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Old 29th April 2009, 06:34 PM   #1
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Default Output transformer current and power ratings

A couple of questions about output transformer current and power ratings:

In attempting to determine an appropriate OT for a project, I've noticed that power and current ratings for OT's vary quite a bit between mfrs, for instance for a 5K PP OT:

James 6235 is rated 100ma/30W

Hammond 1615 is rated 100ma/15W (the hammond datasheet says "per side")

So they both have the same current rating but one has twice the power rating of the other........Is one rated per tube and the other per pair of tubes?

What determines the max power rating, flat FR or max current X voltage? Can one handle higher B+ voltages than the the other?

The current and power ratings are obviously related, but I'm not quite sure exactly how.......
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Old 29th April 2009, 08:16 PM   #2
pointy is offline pointy  United Kingdom
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i would think that per side means each side of the centre tap.
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Old 29th April 2009, 08:31 PM   #3
kuroguy is offline kuroguy  United States
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transformers have a maximum current rating as well as a maximum voltage rating. I believe multiplying them together gives the maximum power rating.
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Old 29th April 2009, 09:00 PM   #4
rknize is offline rknize  United States
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That current rating is describing that about of DC current the transformer is designed to handle for tube bias. The reason this is important has less to do with the windings and more to do with the core. The goal of the transformer is to pass AC signals from the primary to the secondary and it does this by converting electrical current into magnetic flux and back again. The transformer core can only handle so much flux before is saturates. Manufacturers install an air gap in the core to prevent high DC currents from saturating the core before any useful AC signal is passed.

This is important with SE amps because all of the plate current of the tube is going through the transformer. PP amps also handle bias current, but it goes from the center tap to each end of the primary in opposite directions. The result is that no flux flows as a result of this as long as the tubes are balanced.

The power rating is referring to the amount of signal in the AC domain that can pass through the transformer within a certain frequency range. It has nothing to do with the DC power it can handle through a winding, which is generally not a useful piece of information.

EDIT: I just noticed you are talking about a PP transformer. In this case we are still talking about the amount of bias current the primary is rated to pass, but now it has less to do with core flux and more to do with the thickness of the primary wire.
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