Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dynaman1 on October 01, 2010, 06:14:28 am
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I'm converting an old integrated amp into a guitar amp. The amp's rear panel has large (1-3/16") holes that once supported octal sockets used for speaker connections, preamp power, etc. To maintain the aesthetics of the amp, I'm going to use these holes for 1/4" speaker jacks and toggle switches for the AC power and DC standby. I planned on fitting a 2" strip of 1/8" phenolic across the inside of the rear panel to cover the holes. My switches and jacks will be attached to the phenolic and centered in the chassis's old holes. The phenolic will isolate my speaker jacks, and that's perfect for me. However, the power switches will also be isolated from the metal chassis, as they'll be "suspended" by the phenolic.
Is this safe? Is it possible that a switch malfunction could put voltage on the toggle handle and short out once it's touched??
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Is this safe? Is it possible that a switch malfunction could put voltage on the toggle handle and short out once it's touched??
It's unlikely, but not impossible.
A couple of suggestions:
1. Solder a lead onto the switch body and attach this lead to a chassis earth point.
2. Solder an earth wire to a large steel washer and fit this washer between the switch body and the phenolic strip.
The first idea might not be feasible though if the switch is mostly plastic...
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Here's a crude possibility: Assuming that you are mounting the phenolitic board inside the chassis, get a 1-1/2" wide metal washer (hopefully with a 1/2" hole in it) and put that on the outside of the chassis. In my experience, there's always extra room on the switch bushing although the thickness of your board comes into play here.
Anyway, that would solve any potential grounding issue, look better, and be a more secure mount for the switch as well.
HTH
Chip
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Just wrap the back side of the phenolic board with heavy duty aluminum foil in the area of the power switch. Let the foil lap over the edges onto the face of the board. Trim so the foil doesn't show thru your big hole but still makes contact with the chassis.
If you have access to aluminum duct tape, use that for an even neater solution.
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I have never HEARD of a switch failing in that manor.
I see so many products built today out of plastic and I do not ever see any "safety" grounding lugs on switches.
I've tore apart countless switches and cannot fathom how any signal that is on the lug can possibly make contact to the lever.
HOWEVER, those are just my opinions and I can't back that up in anyway.
SO if you really insist on pursuing this further I SUGGEST(take it FWIW) to go to the hardware store, buy a piece of brass shim stock, drill the appropriate sized hole to pass the switch through, file, deburr etc. then take a small drill bit (1/16 - 3/32) and make a hole maybe a half inch away from the edge of the big hole. with a pair of scissors cut around the hole and the the small hole to make a proper grounding lug.
Ray
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OH!
PLEASE be careful when drilling the brass, especially the big hole.
Brass machines in a way that tools like drill bits grab the metal and will spin the sheet with violence.
Brass shim stock can be very SHARP and you wind up with a razor blade of death being whirled around at many hundred RPMS on the drill.
For safety sake cover the edges of the brass sheet with a few layers of duct tape and be aware of how you are working.
Ray
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OK,
I've looked and looked and can find ground lugs for Pots, RF connectors, test points, chassis etc, I do not see any grounding lugs for switches.
If there were A likelihood of the switch failing the way we're imagining I bet someone would be manufacturing switch grounding lugs and selling them through Mouser Newark Digi Key or someone.
Ray
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I've had a good quality toggle switch to come apart while flipping the toggle. The inside switching mechanism disappeared and left the back of the switch dangling by the wires and the toggle between my fingers. There was no dangerous voltage on the switch, but I wouldn't want to repeat that with a hot switch.
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Thanks for the replies. I almost don't want to reply cuz I wanna see who comes up with the most elaborate and time consuming fix!! Take no offense - that's why this forum is my favorite! I didn't realize there were 782 ways to skin a cat 'til I found this place.
I think I'm just gonna make a single wrap of bare 22ga around the switch shaft and squeeze it between two nuts on the backside of the switch. Solder the wire tail to ground.
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I've had a good quality toggle switch to come apart while flipping the toggle. The inside switching mechanism disappeared and left the back of the switch dangling by the wires and the toggle between my fingers. There was no dangerous voltage on the switch, but I wouldn't want to repeat that with a hot switch.
I now have heard from a reliable source that this could happen.
I'll not take it lightly any more (possible dangers) if I'm running HT or mains through a switch.
Thank You Steve.
Ray
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I didn't realize there were 782 ways to skin a cat 'til I found this place.
I think I'm just gonna make a single wrap of bare 22ga around the switch shaft and squeeze it between two nuts on the backside of the switch. Solder the wire tail to ground.
There usually are many many ways to skin a cat. (watch out for PETA, the ASPCA, and The Humane Society when you do it. :wink: ) As you see, some are more elaborate (and sometimes convoluted) while others are simpler. I like simpler myself. Your idea can work... BUT I personally would feel safer knowing that I couldn't inadvertently pull it off. I'd suggest using a large enough washer that has some kind of solder lug (toothed or not) and secure the other end to chassis ground in a similar fashion. It's still simple, but more reliable IMHO.