Ohms needs holmes..

Hi... story goes, i have been slowly repairing and bringing back to life this old Reslo class A valve amp,
am no expert mainly an exchanging like for like with dodgy resistors and what ever help i can get or facts i can glean..
The amp is from the late 50's and says 240v 10-15 watts on the output on the back. I am stuck now because i don't
know the ohms rating so i can get the correct speaker... No info what so ever anywhere on the net and have run dry
of anyone with possible knowledge on this matter.. Is it difficult to work out the rating ? I know its all a bit vague, it is what it is...
This is my first post here although have been hanging around for a year or so
just looking and wondering before making the plunge and joining last November...Tony..

If you have 2 x VAC meters and a 1-12 volt VAC supply (like from a kids trains set - or even just using the heater winding of your PT), you can determine the impedance ratio of the OT.

You (carefully) hook up the VAC supply across the OT secondary and switch the VAC supply on, and measure the resulting VAC across both the secondary and the primary (with your pair of VACmeters). The voltage of Pr:Sec is the VAC ratio. Square it to get the Pr:Sec impedance ratio. Then multiply the impedance ratio by whatever speaker load you choose to run on the secondary in order to get the reflected load at the primary.

The 10-15 watts is probably the total wall power dissipation of the amplifier, being a single ended amp I doubt the speaker output is anything over 5 watts (probably closer to 2-3 watts.)

What tube type is the output tube? That knowledge, combined with tubeswell's method for determining the tranformer's ratio, will tell you where the speaker load should be.