chacham: I thank you for finding this letter. I did no say categorically that R”Chaim Ozer was one of the makilim. I only had a thought about it. I do know that the Maharsham (the gaon of Brezahn, Rav Shwadron) who was considered THE Possek in Galicia, POland and Rumania allowed it on yomtov.
My question on R’Chaim Ozer zz’l ,if it is “mavir” why is it ossur on yomtov? You are allowed to make a fire on yomtov, and ,at the minimum, you can use “ho’il”.
You should also know that R”Shlomo Zalman (to whom this letter is addressed) thought thst using a microphone (on shabbos mind you!)is only miderabbbonon (check Rav Ovadia Yossef’s shelita, teshuvos)
Thank you for providing the letter (an assist to hebrew books!)