Have doubts on transformer rating

[exztinct01]

I just bought a transformer made here in the Philippines. It's 220vac to 18-0-18vac and rated at 2 amperes. (i don't know why they don't use va for transformers rating here)
I'll be using it for a power amp (Rod Elliott's Project 72) which asks for at least 50VA. Problem is I don't trust its rating since it's on the small side. Photos below shows the laminations' dimensions. Can anyone confirm if it really is 2 amperes (72VA for 18-0-18)?

[J M Fahey]

No way.
Rough power calculation is: core width (in cm) * core thickness (in cm) which gives you core area in square centimeters.
Here 2.2*2.4=5.28 sq cm

Then you square core area and you have basic VA.
5.28*5.28=28 VA.

Core center leg widths in Philippines must be same as here: expressed in mm but based on some old "inch" one, so typical core widths are 19mm , 22mm , 26mm , 29mm , 33mm , 38mm , which jump from size to size by a certgain "inch" step (1/8" or 1/4").

Next larger is 26 mm (full E width is always 3x center leg width as you already found) and smallest stacking is "square" stacking, meaning it's equal to center leg width.

So next size core : 26mm center leg, will allow, if "square":
2.6*2.6=6.76 cm^2
VA=46VA ... still short

if stacked 32mm:
2.6*3.2=8.32 cm^2
Power: 69VA ... now we are talking.

Now to actual needs:
* for a single LM1875, listening to music (CD/MP3/etc.) the one shown is ample.

* for a single LM1875,driven to clipping as is n ormal with a guitar, the transformer shown *will*  still "work" , don't think it will actually burn but may work hot, and rail voltages will drop , so you won't get full 25W but more like 18W or so.
As Enzo says, "the amplifier is the thingie between the power supply and the speaker" , it will never deliver what it does not get in the first place.

* the 26*26mm core one (full E width 3*26=78mm , 26mm stacking) will be perfect, and matches Rodd Elliott's spec.

* the larger 69VA one is overkill, but of course does not hurt at all if you get it for a good price.
And later you'll be able to add a second LM1875 and have a stereo amp or at least double the power (into an extra speaker).
Not bad, huh?
Practice/garage amp suddenly grows into a giggable one

So go shopping with a ruler and a calculator and impress those sales guys (as if they cared  :grr )

[exztinct01]

I wonder if they do it intentionally or maybe they just don't know what they're doing  :duh
Anyway, I'll be using the first transformer I've shown in the other thread since the amp's main purpose is for practicing guitar with distortion, although it won't happen anytime soon. I'm having a hard time creating a layout for the P27 preamp. I don't consider buying a pcb from australia since customs and shipping fees are a considerable increase.

[J M Fahey]

Agree with customs and shipping fees *usually*  being a problem, I mention that time and again, BUT in the case of a simple unstuffed PCB, Rodd can simply put it in a plain brown paper envelope taped to an envelope sized piece of cardboard so it does not jump around or a bubble plastic inside brown paper envelope and send it as "small parcel" or "small packet", labelled as "gift" or "free sample" and "value: U$5" , using standard Post or EMS, which is the "company" made by all "Official/State" Post Offices in every Country.
Shipping is inexpensive and such low value and weight parts go straight through Customs unopened (they are no fools, they XRay them, ... and see it's a plain unstuffed piece of "plastic") and most probably will be slipped under your door or worst case delivered with no problems at local post office.
He will have to fill a small green form which is glued to the envelope.

Don't worry, it exists in most Countries of the World, it's an agreement backed by "Universal Postal Union" or something like that, to simplify mail delivery.
http://www.ems.post/operators
https://auspost.com.au/parcels-mail/calculate-postage-delivery-times/#/

Notice both Australia and Philippines are full members  :tu: