[RadioSvit]

Hello Arran. There were two types of Ketkin tube designs, - with an aluminum protective screen like the left tube in the photo and with an open anode like the tube in the center. The protective screen was used only on low-power pentodes, perhaps it had another function - shielding the low-power anode from interference, and this screen was absent in the output pentodes, perhaps for better cooling of the anode. This is just my version and I am not convinced of this.