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# Transformer current rating question.

Hi all,

A while back I built a Morgan Jones tube amplifier for my electronics lab's final project.  It worked beautifully until my transformer burnt out.  I've been looking at transformers and I have a questions about how they are rated for power.

I need a 500V CT transformer with a 6.3V heater tap.  My question is what do the current ratings mean for these voltages?  Are they the maximum current that the transformer can run at or the current that the transformer should be running at?  I only ask this because transformers are expensive and I don't want to burn another transformer out.  My hunch is that the current ratings denotes the maximum allowed current, but I wanted to be sure.

Best regards,

Keabler

Transformers are usually advertised with a voltage rating and a VA (power) rating. A current rating is not usually given, but it can be quickly calculated from the power rating divided by the voltage rating.

The voltage rating is the RMS voltage that will be measured across the secondary winding under full load.

The VA rating is the average power that that can be safely demanded from the transformer, and if only one figure is quoted then it is for the whole transformer.  If there are several secondary windings then the total core power is divided among them.

The power rating is given in volt-amps, rather than watts, because it takes into account the fact that the load current might not be in phase with the secondary voltage. This is usually the case when a rectifier is used, so the power that the transformer actually has to handle (in VA) is more than the actual power the circuit consumes (in watts), so the transformer VA rating will need to be greater than the circuit power for safe operation.

Excerpt From: Blencowe, Merlin. “Designing Power Supplies for Tube Amplifiers.” Lulu.com, 2011-05-25.  He explains it better than I could.

Thanks for your reply.  I'm familiar with VA ratings.  I thought that they rated the maximum output of a transformer, but I wanted to be sure.  My previous transformer should have been able to handle the power that my circuit needed but it burnt out so I wanted to make sure that it was the transformer and not my understanding of the circuit.

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