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How to guess the volt-amp rating of the transformer in my amp

I'm the owner of a Marantz PM 6006 integrated amp, which the company touts as having a "high-current toroidal power supply" without actually giving a rating of the volt-amps the power supply is capable of. It's something I'd like to know, because for better or worse I'm just a numbers-oriented person.
I've used Google, service manuals, parts numbers and Marantz customer support without success. Instead, I've come up with two very rough methods of guessing what the transformers VA rating might be. I freely admit these are error-prone guesses by a non-expert, and I'd like to ask if anyone with more knowledge than me could comment on the validity of these two attempts.

Method 1 (Use the approximate size of the transformer)
The transformer is in an iron case, with a radius of 11 cm and height of 7 cm. I spoke to someone at a company that builds custom audio toroidal transformers, and he thought a transformer of those dimensions would likely be in the range of 250VA -300VA.

Method 2 (Use the power consumption rating on the back of the amplifier)
The back of my integrated amp says "Power consumption 395 Watts". If I assume this represents the maximum, or transient peak power consumption, the Volt-Amp rating of the transformer would be equal to the product of 395 watts and the "power factor" constant, which could range from about 0.6 to 0.8. By that math, the VA rating would be between 237 and 316 volt-amps.

If correct, either of these methods would lead me to the conclusion the transformer is either 250VA or 300VA. Thanks in advance for any perspective you can provide.
johnanguswebb

Showing 4 responses by johnanguswebb

johnanguswebb OP
7 posts
 
Thanks for your help!
On the circuit board by the fuse (next to the transformer) the following is written:
100V T5AL 250V
120V T3.15AL 250V
220-240V 1.6 250V

I assume these are for different countries with different mains voltage. The bottom two both work out to 352-378 VA, which seems in the ballpark. The top one is 500 VA. I’m surprised it’s so high. I’m also thinking the fuse wouldn’t be set to blow at a current higher than the transformer’s rating, but that’s not my expertise. Could I ask how well these numbers fit the hypothesis that it’s a 300 VA transformer?
johnanguswebb OP
7 posts
 
To answer my own question: I read online that a rule of thumb to find the fuse rating for a given circuit is 1.25 x Power/Volts. (I’m guessing 1.25 is some kind of fudge factor.) Applying that formula for 300VA gives 1.25 x 300VA/120V = 3.13A. The closest available fuse rating would be 3.15A, which is what’s on my circuit board. If I left out the "fudge factor" the rating would be 2.5A, as imhififan predicted. So I think I do have a 300VA power supply.
I still wonder why the power comes out differently following the fuse recommendation for 100V mains power, but I’m not going to try to figure that out.
johnanguswebb OP
7 posts
 
Thanks, that’s really interesting! I just learned what the capacitors are for.
johnanguswebb OP
7 posts
 
A look inside my PM6006 reveals it has 10,000 uF capacitors paired to its presumed 300 VA transformer, which is humbling by comparison. 
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