Two nice pieces were added to my collection of tubes :). Theese are a Russian (Soviet) pentodes GU-81M with the anode dissipation of 450W (600W max 3 minutes). They have a diameter of 10 cm, height 24cm. They are directly heated. Voltage 12.6 V and current of 10A. Max. anode current is 600 mA, voltage 3000V up to 6MHz, 24MHz up to 2500V and 1500V up to 50MHz. Max. peak anode voltage (anode modulation) may be twice that. Minimal guaranteed lifetime is 1000h. Guaranteed power output (up to 25 meters range) 750W, and at the end of life 675W. Loss of g2 120W, loss of g1 10W. Slope 4,5-6,5 mA / V. Max. bulb temperature of 350 deg. C.

Here you can find the datasheets of theese tubes: datasheet GU-81 (1), datasheet GU-81 (2), datasheet GU-81 (3).

GU-81M (GU-81, GU81M, GU81) with sockets


inserted into slots





heating - glowing heater


glowing heater


Both GU-81M tubes with heater on


Marking of GU-81M (one of them is probably vintage 1971 and second one is 1986) and the "sdelano v CCCP (SSSR)" inscription


Socket of GU81 from bottom


slot for GU-81M From top


tube from top


both tubes from top


Video - testing heaters of GU-81 and inserting into sockets.


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