Stepping up the voltage of a transformer safely.

[Thecomedian]

I found what is probably a 15 lb transformer with 10 lugs and a barely schematic of 220v in and 14v, 20vct, and 8.5v out windings. The hand drawn instructions also say its 15 amps.

What should I look into for stepping up the voltage to fully power some tube based projects? Will normal voltage 2x-4x doubling circuits work?

Okay, I've been spending the day thinking about it, and I guess the voltage coming out from the secondaries would be 210 @1A, 300 @1A, and ~127 @1A, wouldn't it? So I might not need the voltage doublers after all. Is there anything I'm missing about this kind of potential conversion from high amps to high volts?

[merlinb]

I found what is probably a 15 lb transformer with 10 lugs and a barely schematic of 220v in and 14v, 20vct, and 8.5v out windings. The hand drawn instructions also say its 15 amps.

What should I look into for stepping up the voltage to fully power some tube based projects? Will normal voltage 2x-4x doubling circuits work?

Okay, I've been spending the day thinking about it, and I guess the voltage coming out from the secondaries would be 210 @1A, 300 @1A, and ~127 @1A, wouldn't it? So I might not need the voltage doublers after all. Is there anything I'm missing about this kind of potential conversion from high amps to high volts?

Wut? The voltage coming out will be 14V, 20V and 8.5V as you first described...

This transformer is totally unsuitable for tube projects. You don't use 15 amps for tube projects.

[Transmogrifox]

Well you don't need 15 amps, but if this was the only downfall, it would be a useful transformer.

You could get some useful voltages out of it if you drove the 8.5V tap with 120 VAC, you would get 197, 282, and 3.1 kV respectively.  The 3.1 kV would probably break down the insulation in the transformer unless its insulation system is rated for something in excess of this.  

In either case, the 3.1 kV voltage would be extremely dangerous, so there isn't a safe way to use this transformer for tube projects unless you take it apart and rewind it.  Then you could change one of the windings to make a useful heater voltage.


Unless you're making a power amp the most straighforward way to get useful tube project voltages is to do back-back 500 mA "wall-wart" transformers.  

The next best way is to make SMPS boost converters to go from 9 or 12V up to the desired plate voltage.  Pretty easily done with a 555 timer, inductor and MOSFET.

If doing power amps, then you have a great core in your 15 lb transformer -- more than you need, but with some rewinding you can get the voltages you need at the power you need.  You would have one heavy amp

In either case this transformer presents you with a very dangerous situation unless you really know how to wind them to the correct number of turns and derive correct ratios from that.

[R.G.]

You could get some useful voltages out of it if you drove the 8.5V tap with 120 VAC, you would get 197, 282, and 3.1 kV respectively.

It doesn't work that way. It might if the core did not saturate, but it will. The core of a transformer saturates at a given number of integrated volt-seconds. Economics being the dismal limiter that it is, power transformers are designed to work just barely within core saturation at the intended frequency and voltage. If this one is designed to work with 8.5V coming out of that winding, it's unlikely to have more than about 15% "headroom", especially with this large a transformer. So putting in the same power-line frequency and any voltage over about 8.5*1.15 = 9.8Vac will start pushing the core into saturation. Putting 120V on it will certainly saturate it, and the current flowing into the winding will no longer be limited by the inductance of the winding, only the resistance of the winding, so you'd get 100+ volts across a few ohms, and it will be a race between the heating of the wires inside burning apart and the wall-power breaker opening.

Have I mentioned that Mother Nature is a *mother*? 

In either case this transformer presents you with a very dangerous situation unless you really know how to wind them to the correct number of turns and derive correct ratios from that.

I agree - and would add that it would take a very great amount of work to make any useful high voltage project from it.

[thelonious]

Put that beast on feeBay! Someone will pay you bucks so you can get iron with the right secondary voltages. It might cost a kidney to ship it, but you gotta try... 

[GibsonGM]

Put that beast on feeBay! Someone will pay you bucks so you can get iron with the right secondary voltages. It might cost a kidney to ship it, but you gotta try... 

Someone may want it for a transistor project or something...I vote sell it! 

Then get yourself a nice Hammond HV transformer for tube work

[Thecomedian]

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Thanks guys, I'll figure out what to do with it.

I wonder if I should use it for my own amp/transformer/transistor build, although I still wonder. Do you guys think its 15 amp or 1.5 amp? I put some pictures up on the off chance someone might know what it is.

I thought that the way voltage vs amps worked was that if you have high voltage but no resistance, there's no appreciable voltage difference as its all converted to amps, so the reverse could be true. Apparently not for transformers  

Do all guitar amps use a transformer? For the safety of mains isolation if no other reason?

[R.G.]

Do you guys think its 15 amp or 1.5 amp? I put some pictures up on the off chance someone might know what it is.

Transformer size is related to the power it transfers (volts times amps, or VA), how hot you let it get, and what the lowest frequency it runs at. For power line transformers, the frequency is all about the same, and the internal heating is similar, so size goes up with power.

I thought that the way voltage vs amps worked was that if you have high voltage but no resistance, there's no appreciable voltage difference as its all converted to amps, so the reverse could be true. Apparently not for transformers

It is true - excepting that the voltage ratio is set by the number of turns of the winding, not how much or little it's loaded.

Do all guitar amps use a transformer? For the safety of mains isolation if no other reason?

Yes. In most cases they need a different voltage than AC line voltage, so the transformer gives them that, but even if the circuit needed the AC line voltage directly, you'd have to use an isolating transformer to avoid electrocution.

There were some transformerless guitar amps in the 50s and 60s, but these have come to be known as "widowmakers". Two guesses why.   

[GibsonGM]

Uh oh, R.G just beat me to it! 

Hey, Comedian, I'll take a stab, and *GUESS* 15A, given the size of the core and the low secondary voltages, and the fact that *someone* wrote that down...core size in relation to how much current you can draw is actually more of a "power" thing...the size (and gauge of the windings) determines what you can get out of it in power.     You could use this transformer, humble old thing it is, to learn more about them!   R.G., among others, has info on Geofex about how to work safely with a transformer (Google "transformer load test"), SAFELY, to see how much current you can draw from it.   You'd test each winding.  That will tell you a lot more about what you can use it for.   Simply put, you load it with resistance until it sags a certain percent, at which time you note the draw and get your answer.    

Make sure you are OK using mains voltage if you decide to do this!!  <Disclaimer: it can be VERY dangerous if you don't do this properly!

I am willing to bet you could make a pretty nice SS amp with that

Your thinking on power (voltage vs. current) is essentially correct, but 'transformer action' involves a few other variables, too.  We move out of our "standard electronic thinking" when dealing with them.   R.G. points out "saturation", and how the thing is probably built near its design point....to work it 'backwards' would clearly go over that design point, and cause bad things to happen    

As you are using a magnetic field to induce a current in the secondary coil, you're really talking about inductance (and a few other dark arts sort of things!!) and magnetism....these phenomena kind of change the normal rules we're used to.  There are other effects at work besides simple Ohm's Law.   Some people DO get away with playing the backwards game, but it's a very poor idea, where saving a few lousy dollars can burn your house down...much, much more worthwhile and less frustrating when building to buy a $60 transformer meant to do what we want!!

If you went the transistor route with that transformer, once you learn to get the info from it, it would probably do very well. We don't need HIGH voltages to make output power....just the ability to use our supply to provide power to drive speakers.  That trafo can deliver the CURRENT needed to run a speaker...  Read up!  Lots of info on trafos on the net...look at some SS designs, too.

All MODERN and COMMON amps use a transformer of some sort.  A tiny one might use a plug-in wall wart, and the others will use a real transformer like you have there.  You hit the nail on the head....mains isolation is a HUGE reason to use one!   Voltage conversion is the other.

Some old circuits (the "NOT COMMON" case), and radios, etc., did not use one - if you plugged it in backwards, you could get a hot chassis!   Anyone coming across one would be well advised to get a 1:1 isolation transformer ASAP.