I did both things, a physical measurement and some calculations.
The core of this one is 1 cm x 1.5 cm, which results in a 1.5 cm² area. The equation for the core area is Sg = 10 * (P/f)^1/2, where P is power andf is the lowest frequency. Some books use 7.5 instead of 10 as the constant, but 10 is a better number since you'll get a bigger core area, and it's better to use a overdimensioned transformer than an underdimensioned one.
Solving that equation for Sg = 1.5 cm² and f = 60 Hz, I got 1.35 W as the output power. We don't know anything on the windings, so it's not a good idea to use this full number. And since my previous PCL84 amp gave me about 1/4 W, it's safe to use this "1.35 W" as the output transformer.
For the impedance, the equation is windings ratio = V1/V2 = (Z1/Z2)^2. This is a 220 V to 9 V transformer, which gives a 24.4 winding ratio. For a 8 ohm speaker (Z2), that gives Z1 = 4K8. The PCL84 asks for a 10K transformer, so it'll result in a impedance mismatch and lower output power (about half of the power). Since I used a 6K transformer before, I just gave a go on the 4K8 one and it worked pretty well. But if you want to use the 10K impedance, you can use a 6 V transformer and a 8 ohm speaker. Use the 220 V side as the primary and the lower voltage as the secondary. Don't know how bad is that mismatch in a bigger amp.
Then you have to rearrange the lams because a single ended amp asks for a transformer with an air gap. Basically, you put all the E lams, then add a layer of paper (photo paper or something like that) and then the I lams. Then superglue them together or something like that
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