much easier in a house with an 88-year old cast-iron radiator heating system with a good 'ground' in every room) if one is truly concerned. And if there are fireworks, or a hum as a result, something else is wrong.

I connected chassis ground on my ST-120 and it is very quiet.  The main thing to consider with ground loops is to connect all audio equipment to the same wall plug source to keep the ground impedance low.  Otherwise the ground connection point is all the way back at the AC breaker box.

25 years ago I fried the probes on a Tektronix scope.   The scope was AC grounded and the power supply I was working on was not.   The scope probe ground connection could not handle the "floating" ground of the power supply.   And it all ended up with a lot of smoke and sparks.  The solution was scope needed a cheater plug to float its AC ground.  So I always scarily envision my fancy audio interconnects providing the same drama when connecting AC grounded to ungrounded audio equipment.

And inside the amp I wire the AC "hot" immediately to the fuse and then to the switch.  This is what "UL" likes to see when inspecting equipment under consideration for UL safety approval.

Good evening everyone,
    This is what I did with mine, since the chassis I have is Stainless, I had to have a machine shop do the hole.

I used a Neutrik PowerCon which has a really solid and locked connection.

here are a couple of webpage I found that have good info on AC polarity . . .
this one talks about audio quality . . . http://www.gcaudio.com/resources/howtos/acpolarity.html
this one has excellent diagrams . . . . http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html

ejc wrote:We started working on my my kids ST-70 tonight.
Early progress is going smoothly.
The input jacks went in tonight and the stereo-mono switch came out.

My son raised the question about the line cord. Should we replace it with a new polarized line cord? And if so should we put a 3 prong grounded ? Or will that potentially cause a ground loop?

No you won't have a ground loop with a 3 prong grounder plug.

Glad to help in any way I can, if you have more questions.

fagan5982 wrote:
No you won't have a ground loop with a 3 prong grounder plug.

...you WILL if you have another three-wire AC mains cord in the system, that is also grounded.

Let me see if I can clarify a few behaviors that have served me well over the years.

a) One wants to determine the *BEST* ground available at the intended location.
b) If that becomes the reference ground, it must be one of two things: Either the only ground used, or  a ground shared by all equipment.
c) Because: If Unit A uses the reference ground and Unit B uses a different ground not quite as good Unit B will be *above ground* with reference to Unit A.
d) Now the issue of a 'ground loop' comes into play. Sometimes with no consequence, sometimes with significant consequence. Much depends on the nature of the equipment and how far apart the grounds are. To determine the latter, just use your VOM (Ohms) between the reference ground and the 'other' ground. If you get _ANY_ reading at all, you will now see the point of all this.

So, cutting to the chase: I will daisy-chain all my chassis together with an external ground wire to my reference ground. At which point it makes no difference whether the equipment has a 2 or 3 prong plug - if that third ground is chassis connected in the first place - as all the external metal is at the reference ground. Where things get squirrelly is when user-installed grounded plugs come into play. Things may be rendered equally safe without altering the circuit from what either the Engineers designed or UL/CA/CE approved when it was originally made.

One last note: Those of you who still support vinyl. The turntable ground should NOT go to the reference ground but to the headshell of either input jack. Several manufacturers back in the day did this as a matter of course, AR, Rabco and a few others.

As always, each situation is unique. What I have detailed above is the case for the six systems I support, four long-term, two constantly changing at home and at our summer house.