Monsoon PM-9 Transformer

[thoglette]

Is there some way that I could determine the VA rating of the original PT so that I can confidently fit a properly spec'd replacement?

Weigh it. Similar architecture (toroid if i recall correctly) transformers of a similar weight will, generally, have similar VA ratings

The 100VA is the max rated draw from the mains, not the rating of the transfomer. A quick examination of the circuit will show you that there's no way in hell the PM-9 amp draws 100VA (quite what it is I leave as an exercise for the reader but rail voltages and DC woofer resistance will get you close)

Also, the original PT has a thermal fuse while most of the PTs that I saw at the shops did not have one. How critical would this be?

It's critical for most of the markets that the product was sold into. It also allows a smaller VA rating (as if it gets hot POOOF rather than fire)

Have either have a thermal fuse or a separate mains fuse (200mA slow blow) for safety's sake.

In the absence of a fuse, a genuine short across the mains will generate several thousand amps at 230VAC, which in a domestic setting will be shortly followed by your smoke alarm going off and an unpleasant conversation with your insurers.

 

[yorab]

Ok, so I think the rail voltage is 12.5VDC. At least that's silkscreened to the board at the output of the bridge rectifier (see pic). I also know from measurement that the original PT secondary is at 12VAC.

I don't have access to the subwoofer at the moment to measure, but a quick search of a similar Eastech driver shows 3.56Ω (DCR) for a similar 4-ohm (Z) speaker.

So it looks like a ballpark of 12.5V/3.5Ω= 3.57A across that load.

That means a PT secondary coil would need to carry 3.57A*12VAC= 42VA. Then there are some losses to be concerned with.

Am I on the right track here?

 

[thoglette]

That means a PT secondary coil would need to carry 3.57A*12VAC= 42VA. Then there are some losses to be concerned with.

Am I on the right track here?

Yup. Throw in a few watts for the planars & preamp plus a bit for losses and you're in the right ball park. 60VA would be about right, based on what you've said