Which step down transformer for my lovely little u.s.a guitar amp?
You may get lucky or you may need to change the transformer in your amplifier to suit your country's operating frequency if you use 50 Hz.

Your transformer is for 60 Hz and, if you run it at 50 Hz, it will take a magnetization current that is 6/5 times higher. This could be enough to cause excessive saturation and your amp may burn (worst case scenario). Here's the idea: -



The core magnetization characteristic is the fixed red curve. Saturation is where the core cannot handle increases in the applied field (shown in green). This is equivalent to putting too much voltage on the primary or running at too low a frequency. The resulting blue waveform shows the magnetization current and would normally not have such big non-linear excursions - these big excusrsions are due to excessive saturation and results in too much current being taken. This can lead to catestrophic failure because too much current means higher peaks in the green waveform and this can quickly spiral into a fire.

However, if you ran it at 5/6 of the voltage i.e. 96 volts then the magnetization current (and level of internal core saturation) will be identical.

You could argue that in fact it's 5/6 of 120 Vac and this means 100 Vac but that is pushing it. Based on the power supply spec you should run it nominally at 96 volts and 50 Hz (to be safe).

The big question is, will your amplifier function at this lower voltage and that is impossible to say given what we know about your amplifier.