Non polarized AC

[Karl5150]

I have an early '60s Scott amp with the stock, non polarized cord that has seen better days. I'm looking for suggestions on replacement direction.
Thanks in advance,
Karl

[Paul Birkeland]

I'd start by posting photos of the area where the power cord comes in.

Generally, I buy a new cable gland, then install a 3-wire power cord through that gland, so that no metal work needs to be done to the chassis.

With a couple of photos, better directions can be provided.

[Paul Birkeland]

First you determine which orientation of hot an neutral gives lower voltage on the chassis.  With that nailed down you put a grounded cord on it and it will be much safer.

If you don't follow this I can give step by step instructions.

Generally we want to lift the neutral connection to the chassis and add a dedicated earth connection to the chassis. 

Leaving the neutral tied to earth inside the amplifier will mean that you'll get a breaker pop on a hot/neutral swapped outlet.  (Not the amp's fault, but still...)

[Grainger49]

I was flashing back to an article that indicated the AC on the chassis was a large source of noise.  It assumed a two prong plug as Karl has. 

Since the Scott has no designated neutral inside the hot and neutral can be swapped at the input to find which has lower chassis voltage.  Then tie the ground to the chassis. 

Bottlehead doesn't tie the neutral to the chassis that I am aware of.  I wasn't suggesting connecting the neutral to the chassis.

[Mike B]

If everything goes thru an isolation transformer it won't matter, just make sure line gets switched.  Yes, chassis shound be earth ground, if a transformer primary shorts to case, chassis won't be hot.

[Karl5150]

Thanks everyone. It is switched. Instructions were to listen with plug in both directions and use in the direction that sounded better, I can just trace it back to see which side of the existing plug goes where in the better sounding (lower chassis voltage) orientation. I'll remember to include more info in the original post next time, see attached schematic of the power input area FWIW.
Thanks again,
Karl

[Paul Birkeland]

I would recommend tweaking the amp so that the fuse comes between the power switch and AC line.

If either of those wires on the primary of the power transformer connect to the chassis, lift that connection, then connect the earth from your 3-prong cord to that chassis connection. 

[Karl5150]

Paul,
Our electronics guy at work thought it was "interesting" that the switch and the fuse are on separate legs. Would you expect that the power supply topography used would be a contributor to the overall system hum? (It is certainly not quiet by BH standards)
I guess it will boil down to how much I want to do to muck with what is, despite the background, a pretty sweet sounding amp.
I guess I was looking for the easy answer like "so-and so makes a decent non polarized, two wire ac cord." Foolish me
Thanks again,
Karl

[Paul Birkeland]

Oh yeah, a lot can be learned from working through these vintage amps, especially if you take one and apply some of our hardware tricks. 

Inside these vintage amps, we will often find multiple different solder lugs connected to the chassis plate being used as grounds (including ground lugs built into the mounting rings of tube sockets).

If you were to isolate all but one of those solder lugs, then wire them all together (without creating loops), you would likely knock the hum down a fair amount. 

[Karl5150]

 The only chassis ground from the transformer is tied to a leg of the rectifying device so I was hesitant to lift it. I ended up lining up the switch and fuse on the hot leg of the new cord and grounding direct to a chassis bolt, no loop and it is WAY quieter.
 I don't have 'phones or measurement equipment, just a dB app on my phone that showed >4dB at idle. (hum wasn't volume dependent)
 I'm sure cleaning up all the multi point grounding on the unit would improve it more but for now I am satisfied just getting rid of the antenna that was powering it before.
Thanks again for the info.