It is an allen screw, hex head. I can't tell you the size off hand, maybe 3/32" not sure in metric. These wrenches usually come in sets and are readily available.
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It is an allen hex head, the outside circumference is round, look closely, unless Berning changed it but I doubt that. A 3/32" works, I checked it, try it or if you are in Europe, buy a metric set. A corresponding size will be adequate or to be exact simply e-mail David Berning directly from his site.
My screws are Allen screws and my unit was the last ZH270 model David Berning made and he built it with all of his latest upgrades. Assuming your ZH270 is recent vintage, I would assume the screws are Allen screws. If your unit is pre-owned, some goofball could have rounded-off the inside of the screw.
I don't believe he used a particular manufacturer for the driver/input tubes. The original 12AT7 and 5965 (12AV7)tubes in mine were GE. I am using GE 5 Star Platinum 5965 and a combination of Amperex and Tung Sol 12AT7 (RCA is the manufacturer of the Tung Sols, same tube different label). These tubes last forever, I recently checked them out after 7 years continuous use with the SAME TUBES and they test darn close to NOS, same with the output tubes, just amazing how efficient this design is. Not only that but replacing these tubes is so inexpensive relative to what you would spend for other driver input tubes 12AX7, 12AU7 or 6SN7 for example. Tube rolling is quite inexpensive, a beautiful thing.
Enjoy your amp, it is a gem and a keeper and will never be the weak link in making music!