12v DC Adapter Current - How much is too much??

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nomorebetts:
Hi All,

I'm planning on building a Valvecaster and after reading different posts have decided to run it at 12v.
The question I have is in regards to the amount of current my adapter provides.  It's rated at 12v 1000ma.

I know that the 12AU7 tubes when in Series draw 12.6v @ 150ma and in Parallel: 6.3v @ 300ma.

Will I destroy anything if I plug in 12v 1000ma??  What would be an ideal current for the adapter?

Thanks!

PRR:
The current rating on a power supply is usually "up to" that current.

You can pull less current. This is normally fine. Just do it.
My house has 100 Ampere service wiring.

When I plug in one 120 Watt 120V lamp it sucks 1 Ampere.

I could plug in up to one hundred 120 Watt 120V lamps to suck  100 Amperes, the most my house wiring is allowed to pass.

The main "problem" with excess current rating is that you paid too much. If I only ever ran one lamp, I could have used much smaller 1 Amp service wires, maybe 2A just for some margin. However I run a lot more than one lamp! And my power company knows that. So they won't connect a house for less than 60 Amps, and 60A service is the same cost as 100A service. Likewise a 12V 0.5A adapter may not be any cheaper than the 12V 1A adapter in your hand.

> draw 12.6v @ 150ma

That's when running. At cold-start it will pull about 3 times as much for less than a second. The old battery and transformer supplies did not mind this momentary jolt. Some modern power supplies have no reserve on the current rating, not even for an instant. So you may really need about 0.45A rating. Better round-up to 0.5A to be sure. And if a 1A is handy and affordable, that's an excellent choice.

darron:
one small other thing to consider is that the power supply should give out the correct (stated) voltage on the secondary when you are drawing that much current. at least that is what i believe. for example of you are using a supply 12V @ 1000mA and you are only drawing 100mA, then expect the output voltage to be higher than f you were drawing the full 1000.

If you can use it to power a whole pedal board then 1000mA would be a very nice supply letting you load it down with lots of pedals.

composition4:
I think another main point to consider here, and Darron hinted at it, is that not all PSUs are regulated.  Switchmode PSUs are, but everything else may or may not be.

If unregulated, your power supply might be 12V when drawing 1000mA, but if drawing say 100mA it might be at 18V.  Depending on the circuit's tolerance to overvoltage this can be bad news.

If the power supply is regulated, you should be able to draw up to 1000mA safely as PRR said.

You can easily check this with a dummy load at 100mA, and if the voltage is much higher than 12V, it's unregulated.

Jonathan

nomorebetts:
Thanks for all the info guys!  It's been a great help  :)

I was planning to incorporate a 7812 regulator into the circuit as other people have suggested this to prevent noise.

So I'll go ahead with using the 12v 1000mA adapter and use a regulator.

Thanks again!

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