Power Transformer Selection

[Bill Mountain]

I want to set up a prototype rig for simple tube preamps, overdrives, etc...

I have a transformer that I've used in the past:

Hammond 262B12

http://www.hammondmfg.com/261.htm

But it got super hot (burned me more than it shocked me) and I couldn't figure out what the problem was so I stopped using it.  I still have it and can use it again but I was wondering if anyone had found a decent power transformer that can fit in a pedal enclosure that I could use instead.  I don't need super high voltage.  I can work with what ever the transformer provides but I would like 60V as a minimum.

Any ideas?

[Bill Mountain]

Maybe I could find a small 120V to 9-12V  toroidal transformer and use a voltage multiplier for the anode voltage and tap the it at the beginning for the heater voltage???

Can I take two paths from the same secondaries?  One path gets rectified immediately for the heaters and the other uses a voltage doubler/tripler/quadrupler for the high voltage?

[Bill Mountain]

Something like this?

http://www.antekinc.com/an-0112-10va-12v-transformer/

Now...this seems to have two outputs but it also has two inputs.  Can I get both outputs from one primary connection?

[PRR]

> it got super hot

Just sitting there no-load?

Maybe you don't know how to hook it up?

[petemoore]

 Test it !

[Bill Mountain]

> it got super hot

Just sitting there no-load?

Maybe you don't know how to hook it up?

Quite possibly.  I've built a tube amp before with no problem and I used this transformer for a phone preamp with no problem but when I used it in my prototyping rig I may have hooked something up wrong.

[darron]

Now...this seems to have two outputs but it also has two inputs.  Can I get both outputs from one primary connection?

Depending on which country voltage you are feeding it, you'll need to wire it up accordingly.

IE if you wanted to feed it 230V at the primary you could run the 2x 115v windings in series to make it happen. They are an equal load so the voltage will get halved between them.


if you want to feed it the slight tingle 115v () then you could run them in parallel.




same goes for the secondaries. you could run them in series to get twice the voltage, or it could be useful to join them up and make a centre tap transformer. or run them in parallel to get the full rated current. whatever requirements are.


at any rate, it seems a little silly if you only want to use one half of the transformer.

[PRR]

> has two inputs.  Can I get both outputs from one primary connection?

Yes; but at very reduced total power output.

If an airplane is designed for twin engines, you should use both engines.

Why would you wanna do just one?

Darron may snicker at our faint voltage, but he's living recklessly (or paying a lot for safe plugs).

To you both primaries you MUST wire them the right way. Get it wrong, the transformer is a near-short across the line (and our lines have more Amps than most of Darron's).

The Anteks make it very simple.

In general, a Lamp Limiter is good to have when trying to get your wall-voltage wiring right.

[Bill Mountain]

Thanks guys.  I'm gonna take this slow and follow the original power supply schematic for the phono preamp that I built before.

[darron]

What am I doing recklessly? And I suppose I'm not sure what a plug is. I don't get it, sorry?

[darron]

Most of the household GPO RCDs here are rated 16A. So 3,840W. If that helps.


Edit/addition. You could also make up a isolation unit to help make the task less likely to be lethal. But to do that would require wiring up a transformer which brings us right back to the beginning.

I hope my description helped with the correct wiring but please do read up first.

[merlinb]

For those who live in the US, one possible way to get an HV/LV transformer for cheap, is to use an ordinary dual-primary transformer, but use one of the primaries as a secondary. The total power handling of the transformer is halved, however, and this power is shared between the HV and LV outputs. But it's certainly cheap!
SAFETY WARNING!
The insulation between the two primaries may not be as good as that between primary and secondary, so I can't vouch for the actual safety of this method.

[Lurco]

2 inputs? Be aware of the coil orientation! ("polarity").

[PRR]

> What am I doing recklessly?

I am joking about your 240 Volts in the walls.

Here, many people are scared spitless of just our 120V.

> what a plug is

A US plug is a 19 cent hunk of junk which begs you to put fingers across the live blades.

A UK-tradition plug is a much more serious and safe thing.

I suspect, to the average user, UK-style 240V on a "safe" plug is safer than 120V on a US-type plug.

(Our plugs vary. Some are less unsafe than others. History has been toward better plugs. I have a 1960 alarm clock which I would not leave around children. I have some modern plugs which make it very difficult to get fingers onto the 120V. Difficult but far from impossible, as in the UK plugs.)

Typical US 1930-1940s wall plugs.
http://www.antiquelampsupply.com/product/1900_early-electric-style-heavy-bakelite-wall-plug
http://www.hudsonscustommachining.com/vintage.plugs.3.jpg

Note complete lack of finger-grip:
http://www.hudsonscustommachining.com/plug.eagle.1.jpg

Then they got smaller. This is a (reproduction of!) a US 1950s plastic wall-plug. Many lacked the slight ridge between fingers and blades.
http://i00.i.aliimg.com/wsphoto/v0/1761965744_1/R1140-Vintage-Antique-Style-Bronze-Electrical-outlet-Line-Finger-Ring-Size-5.jpg_350x350.jpg

(For US lurkers) A UK-type plug has massive finger grips and also plastic-wrap prongs so you CAN'T get a finger on a live pin:
http://images.wisegeek.com/uk-outlet-plug.jpg
By the time it is out far enough to touch live brass, the brass is out of the socket contacts.

> here are rated 16A

Nearly any US wall outlet a guitarist would plug into has 20 Amps available. (15A general lighting circuits are out of style.) It may be slightly easier to start a fire in the walls with 20A than with 16A, though much depends on the wall wiring which is very different in the two systems.

[duck_arse]

the australian standard mains plug is rated 10A. if you want to plug-in your (single phase) caravan or fork-lift charger, the 15A plau has an enlarged earth lug, won't fit into a 10A socket.

we're funny that way.

from where I sit I can see a lamp (from the 70's) with a two-piece bakelite mains plug, earth pin but two-core flex.

[R.G.]

SAFETY WARNING!
The insulation between the two primaries may not be as good as that between primary and secondary, so I can't vouch for the actual safety of this method.

I can vouch for the NOT SAFETY of this method. To the audience: DO NOT DO THIS. YOU'RE RISKING YOUR LIFE AND THE LIVES OF OTHERS IF YOU DO.

I designed transformers as part of designing power supplies for a living for a number of years. Transformer designers are always under pressure to stuff more power and performance in the very limited spaces in the winding window of a transformer and to make that window smaller to save money. It's the fundamental point of transformer design once you get past being able to figure out the number of turns needed. There is a built-in motive to NOT isolate two primaries by more than functional levels of insulation, because this detracts from the insulation you can put between primaries and secondaries in places you have to have it to meet safety specs.

It is possible that some transformer might have enough insulation between two primaries to withstand AC line transients, but it is an accident if it does happen. Primary to primary safety insulation strength is not tested for in any safety review or testing of the transformer unless it was specified in the original design. I've never seen one specified for this, and I saw a lot of whacky transformer designs.

I have used a dual primary transformer for isolation before, but it was under pressure of time in a lab setup where everything could be nailed down and the "isolated" power could be viewed as being as dangerous as non-isolated power; and the setup was torn down when the testing was over so a less aware/informed colleague would not kill themselves with it.

You get justifiably paranoid when touching any wire might just kill you.   

[darron]

the australian standard mains plug is rated 10A. if you want to plug-in your (single phase) caravan or fork-lift charger, the 15A plau has an enlarged earth lug, won't fit into a 10A socket.

we're funny that way.

from where I sit I can see a lamp (from the 70's) with a two-piece bakelite mains plug, earth pin but two-core flex.

even though regular plugs are 10A the breaker is often 16. for 15A plug they like the fatty earth so that you're forced to use a point that has a dedicated safety (not shared). i like the 56 series plugs though, too. and yeah, they are costly. ours is different from the UK


in my partner's original country, Bangladesh, there's no standard, so you're plug and point need to match out of about 5 selections.

sorry... OT.

[PRR]

> plugs are 10A the breaker is often 16

In the US, most plugs and outlets are 15A on 20A breakers.

True 20A outlets are quite rare. (The blades are not parallel; 20A outlets are usually combo 15A/20A.)

I assure you that I can pull 20A+ out of a 15A plug. For a while.

[darron]

I assure you that I can pull 20A+ out of a 15A plug. For a while.

ahhh i love that
true, true