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Old 19th February 2012, 08:06 AM   #1
deuce42 is offline deuce42  Australia
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Default Minimum rating for stepdown transformer

Hi I am wondering if someone could assist with my confusion regarding stepdown power transformer minimums.

I have a 45watt output guitar amplifer (Fender Super Reverb). It runs on 117VAC I believe, however my wall socket in Australia puts out 240VAC. I have a stepdown transformer which says rated at 250watts. Am I am correct in thinking this stepdown wouldn't be rated high enough? - since I would need 240volts x amperage = watts. I am not sure of hte amperage of the amplifier but am still thinking a 250watt stepdown wouldn't get enough.

Can someone assist me understanding how this works?

I am sorry its such a novice question but appreciate any help offered.

Thank you

Last edited by deuce42; 19th February 2012 at 08:51 AM.
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Old 19th February 2012, 10:05 AM   #2
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The way I look at it... if the circuit input takes 250 watts from an AC source and delivers 45 watts output to an AC load, it only needs to be 18% efficient to do so. So is the stepdown transformer + Fender amp circuit >18% efficient?
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Old 19th February 2012, 10:51 AM   #3
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There are a number of ways that you can assess the power consumption of a valve amp.

1. Borrow a 240/115V stepdown transformer from your local Tool Hire shop and measure how much current the amp uses.

These will normally be rated at 1KW or possibly more.

2. Make a note of ALL the valves that are in the amp. Then, using the datasheets you can calculate the maximum current that could theoretically be pulled by each one, both heater and anode currents.

If you know the HT voltage and Heater voltages of the amp you can apply Ohms Law and work out the total power of the amp.

This wont be accurate but will give you some idea of what you require.

Best method is method 1.
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Old 20th February 2012, 12:54 AM   #4
dgta is offline dgta  United States
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If the amp was a commercial product, the plate on it should give both voltage and power (watts) or current (amps) consumed. Go from there. If the plate is missing, the spec should be in the owner's manual.

The 45W audio output CANNOT be reliably used to deduce anything about the power consumption at the mains.

If it's a DIY amp you need to measure the power consumption.
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Old 20th February 2012, 02:39 AM   #5
kevinkr is offline kevinkr  United States
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I'd consider a 500Va to 1kVa step down transformer a reasonable choice based on experience trying to run a 117V stereo system on 230V mains in Europe as a teenager. The total amplifier power consumption was about 200Va and there were a couple of other pieces of gear as well, the higher rated transformer resulted in better overall regulation during operation than a smaller transformer I tried.

Find out what the maximum power consumption is, and note that on 50Hz mains the losses in the power transformer are likely to be somewhat higher than on 60Hz. Most people make the mistake of under sizing the auto-transformer rather than the reverse which actually has no negative performance consequences although it will be heavier and more expensive. (The only negative consequence is what it does to your wallet.. )
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Old 20th February 2012, 02:49 AM   #6
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From the "plate" on a 67'....117VAC@1.5A.A 2.0 Amp Slo-blow fuse. That makes it 175.5 Watts.
Your 250 Watt Step-down will hold it.....A little bit on the edge at 70% . Your load should be 50% to 66% of step-down rating.

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Old 20th February 2012, 02:51 AM   #7
deuce42 is offline deuce42  Australia
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Thank you for your responses. I appreciate it guys!
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