I'm on my way toward building my first tube amp & the most straightforward set-up would be using an autotransformer. Down here I can get a 220-110 at 300W for some $15 USD(60 Pesos) With a core of 5" by 5" by 2 1/2"....
Just how unsafe are these transformers & am I commiting the ultimate sin here by not using a "true" transformer....am I about to be censured?
Perhaps I could fuse it three ways to Sunday! Just to throw gas on the fire the grounds here are a hit & miss affair. Household wiring here is shoddy to say the least!
_______________________________________________________Rick.....
Just how unsafe are these transformers & am I commiting the ultimate sin here by not using a "true" transformer....am I about to be censured?
Perhaps I could fuse it three ways to Sunday! Just to throw gas on the fire the grounds here are a hit & miss affair. Household wiring here is shoddy to say the least!
_______________________________________________________Rick.....
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Gas on the fire is a good enough analogy
I would recommend a proper step-down isolation XFMR.
In-fact any time I use an auto transformer it's paired with an oversized isolation XFMER.
EDIT>A Fuse or more than one especially is not a panacea for shocking hazard protection.
If any part of that circuit shown above and its connected circuitry interfaces with the outside of a grounded enclosed metallic or doubley insulated case then it's considered a safety hazard in most industrialized countries.
I would recommend a proper step-down isolation XFMR.
In-fact any time I use an auto transformer it's paired with an oversized isolation XFMER.
EDIT>A Fuse or more than one especially is not a panacea for shocking hazard protection.
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An autotransformer is about the same thing as connecting a wire directly from your circuit to the wall outlet. It has zero isolation. Don't do it. An autotransformer doesn't need quite as large a core (not sure why), so is often cheaper. Is there any part of "Don't do it" that needs additional clarification? Did I mention not to do this? We'd like you to keep posting and the internet connection from six feet under is really bad. 
(the Tolkien lost manuscript)Richard,
If you like life, you'll heed the other posters and acquire a proper power trafo. Depending on the B+ draw, either a Triad N-68X (50 VA) or a Triad N-77U (100 VA) will get the job done for you, SAFELY. Both of those isolation trafos have dual primaries, a "115" V. secondary, and they are inexpensive.
If you like life, you'll heed the other posters and acquire a proper power trafo. Depending on the B+ draw, either a Triad N-68X (50 VA) or a Triad N-77U (100 VA) will get the job done for you, SAFELY. Both of those isolation trafos have dual primaries, a "115" V. secondary, and they are inexpensive.
"extrema ratio"
The best solution are the Triad xformers (I'm using these), but if You are by no means able to access to imported material, a couple of 220/12V cheap xformers, tied back to back, can do the trick. When feeded trough the step down autoformer ( this one rigorously outboard), You'll get half the socket voltage, and a safe chassis. I've just done this, this morning, to be sure that it works.
The best solution are the Triad xformers (I'm using these), but if You are by no means able to access to imported material, a couple of 220/12V cheap xformers, tied back to back, can do the trick. When feeded trough the step down autoformer ( this one rigorously outboard), You'll get half the socket voltage, and a safe chassis. I've just done this, this morning, to be sure that it works.
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In my earlier days, anyone on my watch caught using an autotranny directly on the workbench without an isolating tranny where others also working was a punishable offence because it endangered others. Back to Tech evening classes on safety.
Simply put, engineering wise it isn't the done thing and contravenes a raft of IEEE regulations.. In many domestic European locations one often finds there isn't a neutral, but two live phases and earth. Don't blow ones chances.
richy
OK, OK Got the point!
Besides changing the transformer I will be wiring a ground for the one outlet this amp will be "plugged into".
The house had two conductor outlets thruout , but the kitchen was remodeled when we got here and three conductor outlets(Modern) were used. I would bet real money these new outlets have no ground hook-up.
In the living-room all my U.S.A gear runs thru one of these evil Autotransformers...a big one... rated at 750 Watts. Perhaps it too should be 'isolated'.
Almost all appliances here will give one a low level shock. The outlet in question...I can run some 12 Gauge solid thru the wall onto a grounding rod(Rebar?) driven into the soil. I know one MUST paint over or 'seal' the connection to the grounding rod lest corrosion or rust start to 'open up' your resistances.
I found a transformer which I can use for my amp & perhaps as an isolator for the rest of my system.
Antek 64115, dual 115V Primaries, dual 115 Secondaries, 12V, 5V.
The Antek is a really good case of overkill....but that what tube building is all about!
_____________________________________________________Rick......
Besides changing the transformer I will be wiring a ground for the one outlet this amp will be "plugged into".
The house had two conductor outlets thruout , but the kitchen was remodeled when we got here and three conductor outlets(Modern) were used. I would bet real money these new outlets have no ground hook-up.
In the living-room all my U.S.A gear runs thru one of these evil Autotransformers...a big one... rated at 750 Watts. Perhaps it too should be 'isolated'.
Almost all appliances here will give one a low level shock. The outlet in question...I can run some 12 Gauge solid thru the wall onto a grounding rod(Rebar?) driven into the soil. I know one MUST paint over or 'seal' the connection to the grounding rod lest corrosion or rust start to 'open up' your resistances.
I found a transformer which I can use for my amp & perhaps as an isolator for the rest of my system.
Antek 64115, dual 115V Primaries, dual 115 Secondaries, 12V, 5V.
The Antek is a really good case of overkill....but that what tube building is all about!
_____________________________________________________Rick......
Hello Richard, For a grounding rod, I would really suggest a piece of copper rod, not iron rebar. And for corrosion prevention at the connection between the ground rod and the incoming ground wire, I would suggest covering the connection in grease after you hook it up, then put a plastic or rubber jacket of some sort over that connection. That should keep most of the corrosion at bay.
Peace,
Dave Gerecke
P.S. I believe that current USA standard is a 1/2"(13mm) diameter copper rod, minimum of 6ft(~2m)long, pounded into the ground.
Peace,
Dave Gerecke
P.S. I believe that current USA standard is a 1/2"(13mm) diameter copper rod, minimum of 6ft(~2m)long, pounded into the ground.
Grounding Rods
According to this McMaster/Carr page, grounding rods are made from plated steel. The plating metals are Nickel and Copper. The construction makes considerable sense, once the softness of Copper is taken into account.
BTW, notice the 8 foot and 10 foot lengths. That suggests the rod is pounded into the soil, until only 2 feet are left sticking out.
According to this McMaster/Carr page, grounding rods are made from plated steel. The plating metals are Nickel and Copper. The construction makes considerable sense, once the softness of Copper is taken into account.
BTW, notice the 8 foot and 10 foot lengths. That suggests the rod is pounded into the soil, until only 2 feet are left sticking out.
Not necessary if said gear has some safety markings i.e. "isolation inside". Most stuff is 2 pronged then it's double insulated as well. Except for SELV circuits, thingys that use wall warts.
EDIT> It could NOT hurt tho as the Variac itself is probably not too safe.
FWIW The variac rating is for current so the posted rating at 1/2 V out would be 375 VA only!
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Ideally, your copper clad steel ground rod is pounded in until 2 *inches* are sticking out. I've never used anything to prevent corrosion but certainly the typical greasy compounds (Crouse-Hinds STL, NOALOX, etc.) could be used if needed.
BTW, the schematic in the first post, besides being a "widowmaker", will short out on the bonding of neutral to ground if present...
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