Bucking transformer rating question

[Randall]

Would this 12.5v 100VA transformer be robust enough to handle two tube amps in a HiFi configuration, both fused at 2 amps? No standby switches on the amps, so slow inrush current? It's going to be for the Bell Labs amps I have in another thread here.

FD8-12 Triad Magnetics | Mouser

[Mick Bailey]

Yes. As only the secondary carries the load current you have more than enough current capacity. Remember the VA rating of a bucking transformer is far greater than the nominal VA rating because we're only really interested in the secondary. Depending on your mains voltage you're looking closer to 860VA for this transformer in bucking mode.

[Tom Phillips]

Yes. As only the secondary carries the load current you have more than enough current capacity. Remember the VA rating of a bucking transformer is far greater than the nominal VA rating because we're only really interested in the secondary. Depending on your mains voltage you're looking closer to 860VA for this transformer in bucking mode.

Mick - I get what you are saying but for others reading this they really need to understand the explanation given in your first two sentences. For those who do not understand be aware that the VA rating of this transformer did not actually change. It is still a 100VA transformer.

Randall - What this means for your application is that you can get by with a much smaller bucking transformer. This gives you the opportunity to mount a small bucker inside the chassis of each amp as I suggested in the other thread. That would be a much cleaner, fail safe setup.

[J M Fahey]

Just to introduce some Math (sorry ) : 100VA/12.5V=8A current capability.

If your amps are fused at 2A each, possible total consumption=4A , we are comfortably within rating.

[Mick Bailey]

That's how I arrived at 860VA. As per Tom's observation, the actual transformer VA doesn't change but because the secondary current rating is really what we're interested in the effective VA rating increases. Or should I say the load rating. This is the supply voltage minus buck voltage x secondary winding current rating. So for a 120v supply and 12.5v secondary voltage we get 120v-12.5v x 8A = 860VA.

One of the issues with a bucking transformer is the load regulation can be quite poor if the transformer is too small. A larger transformer (one whose secondary current rating is much larger than the full load current) will give more consistent voltage output. The voltage reduction can be higher than expected with a larger transformer though, as a consequence of having a relatively lightly loaded secondary.