This humble little transformer is a favourite valve amp OT among some DIY folks in Australia, as it's cheap at only $27 and readily available from Altronics.   It's sold as a 20w 100v line transformer, but it also works really well as a 8k P-P output transformer.   Based on real world testing, it's thought to be closer to 15w, rather than 20w.

Here’s how you connect it: just follow the primary side and pick an output impedance that works for you (4, 8, or 16 ohms).

P: 1.25w 
CT:  5w
P: Common

These are perfect for replacing transformers in lower-power amps. I’ve even used one in an 8w push-pull PA head, and it sounds great.  Some old PA heads have output impedances that were meant for a bunch of 100v line speakers, not a single 8 or 16 ohm speaker. 

There's a few more versions available in the line with higher and lower wattages, but this one is probably the most commonly used.  The M1130 is larger, and with an impedance mismatch on the secondaries, it can be taken up to a 4k plate to plate load suitable for a pair of 6L6 not running too hot - they're used in the valve Heaven Lamington II.

Further reading:  Ozvalve amps

ALTRONICS M1120 OUTPUT TRANSFORMER

Frequency Response: 30Hz - 20kHz ±3dB
Secondary Taps: 4, 8 & 16 Ohm
Power taps: 1.25W, 2.5W, 5W, 10W, 15W, 20W
Frequency Response: 20Hz - 20kHz ±3dB

HOW IS THIS CALCULATED?

Note the transformer’s rated voltage (100v). Square this voltage: 100 squared is 10,000.

Divide 10,000 by the wattage on each tap. The smallest wattage has the largest P-P value.

10,000 divided by 1.25 equals 8,000. Repeat for the rest of the taps.

To calculate the centre tap (CT), divide 8,000 by 4 and match it to the relevant tap. In this case, the CT is the 5w tap.   

If for some reason 4K P-P was required, then connections would be as follows:  P 2.5W, CT 10W, P common.   

ALTRONICS M1120
PRIMARY	Watts	P-P Value	CT
6 (P)	1.25	8,000	2,000
5	2.5	4,000	1,000
4 (CT)	5	2,000	500
3	10	1,000	250
2	15	667
1	20	500
C (P)

Which is conveniently confirmed by the specs on the box.

and here's a youtube clip explaining the theory