How to tell VA rating of xmfr?

[et...@whidbey.com]

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

[Fox's Mercantile]

On 12/1/18 2:47 PM, et...@whidbey.com wrote:
> I am looking at a 120 volt to 12 volt center tapped 5 amp rated
> xmfr at PartsExpress.

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
WA6FWi
http:foxsmercantile.com

[Phil Allison]

** 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

[Tom Biasi]

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

[John-Del]

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).

[Tom Biasi]

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

[Ralph Mowery]

In article <ptvkht$17j5$1...@gioia.aioe.org>, tomb...@optonline.net
says...

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

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

[Tom Biasi]

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

[tabb...@gmail.com]

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

[Tom Biasi]

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

[John-Del]

Zeners are a neat trick but I never liked using them in high current situations where a failure causes collateral damage. Consumer electronic designers 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 handle no appreciable current and I've also used them as crowbars following a fuse or low value resistor as a safety.

[et...@whidbey.com]

On Sat, 1 Dec 2018 17:36:04 -0600, Fox's Mercantile <jda...@att.net>
wrote:

>On 12/1/18 2:47 PM, et...@whidbey.com wrote:
>> I am looking at a 120 volt to 12 volt center tapped 5 amp rated
>> xmfr at PartsExpress.
>
>The only thing that matters is the amp rating of the secondary.
>Equal or greater than the load.

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

[et...@whidbey.com]

On Sat, 1 Dec 2018 20:45:41 -0500, Tom Biasi <tomb...@optonline.net>
wrote:

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

[et...@whidbey.com]

On Sat, 1 Dec 2018 19:27:11 -0800 (PST), John-Del <ohg...@gmail.com>
wrote:

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

[et...@whidbey.com]

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

[Fox's Mercantile]

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.

[Tom Biasi]

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.

[et...@whidbey.com]

On Sun, 2 Dec 2018 19:38:33 -0500, Tom Biasi <tomb...@optonline.net>

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

[Tom Biasi]

Do you have a model number?

[peterw...@gmail.com]

On Sunday, December 2, 2018 at 7:38:39 PM UTC-5, Tom Biasi wrote:

> 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.

The US power grid is a strange and wondrous thing!

The last major building (NYC) still on the original Edison DC grid went over 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 average 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 about 1919, and extended as development took place.

As power usage increased over time (our house was built in 1890, first wired in 1913, upgraded in 1928, and very nearly entirely rewired (including those pesky ground wires) in 2006), the power companies had two choices - increase the size of the conductors, leaving the base voltage the same *OR* increase the voltage so as to get more current delivered, and upgrade the transformers 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 voltage swings.

Most (US-Origin) tube equipment extant - that of vintage variety in any case - 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 it. 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 conduct, the load on the power transformer can spike. Fine on a transformerless AA-5 or TransOceanic or similar, but not on a vintage Scott, Dynaco or Fisher amp.

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

Peter Wieck
Melrose Park, PA