My Acom 2000A amplifier started complaining “bias too low at power on -56 V” when turning it on. I pulled out the tubes (GU74B / 4CX800A) and did some simple resistance checks, nothing stood out. Then based on the advice from Acom (fast and good customer support) and a local ham, I decided to put in another pair that was known to work. Well, the amplifier came back on without any issues. Here is the amplifier with the new tubes put in:

And these are the old tubes (of which at least one is bad):

Now I need to finish plate current calibration and I should be all set to go again! If you have to change your tubes, drop Acom an email and ask for instructions. They have a very nice writeup on how to do the procedure. Even though Array Solutions is cooling down their operations (retirement), Jay can still do tube testing. However, there is a bit conflicting information on “gettering” this type of old tubes. On the internet most people say that one must do this for old Russian GU74b’s whereas Acom technical support said that it is not needed. Go figure… Just to be safe, it is probably a good idea to getter them before using.

Except then leads of one of the temperature sensors snapped. This has LM335GE label on it. Now need to replace that. According to Acom, LM335Z will work. I will try to put flexible leads on them in order to avoid breaking them again.

Update: New LM335Z is in and the amplifier seems happy. Also the “new old” pair of tubes look produce full power output.