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Old 9th January 2016, 06:19 AM   #1
cl1238 is offline cl1238  Australia
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Default 220V Transformer question

Hi, I bought a Tube Amp kit which come with a power transformer which primary is 2 x 110V.
Normally, I would consider 220V is good enough to plug into the 240V GPO. However, I think this would affect the output due to winding ratio.
There are various opinion regarding this issue (such as hum, transformer degrading and cause component failure), there are also mention of Bucking Transformer to step down the 240V to around 220V.
My question is it worth while to create a step down based on Rod Elliott's Bucking Xfmrs
Anyone know how to calculate the VA rating for this bucking transformer?
I can only get 240V primary with the following secondary:
a) 2 x 9V 20VA
b) 12 - 18V 18VA (multitab - which I can use the 18V)
c) 24V 72VA
d) 9 - 24V 60VA (multitab - which I can use the 18V)
e) 6 - 15V 30VA
Thanks
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Old 9th January 2016, 06:29 AM   #2
tomchr is offline tomchr  Canada
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Since you're bucking the primary voltage, you need to select a bucking transformer which can handle the expected primary current (plus margin).

Tom
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Old 9th January 2016, 07:09 AM   #3
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Are you quite certain that there's an issue? If it's a modern kit that you just bought, it might be OK as is. I would check the secondary voltages as a first step. Do you have the know how to put a power resistor across the heater winding to measure it's voltage? Just picking whatever power resistor you have on hand to put a 0.5A to 1.0A load (assuming we are talking tube power amp here) on the heater secondary and measure it's voltage.
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Old 9th January 2016, 07:26 AM   #4
Koonw is offline Koonw  Malaysia
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If (c) 24V 72VA is used, I=72/24 or 3A, resulting voltage is 216 (240-24)V, VI=216* 3=576VA is total you can safety draw.

Normally you can connect 220V to 240V supply during the day when 240V can drop to below 220V but at midnight the voltage can rise to 265V or so, that is bad news.

Last edited by Koonw; 9th January 2016 at 07:31 AM.
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Old 9th January 2016, 08:20 AM   #5
AJT is offline AJT  Philippines
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with tube amps it is easier to drop some volts at the secondary, it has never been a problem for me, so i am not sure one even needs to do something about the primary voltage...

unless of course when running 240 volts, that power traffo gets uncomfortably hot...
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Old 9th January 2016, 08:23 AM   #6
cl1238 is offline cl1238  Australia
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Thanks everyone. I'll check out the secondary without step down to see the voltage difference first, if there's problem, then I'll use the buck transformer method to reduce the voltage. I can safely assume all I need is to make sure the secondary of the buck transformer must match (as a minimum) the current of the power transformer primary current.
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Old 9th January 2016, 08:27 AM   #7
AJT is offline AJT  Philippines
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i suggest that you build it without any modification first...
then when everything is wired up, powered up, then you can take voltage readings...
tubes are able to operate over a wide range of voltages within reason...
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Old 9th January 2016, 11:44 AM   #8
Funker is offline Funker  Germany
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Hi,
a rough calculation for bucking/increasing the mains voltage from 240 to 220 or vice versa is that your bucking trafo must capable to handle 10% of the rated power of the load.
I had the problem years ago before we turned to 230V. A live music pub here had loads of bands from Britain gigging. They got their own eqipment from England. So I installed a 24V/25A Trafo behind the stage and step the 220V up to 240V. To avoid issues with local equipment I put a original british 2.gang socket on the wall. It works perfect. The guys were amused!

73
Wolfgang
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Old 9th January 2016, 12:02 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by cl1238 View Post
................. to step down the 240V to around 220V.
My question is it worth while to create a step down based on Rod Elliott's Bucking Xfmrs
Anyone know how to calculate the VA rating for this bucking transformer?.................
ESP autoformers/bucking transformers.

Do NOT copy fig3.

Go to fig4 and wire up the step down autoformer correctly as shown in the right hand half.

220:240 & 240:220 are roughly a 10% adjustment of the voltage.
The secondary current of a 230:10+10Vac or 230:12+12Vac will be roughly 10times the primary current.

It's the secondary current that determines the VA of the autoformer.

eg. a 230:10+10 230VA 7% regulation transformer will have a winding ratio of 230: (10*1.07) + (10*1.07) for the three windings.
On open circuit the output voltage will be 10.7Vac when the primary voltage is 230Vac. Note that esp shows a winding ratio that does not agree with my explanation.

The current rating of the secondary will be 230VA/(10+10) = 11.5Aac
Wire this up as fig4 right hand side and you can input 250Vac into the three series connected windings and the you have taps at 250 & 240 and 230Vac
If you use the two lower taps the output current must be less than the secondary rating i.e. <11.5Aac.
Using the top tap (250Vac input and 250Vac output), you can draw whatever current your mains wiring/fuses will allow.

The effective VA is 230* 11.5A = 2645VA
Yes, that small 230VA autoformer has an enormous effective VA
And you don't have to input exactly 250Vac to the input tapping. You can use 240Vac or 234Vac or 216Vac.
Using a lower input voltage just results in a lower output voltage. And using a dual secondary allows for three output taps, making it very flexible for a variety of uses.

The smaller the step up/down ratio, the higher the effective VA.

But beware a 115:230 or 230:115. The step up/down ratio is 2:1
The effective VA is only double the donor transformer.
i.e that 230:115Vac 230VA transformer has an effective 460VA when using the 2:1 or 1:2 ratio.

And use a Mains Bulb Tester to ensure no damage, while you try to get the taps in phase.
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Last edited by AndrewT; 9th January 2016 at 12:12 PM.
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Old 9th January 2016, 12:06 PM   #10
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If you are within EU, the mains voltage is approximately 230V and should not pose a problem to be 5% over voltage.
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