# Whats the best way to test the output of a transformer? Can I just use a multimeter?

I have a guitar amp that I want to make portable but the transformer isn't labeled, so I don't know what kind a batteries I need. Is the multimeter a valid option, or is there another way to find out how much power is coming out of It? Unless you can identify the writing on the back of the transformer (which I already googled).

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Sandisk1duo7 years ago
either use a multimeter, or look at the hieghest voltage printed on any capacitor
7 years ago
The capacitors should be rated higher than the required voltage, so they aren't a good indicator of voltage.
7 years ago
a good range
seandogue7 years ago
you can test the voltage pretty easily, as re-design suggests, but the load capacity is best determined by contacting the mfg (of course, if you can do that, you can get  spec sheet that outlines all fo the characteristics...)

You can, however, get a bit of a good estimate by checking the fuse, then multiply that value by the input voltage, and once you've determined the full span output voltage, divide the product by the full span output voltage, then multiply by 1/2 to get a ballpark idea of the available output current.

that gives you an idea of how much current you'll need to deliver from your battery packs.

Now that you know the required current at full output, you can determine the AHr rating of your battery packs by dividing the amount of time you expect the amp to work by the current you calculated in the prior step.

This is of course ballpark.
cyberpageman7 years ago
Hi, I've been thinking about your question, and glad Re-design answered. It also occurs to me that the transformer may be center-tapped--it seems to have red-green-red wires, so you might find it measures 0, 6, and 12 v for instance, or some multiple. If this happens, it might mean that green is taken as zero, and you have +6v and -6v (or +12 and -12, etc.), which, after rectification to DC would power a dual power-supply amplifier (I believe better amplifiers are dual supply).

This would make it a bit more complicated for battery power, but is doable. But there is another problem: amplifiers can take a lot of power. To supply such power with batteries could be costly. Check the wattage of you amp. I'm sure somewhere on the Internet there is info on battery power output.
7 years ago
You're right, I answered the question without thinking about how the q/a fit the scheme.

It's more important to know what the voltage the powersupply is supplying to the amp, then match that.
framistan7 years ago
From the looks of your amp, i am guessing your amp is TRANSISTOR not tubes.. which is good for portable use you desire.  I suspect you have a DUAL power supply because of the 2 large capacitors i see (the blue large roundish things in picture)..  A dual supply might supply plus volts on one and minus volts on the other.  If its DUAL supply,  you would need 2 batteries for portabilty...and if volts is higher than 12 volts, then bateries must be added to equal that voltage.... problems problems...   So... i am thinking a simpler fix would be to NOT MODIFY your amplifier.  INSTEAD, just get a 12 volt INVERTER that is large enough wattage to plug the amp into.  If your amplifier uses 100 watts of AC  then a 200 or 300 watt inverter should do nicely (as an example).  Now the inverter feeds your amp what it wants... and the amp produces dual voltages without modification. This is the easiest/ cheapest/ least likely to damage your amplifier solution to the problem.
Re-design7 years ago
A multimeter set on a/c is perfect for measuring the output of the transformer.

1.  Make sure you're on the secondary side and not the line side.

2.  Make sure your meter is set higher than you think the output might be.

3.  If it's a tube set don't even try.

4.  There is a slight chance that the output might be d/c.  I found a transformer once that had the bridge built in under the metal end plates.
stoney123 (author) 7 years ago
Oops, here's the images I forgot to upload when posting the question.