+1 to what Duke posted... This thread topic is confusing since you do not need a transformer to test a 70V amp. You can test the output like any other audio amplifier.
One important missing fact is the rated power (watts) of your 70V amp, that will determine how much load it can drive. 70V amps can come in sizes from 2W to 200W and more, while all make the exact same 70V output.
In 70V or "constant voltage" systems, loudspeakers have step down transformers to deliver X watts to the individual speaker. This dramatically simplifies the installation math, as you just add up how many watts of speakers you have and just size the amp to deliver a comfortable margin more watts (say 10% safety margin).
You can probably confirm that it works with no load.
JR