Simple power switch question

[zhyla]

I'm looking at a Bassman schematic (http://zhyla.net:4000/files/dev/bass...m_modified.jpg) and I see that the power switch is a single switch (SPDT) and only disconnects one side of the circuit. Another amp that I built previously had a DPDT (http://ax84.com/media/ax84_m276.gif) that disconnects both the hot and return wires. Is there a safety reason for this? Is it in case you wire the switch onto the wrong side or something?

[Enzo]

Breaking BOTH sides of the incoming mains is safer because when the switch is off there is no connection between the mains and the amp circuitry.

Breaking a single side of the mains is simply cheaper, so Fender went that way. To be fair, so did most everyone else.

Wiring designs are not based on the potential someone might mis-wire it in the future. After all, the safest circuit you might come up with could be mis-wired and then no longer safe. Wiring designs are based upon a correctly wired amp protecting you from various sorts of failures.

For example it is proper to wire the mains cord with the ground wire longer than the hot and neutral wires. This is so in the event the amp falls and yanks the power cord out of the chassis, the last wire to break will be the ground. There are many little considerations like that in the wiring code.

[lmolter]

What Bassman model schematic was that? It's quite a bit different than the 5F6A.

[rathmann]

Although much less of a concern nowadays since the advent of building and electrical codes, the DPDT option is safer in that it provides some protection even in the case where the outlet might be wired wrong with the neutral tab slot actually wired hot. In such a case, an amp with the single side switch could have the full 120vac present on the chassis, and depending on other grounding practices as far up the chain as the guitar strings. Grabbing this rig barefoot on a cement garage floor could be rather exciting.

Note that an old vintage Fender amp chord could simply be put in backwards and result in this same situation. The best protection here is to use a three prong chord, and if you're slightly on the side of paranoid or electrohormephobic you'll use a DPDT switch.

[zhyla]

I've already got a three-prong chord installed. Sounds like I'll be fine with just the hot switched.

lmolter, that's the 5B6 "widepanel" Bassman. I couldn't find too much online about it and I've never heard one played... I'm kind of just building it to do something different.

[dskowron]

electrohormephobic

- I love this one!

I actually had this happen to me: I was on stage cleaning up before a gig. I had one hand on a piece of equipment that was mounted to a steel frame - it was a fog machine, and I went to stand up and for support I grabbed a mic stand. BAD IDEA!! It whacked me through the body. Fortunately I was standing on the very edge of a riser and I fell off, which forced me to let go of the fog machine frame. If it weren't for that, I would not be here typing this. I wondered why it happened since all power was off. Well, the doof that wired the fogger up wired it backwards at the plug, which put hot to the chassis. The mic stand was grounded through the cable. I was the fuse. I had a splitter of a headache for the rest of the night.

I like the DPST idea. I don't use it for that purpose, but I do like it. Certainly can't hurt anything.

[Enzo]

Many years ago, we were at come club, and our guitar player walked up to his mic, leaned a little and his guitar neck touched the mic and it burnt his E string in half.

[mooreamps]

Use the DPST.... Doesn't matter if you have a three wire plug or not. Always play it safe when designing a circuit involving High Voltage.....

-g