I have two brand new Eimac 3-500Z’s that were purchased about 20 years ago. They’ve been sitting in their original boxes on the shelf. It’s time to do some much-needed maintenance on my SB-220, so I’ve decided to replace the original tubes which are definitely soft. I’m aware that I don’t want to just install the new tubes and start using them. According to Eimac’s recommendations (below) they suggest “running them for several hours with filament voltage only applied”
Contrary to Eimac’s recommendations, many people are saying that the filament voltage alone isn’t hot enough to do the degassing.
I’m sure among the AM boat anchor community that many of you have been faced with this predicament. What’s the safest way to proceed?
Bob, WA2SQQ
Eimac says:
http://www.cpii.com/library.cfm/9"Degassing Tubes that may have gassed up can be partially degassed by putting them in the equipment and running them for several hours with filament voltage only applied.
After the initial filament-only degassing; operation for an hour or so at reduced plate and screen voltages is desirable. This allows the getter to soak up and hold
any residual gasses. In directly-heated filamentary tubes, the getters are generally zirconium-bearing materials, which depend on heat to activate the gettering action."
Others say,
“You can let the filament run til the cows come home and it will not help. The gettering agent in the 3-500Z is zirconium, and it is coated on the anode, NOT the filament. If you don't want to take my word for it you can look on Internet, but zirconium catalyzes most common gasses optimally at 1000 degrees C or slightly hotter. 1000 degrees C takes about 300 watts of anode dissipation, and would be a bright red color on the anode. The anode has to be at least 500C for the zirconium to have any effect, and that is a faint black red color. Running the filaments is a waste of time. Anode heat is what absorbs gas.”