# How to tell VA rating of xmfr?

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To All, I need to power an amp that draws just under 1.5 amps at 110 volts. I measured the current it draws when powered from my variac. The voltage to the amp should be within 5% according to the documentation. The voltage in my house measures 122 volts to 126 volts, varying during the day. So bucking 12 volts should be fine. I am looking at a 120 volt to 12 volt center tapped 5 amp rated xmfr at PartsExpress. The VA of the xmfr is not listed in the description. I want to connect this xmfr in buck configuration. Since I will be bucking 12 volts at approx 1.5 amps this equals 18 VA. At least that's how I remember how to calculate VA from connecting big buck xmfrs to one of my machines several years ago. I think the 12 volts at 5 amps secondary means that the xmfr is 60 VA. But I'm not sure because it is center tapped. It shouldn't make any difference, right? If I used the center tap and just bucked 6 volts then the VA rating would be 30, wouldn't it? Thanks, Eric

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The only thing that matters is the amp rating of the secondary. Equal or greater than the load.

```--
"I am a river to my people."
Jeff-1.0 ```
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** You need to buck 1.5 amps and you have a tranny rated to do 5 amps.

Whether you use the whole or only half the 12V secondary is up to you.

.... Phil

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If all the 110 volts does is make DC why can't you just clamp the DC?

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What if it's a tube amp?

It's still easier to add a buck transformer than add a regulator circuit (not a great idea to "clamp" the DC).

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I didn't think a tube amp would be so fussy about the supply voltage.

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They are probably more fussy about the supply voltages. The filiments of the tubes burn out faster if the voltage is too high.

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That's why most refurb guys put zeners across the filaments.

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surely that would overheat the transformer if they go into conduction. Also it does not prevent Vf rising too far, only reduces the rms rise.

NT

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Well its been done for quite some time. I would like to know the name of this product that specifies this voltage.

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uit (not a great idea to "clamp" the DC).

Zeners are a neat trick but I never liked using them in high current situat ions where a failure causes collateral damage. Consumer electronic designe rs fell in love with them in the 70s and 80s where they caused all kinds of mischief. I'll use them as high impedance voltage reference where they han dle no appreciable current and I've also used them as crowbars following a fuse or low value resistor as a safety.

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Thanks Jeff. Your answer means I can use the lower amperage xmfr I also looked at. It has a 3 amp rated secondary. Eric

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It's a tube amp. Seems easier to just feed it the correct voltage. Eric

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You got it John. I should have mentioned that it was a tube amp, I gues. Eric

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Yeah, that's apparently correct, from everything I have read and watched about tubes. Eric

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Obviously, what I should have included is adding a margin of safety. A 3 amp secondary used to buck a 1.5 amp load is just fine.

```--
"I am a river to my people."
Jeff-1.0 ```
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I'm just curious as to what product has that spec. 110 VAC on an old amplifier means line voltage in North America. The 5% is odd to me.

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Nobsound. Who knows why the spec but I don't want to burn up expensive tubes prematurely. Eric

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Do you have a model number?

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The US power grid is a strange and wondrous thing!

The last major building (NYC) still on the original Edison DC grid went ove r to AC in 2007.

Coffin-box AC radios from the 1920s often had a dual-voltage switch inside from 110 VAC to 120 VAC. Back when tubes were a week's salary for the avera ge worker, and were direct-heat triodes, voltages were critical.

Most of US power is still distributed via aerial power lines. The majority of the grid in the Philadelphia, PA area (PMJ) was wired between 1913 and a bout 1919, and extended as development took place.

As power usage increased over time (our house was built in 1890, first wire d in 1913, upgraded in 1928, and very nearly entirely rewired (including th ose pesky ground wires) in 2006), the power companies had two choices - inc rease the size of the conductors, leaving the base voltage the same *OR* in crease the voltage so as to get more current delivered, and upgrade the tra nsformers as needed. And, as that happened several times, the latter option became the most cost-effective way to get to the necessary end.

And, over time the tariffs were revised (upward) to accommodate large volta ge swings.

Most (US-Origin) tube equipment extant - that of vintage variety in any cas e - is/was designed against the old ways - so about 120 VAC is the highest comfortable voltage to it - and at the same time, 110 VAC will not starve i t. At more than that, filaments wear out exponentially more quickly, power- transformers will overheat and more at higher voltages. Zened diodes simply do not cut it for transformer-based equipment as when/if they start to con duct, the load on the power transformer can spike. Fine on a transformerle ss AA-5 or TransOceanic or similar, but not on a vintage Scott, Dynaco or F isher amp.

So, bucking is the way to go if one does not wish to invest in a true-Sin-W ave line-conditioner. And, a safety margin (together with the correct fuse) is the proper way to fly.

Peter Wieck Melrose Park, PA

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