#1
Hello everyone,

I have a full tube amp that I bought in Germany and now I'd like to use it in the US. It's 220V, so I need an step-up transformer to use it here. Currently I'm using a 750VA 110-to-220V transformer, but I feel it's really overkill (not to mention OVERSIZED )

I don't know how many Watts its power source drains, but I used a meter to find out how many Amperes it uses. I connected it to a 220V outlet and found out that it's using 0.15A of current.

The thing is, I'm not sure if 220*0.15 converts to 33W. I know that there's this thing called power factor that I should take into consideration when calculating power in an alternated current circuit, but I'm not sure what it should be for a tube amp.

So, is a 50W step-up transformer enough?

Cheers!
#5
No, 50W is probably not enough. My 15 watt amp draws up to 65 watts and I use a 300 watt step-up transformer, which is reasonably sized.
Most amps list the maximum power consumption in watts on the back, often near the power cord.
The transformer I use is a 7-star TC300, it's 6"x4"x3.5" - plenty small IMO.
#7
Most amps list the maximum power consumption in watts on the back, often near the power cord.

Yeah, I tried looking for it. Not a single note. It also didn't come with any manual or anything...

It's also pretty hard to find information on it online
Last edited by strangedata at Oct 13, 2009,
#8
Remeber the equation PIE. P=IxE. P=Watts(power) I=Amps E=volts.

That's how I came up with the 33W figure... but that's DC.

I dont know if that applies to AC current but I would think it would.

I think it needs a power factor for this equation to work for AC. I know that a computer power source should use a 90% power factor and rotating motors should use a 60% power factor.

The problem is that I have no idea what the power factor for a tube amp is :-(
#9
I'd just get a 150 or 200 watt one and be done with it. Power factors vary wildly for tube amps, but they're very inefficient. I wouldn't go any lower than 100 watts on the transformer.
#10
I'd just get a 150 or 200 watt one and be done with it. Power factors vary wildly for tube amps, but they're very inefficient.


According to some equations I've found online, for the voltage and current I've measured (220V and 0.15A) the power factor for 200W would need to be around 16%.

Is that right? Is this tube amp even more inefficient than a rotating motor (power factor around 35%)?

Cheers!
#11
Remember, you're converting electricity into sound, and tubes are very inefficient (think of how hot they get - that heat is all wasted energy). I'd put efficiency for tube amps around 25%, though again it varies quite a lot. Also, those engines you're referencing to are designed to be efficient, whereas Marshall doesn't really have much reason to make their amps draw fewer watts.

200 watts is more than your amp will draw, but I'm just suggesting getting a beefier transformer because cheaping out on a smaller one could end up damaging the transformer and your amp, and the price difference between a 100 and a 200 watt transformer is not all that great.
#12
You need to worry about more than power factor. You have a 15 watt amp, but how is that rated? 15 watts clean? If so then that figure could go way up distorted. Also you have inefficiencies in the transformers and components. Tube amps get hot and heat loss is power. All you have to do is touch the chassis and you can tell it's pretty inefficient in turning power into sound.

edit: haha you beat me Roc. Also power factor is not efficiency. It's the phase angle between the voltage and the current. It's probably the least important consideration when trying to figure out how much power you need.
Last edited by fly135 at Oct 13, 2009,
#13
I'm learning a lot on this thread! But also getting tons of new questions :-D

You need to worry about more than power factor. You have a 15 watt amp, but how is that rated? 15 watts clean? If so then that figure could go way up distorted.

The amp doesn't have separated channels, so the test I did was playing for a few hours and "occasionally" crank it up all the way to 10.

The current draw never got past 0.15A. That looks really small, doesn't it?

Also power factor is not efficiency. It's the phase angle between the voltage and the current. It's probably the least important consideration when trying to figure out how much power you need.

But then how can I go from 33W (that I would get in DC P=IE) to around 200W, if I didn't take power factor into consideration?

Thanks!
#14
Oh dear, you need to stop thinking about this so much.
Power factor is not something that ever ever ever comes up in amp design and is not something you need to deal with. Suffice to say that your amp is probably 25% efficient, and that you most likely don't want to cut it too close with your transformer, since it's not a great idea to run it full bore all the time.

What we're trying to tell you is that you don't need to take "power factor" into consideration. Just multiply volts times amps and give yourself a healthy buffer since amps are quirky.

You can get a 50W transformer, but I'd really suggest 100 for safety purposes.
#15
Quote by strangedata
The current draw never got past 0.15A. That looks really small, doesn't it?


But then how can I go from 33W (that I would get in DC P=IE) to around 200W, if I didn't take power factor into consideration?

Thanks!
OK, you're saying that the current never went past 0.15A when dimed. Based on that you could probably get away with a 50 watt supply. But as Roc says a 100 would be a safer bet. OTOH if you don't normally play with the amp dimed and the next upgrade in size on the xformer is expensive then 50 watts might be the right choice.

Edit: Physically the convertor xformer shouldn't need to be that much larger than the power transformer in the amp. If the 50 watt xformer is noticeably larger and heavier than the one in the amp, then I would feel comfortable using it. If it's smaller and lighter then I wouldn't.
Last edited by fly135 at Oct 13, 2009,