"Odd" looking 845 tube

I was sent these tubes by the gentleman I bought my SET 845 amp from. They are heavy, and have a shoulder about an inch down from the top of the tube. Written on them, although I can't really make it out right is: Ga4AHO CCCP.

There is also a green triangle with the number 7 in it, and also written is rM-70 (not in triangle). If anyone can deduct what this tube actually is with such scant information, I will be impressed and grateful.

Thanks, and stay well,
128x128Ag insider logo xs@2xislandmandan
The "CCCP" is used by our Rusky friends (remember seeing it on their fighter planes, etc.). That's all I've got.
Hi Dan,

I did a bit of research, and the "r" marked on the tube is the Russian language equivalent of our letter "G." So the tube is a Russian-made GM-70. The GM-70, in turn, is basically a higher powered counterpart of the 845, ***BUT*** with some major specification differences. Most notably the 845 operates at a filament voltage of 10, while the GM-70 operates at a filament voltage of 20, with the filaments of both tubes drawing in the vicinity of 3 amps at the respective voltages.

So the GM-70/"RM-70" is definitely not a direct replacement for the 845, and should not be used in your 845 amp assuming it has not been modified to work with either tube. In which case it would have been provided with some means of switching between the very different operating conditions of the two tubes.

Also see the following short video, which among other things mentions that the two tubes are sometimes incorrectly described by sellers as being equivalent. Perhaps that is why the person who sold the amp to you purchased these tubes:


Stay safe, and be well! Best,

-- Al

Al, you are a marvel of your ability to not just acquire information, but to understand it, explain it, and share it.

Thanks again, for being who you are, Stay well and happy,