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

[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!

[anchovie]

I was planning to incorporate a 7812 regulator into the circuit...

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

That won't work - a regulator needs the input voltage to be higher than the regulated output voltage. You'll have to check the datasheet for whatever part you're thinking of using to see what the acceptable input voltage range is. For a 7812, your adapter would need to put out at least 14.5V.

[davent]

I was planning to incorporate a 7812 regulator into the circuit...

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

That won't work - a regulator needs the input voltage to be higher than the regulated output voltage. You'll have to check the datasheet for whatever part you're thinking of using to see what the acceptable input voltage range is. For a 7812, your adapter would need to put out at least 14.5V.

... unless he gets a 12vac  wallwart then he'd be ok.

dave

[MikeH]

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.

Thanks!

I use the same rating adapter for my valve caster- works fine

[PRR]

> incorporate a 7812 regulator

As said, won't work, have to start several volts higher.

Anyway throwing Silicon at a tube amp dilutes the karma.

> to prevent noise.

Hum/buzz on the heater is fairly unimportant. (You can heat with raw AC if you wire carefully.)

Hum/buzz on the plate supply rail matters; but that current is surely so small (maybe 1 mA) that you can use an R-C filter (try 330 ohms and 470uFd).

> expect the output voltage to be higher

It's an issue but IMHO not urgent. The 12AU7 is rated for 12.6V, works nearly the same on 12.0V to 13.2V, works good-enough down close to 11V and up past 14V. Yes, "12.6V" tubes will live many-many-many hours at 15V heat. "Instant death" is several minutes at DOUBLE the rated voltage.

Build it. Check the heater voltage. If it is near 14V, add some 10 ohm resistors in series with the heater to get it down. If the B+ is nominal 12V, don't starve the heater. Low-volt working may be a bit better with the "12.6V" heater run a little hot.

Heater life on 12AU7 types is much-much greater than 10,000 hours. From my experience, over 50,000 hours (small tubes die of other things sooner than the heater burns up). I had a 12AU7 with 100,000 hours on it, and when I put a new tube in (for no good reason) performance was unchanged.

Evaporation of Tungsten goes as the 13th power(!) of voltage. So 15V instead of 12.6V gives 1/10th the life. Instead of 50,000+ hours you may expect 5,000 hours. At 10 hours a week this is almost a decade. You can afford $17 every 10 years, or you can add series resistors next month. 15V isn't a real urgent problem.

Oh, and 15V or so plate supply will probably improve a "12V tube amp". Every little Volt helps when you have only a dozen or so Volts to pull electrons through hard vacuum.

[R.G.]

Evaporation of Tungsten goes as the 13th power(!) of voltage.

You're only the second person I've ever met that has that fact at their fingertips. 

[bean]

The One Spot does 9v @ 1700ma. Not what you are asking, of course, but I just happen to be looking into some PS yesterday and ran across that. I'm looking for a 24v @ 1200+ma for a pedal power supply I want to build, and there seem to be a few.

[KazooMan]

For the OP:

Why not build your own well-filtered, regulated power supply for the voltage you need?  Modify the description at the GeoFX website to suit your needs.  Just (1) use a transformer with an appropriate output power rating and a voltage a few volts higher than you need (and less than the max rating of the regulator), (2) be certain that your filter caps have a high enough voltage rating, and (3) adjust the values of the resistors on the voltage regulator to get the voltage you need.  You may want to include a heat sink for the voltage regulator.  

There are several calculators that you can use to select the resistor values.  Here is one:

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

You can substitute an appropriate trim pot for one of the resistors to fine tune the voltage.  

I recently put together a 500mA 12 V power supply for the Vibratone that I built.  It works great, although I am embarrassed to say that since I was in a bit of a hurry for parts this is an ALL Radio (Rat) Shack build.  

[PRR]

> build your own well-filtered, regulated power supply

Sure... but why?

Tubes work fine without regulation.

The heater is just a dim incandescent-lamp or tiny electric heater. You don't use a regulator on your incandescent lamps?

The plate voltage has a maximum, maybe 350V at the plate (more at the B+), and in any specific application there is a minimum to give the desired result, maybe a hundred volts. Very low voltage work (like 12V operation) is clearly very low output (gitar-level), but the exact voltage is not critical.

What I would do in the old days:

12VAC transformer, FWB, 1,000uFd, to give 17V DC.

2-stage R-C-R-C filter for plate voltage. For low loss I might use 1K resistors. The caps must be over 10uFd, but 17V caps are cheap, grab 100uFd or more. Result is very clean DC which may vary 13V to 16V with load and wall-voltage. It's all good.

Heater may as well be very close to 12.6V. The raw DC is about 4V too high. The heater load is 0.15A. 4V/0.15A= 26.667 ohms 0.6W, use 27 ohms 2W part. That's the right DC voltage but has raw ripple. If the heater hot lead runs near audio (usually does), use the R against a C to cut the ripple. For 27 ohms we need hundreds of uFd to get clean. Since there's a 1,000uFd already, use another 1,000uFd.

Power-up, check heater voltage, is way-off then trim the 27 resistor.

That's old-skool. Today I would dig in my orphan-wart box for some 12VDC wart that looked sturdy and stable (not the $0.99 junk you get with cheap toys) with 12V at more than 0.15A. Maybe not the 12V 20A supply from the old disk-array: an accident with that would weld my ring to my fingerbone! But a 1A wart is fine.

[KazooMan]

I would build a proper power supply so that I could use it with ANY pedal, not just the valvecaster.

[PRR]

Evaporation of Tungsten goes as the 13th power(!) of voltage.

You're only the second person I've ever met that has that fact at their fingertips. 

Reduction in lamp life is a function of approximately the 13th power of the overvoltage (Reference 1). 

Reference - 1. Fink, Donald and Christiansen, Donald, Electronic Engineers’ Handbook, 1975, McGraw-Hill, pg 11-6.

https://www.amazon.com/Electronics-engineers-handbook-Donald-1975-08-01/dp/B01K17AZX8
https://www.abebooks.com/servlet/BookDetailsPL?bi=31080393052&searchurl=an%3Dfink

https://www.edn.com/wp-content/uploads/EDN_Design_Ideas_2003_.pdf
PDF page 107

[Ben N]

It took 12 years to look that up, Paul?!?