• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube amps cost priorities

The calculation is even easier: Zprim = U²/P, where U is the nominal line voltage, here 100 V, and P is the power labelled at the terminal.

Anyway, the Alltronics M-1115 appears to be somewhat whimpy. Here's a specs list of Omnitronic line matching transformers which might be more up to task, especially in twin-coupled designs.

Best regards!
 
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Just my opinions:

Many want to build with parts employing them in a circuit that they were not made to work in, nor designed for that kind of application.
I see 3 reasons for this:
It is a challenge to use the part that way. Just like climbing K2, the number 2 mountain.
To save money $$$
The Original Poster has some parts, and does not know what to do with them. So, they find a way to use them anyway (even if they will not work very good there).

I am reminded of being on a US Naval Destroyer in the middle of the Pacific Ocean, without proper repair parts.
That is why I would use parts improperly . . . to get the equipment running again, and to get the officers off my back.

Most tube amplifier designers and builders are not subject to those US Naval conditions; the tube amplifier designers/builders can get the proper parts.
Just stop drinking those $5 cups of Coffee every day. 30 Days = $150, enough for a pair of inexpensive output transformers that were actually designed for Single Ended, or designed for Push Pull.

Have Fun designing, building, and listening!