Polarity Switch

[slippingglimpser]

ok, so what's the deal with the polarity switches on old Sunn amps?  I think I understand the physics of polarity and phase but what practical use do these switches have?  Should I be able to hear a difference between up and down?, because I can't on my 190L.  It just seems kind of funny for Sunn to make all these incredibly staight forward, sleek and strong, no gizmos amps and then have this little polarity guy.  Although, if anyone knows a practical use for it, I would LOVE to know.
Thanks to anyone who can help!

[Don T.]

Hello:
 The polarity switch is there to provide a ground path for the chasis. Since the amps came with two prong plugs it is posible to have the chasis tied to the "hot" side of the AC. Lets say your PA is plugged in with the opposite leg tie to chasis. Now hang on to your guitar and touch your lips to the mic. ZZZZZAPPPPP. That is the reason for the switch. Plug a guitar cord into your amp and have the other end hanging unterminated. Now flip the switch and you will hear a difference. In proper phase is the quieter of the two positions, however I always test the mics to make sure I wont get shocked. I never trust that club owners know how to wire AC plugs. I bring my own tester and check everything before I plug in and fire up the gear.
Don T.

[slippingglimpser]

Thanks Don!
Yeah, that makes good sense.  I like it.  Although I've never had a need or desire to plug anything into that two prong AC outlet on the amp.  (Although, who knows, maybe I will). Does this mean that the polarity switch would then not effect anything?  Or is the unterminated cord buzz still effected?  Sorry for my lack of electric knowlege and thanks again.

[JoeArthur]

Unterminated cord or not... yes, the buzz is still affected by the polarity switch.  It should always be in the correct position, and correct means lower buzz.  

BTW, the unterminated cord just makes the buzz easier to hear.

[slippingglimpser]

Hmmmm.... yeah, I tried getting it to change the buzz.  I even tried cranking up a distortion pedal before the unterminated chord to see if I could then hear a difference, but nope.  My 190L is actually really noiseless, though, and I think I can't tell the difference because it's just not loud enough.  

[JoeArthur]

You know something... you never mentioned if your amp has a two prong cord... and when Don T mentioned two prong plug, it seems like you thought he was talking about the accessory outlet on the back of the amp.

If someone installed a three prong cord into your amp and you have it plugged into a three prong wall outlet, then the polarity switch will not have any effect - the chassis will be properly grounded through that third prong... the single rounded one.  Polarity switches will only come in handy when you have to plug into a two prong wall outlet.

BTW... plugging a cord into a distortion box "terminates" it.