isolation transformer question.

[jeff]

 Hi guys, been a while.

I recently got a bunch of free broken amps. one of the things I got was a intercom that I'm converting to a guitar amp. The unit has no power transformer, just runs off 117V AC or DC. I lucked out because I also got a free isolation transformer. I notice tha transformer has 120V in 120V out and one wire marked shield. I would like to make a 120V power supply to run anything without a transformer. How should I wire this? Should I get a 3 prong cord and wire hot nuetral and ground to one side then wire the secondary to a two prong outlet, or should I also rewire the intercom unit with 3 prong cord and connect hot neutral out to a 3 prong outlet and hot and neutral in to a 3 prong power cord and wire all 3 ground wires together.

Not sure what an isolation transformer is. Why it has a shield wire as opposed to say a step up or step down transformer with no shield wire. Could you please explain. a schematic would be helpful.

 Thanks
 Jeff
Happy new year.

[eleventeen]

Hot, Neut, Gnd > 3-wire input, Hot, Neut, Gnd > 3-wire outlet (eg; socket)

I don't know if you can even buy a 2-wire duplex wall outlet any more. I guess you can, since they still sell 2-wire extension cords.

Do you have one of those socket testers that lets you know if your (source) outlet is properly wired, Hot, Neut, Gnd? Not a bad thing to own at all. Could save your life one day, literally.

Are you going to try to make the i'com work? How can you know if it works if you have only 1 box? (that is an assumption on my part) If not, if you are just going to rip it apart and use the corpse as a home for (say) a (transformer-equipped) Champ, then I would just go there and not diddle with the i'com as it sits. Or...is it your plan to permanently "associate" the iso trans with only this amp? Myself, I would sort of want to use the iso as an independent entity, a piece of test/bench gear as it were. In that case, I would mount it in some kind of cheapo box, hopefully w/a pilot lamp, fuse, and switch.
 

[kagliostro]

Saying that I don't like the idea to build a new amp without an integrated isolation transformer

in the attached image you see how to wire an isolation transformer in a box for general purpose

in the other image you can see both a standard transformer (A) and an isolation transformer (B)

Where is the difference ? Simply in the quality of the isolation between primary and secondary winding

isolation transformers are build using materials that give a more safe result

K

[jjasilli]

Isolation is a matter of degree.  An ordinary PT has isolation between its primary & secondary coils.  But parasitic capacitance / inductance allows some "leakage" of current.  A true isolation transformer has a shield, which if earth grounded, reduces such leakage.  So it provides better isolation.

Here's some wiring info:  http://vintageamps.com/plexiboard/viewtopic.php?f=8&t=98663  http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/step7/The-Plan/

[jeff]

Wow, Thanks!

 to answer your questions:

I plan to permanetly hard wire the unit to listen mode and use a jack input in place of the other send/recive unit.

Innitally I did want to premantly install the Xfmr but it's too tight a fit(no endbells, don"t like the idea of hot tube next to the paper bobbin)

so I plan to make the iso an "independant entity" for playing around with radios and other Xfmrless tube equiptment. Mount in seperate box w? fuse and switch.

BUT.... my concern is that if I use a standard 3 prong plug someone may someday try to plug it straight into a standard outlet. So I may just hard wire it to the iso box to prevent that. Disable the on/off of the i"com so you have to use the  iso box's switch to turn it on or off.

 

[eleventeen]

"BUT.... my concern is that if I use a standard 3 prong plug someone may someday try to plug it straight into a standard outlet. "

What's wrong with that?

You have continuous ground from source (let's call it the "wall" outlet) to the 3-wire outlet (reco'ed) on your prospective "bench iso box".

Incidentally, I did not make that clear in my prior post. Output ground should connect to input ground. Per Kagliostro's diagram. Shield: doesn't really matter, connect it, too, to ground. 

Think: What is the peril that the iso is trying to prevent? It is that given your i'com has one side of the AC line connected to the chassis, and thus that chassis  in a grounded 1/4" guitar-plug world where your guitar strings (connected thru the 1/4" cable and thru the strings, etc;) *could* under some conditions be the hot side of your AC line. You strum your strings and reach over to another device to adjust a knob which is conventionally chassis-grounded. Zap. Across your heart. Not good. Same could occur while you are working on the innards of the i'com. You are steadying the upside-down chassis and reach over to your metal-chassis signal generator to change something.

Now to be clear: isolated or not, if you, your body forms a/the connection between 120 volts here and the other side of 120 volts over there, shock results. Grounded or not, isolated or not.

The condition that the iso trans is guarding against, is the reversal of "hot" and "neutral" in some (miswired) outlet you happen to plug your i'com into. Or; the same could result if someone replaced the line cord to your i'com at some point and flipped the AC input wires. Theoretically, there is a small blade (hot) and a large blade (neutral) on the AC plug to the i'com which assures the correct polarity of chassis-to-gnd. Unfortunately, a wiring mistake here that say a table lamp would not care about is potentially very dangerous.

[PRR]

> I don't know if you can even buy a 2-wire duplex wall outlet any more.

Getting rare; but not needed. Use 3-hole outlets. Ground the U-hole. The 2-pin plug fits fine in a 3-hole outlet.

However the innercom REALLY should get a 3-wire cord, Green to Chassis or any exposed metal.

> an isolation transformer is. Why it has a shield wire as opposed to say a step up or step down

Some step-up transformers have no isolation, just one winding. So a shield would be pointless.

Most step-up 'formers are "just get it to work!". I have a 230V shaver, I'm in a 120V land, I need a shave. Garbage and hash on the line is not an issue.

When you go out and buy a same-voltage ISOlation transformer, you might really want to be cutting some hash off the line. Or not (we don't). However the shield adds less than a buck to the cost, so they just throw it in.

It should connect to Green from the wall. Not really for shielding, but because it is an additional safety against transformer breakdown.

If the iso-former is like 60 Watts and has 3-pin in and out, some day some fool will plug-in a Marshall Major or Gibson 800. The big amp won't work well, and then the iso-former SMOKEs. Put a half-Amp slow-blow fuse between wall and 'former.

Really: unless you collect the AC/DC junk, you oughta just nail the iso to the intercom any way you can.

[kagliostro]

Innitally I did want to premantly install the Xfmr but it's too tight a fit(no endbells, don"t like the idea of hot tube next to the paper bobbin)

a small sheet of copper may be your solution



K

[jjasilli]

Here's a photo of a Stromberg Carlson PA amp. A bit hard to see - on the far right corner is a 6550 power tube, then an L-shaped protective shield; then a relay in a rectangular, black phenolic case.