Eimac 304TH anode question

[hoangduongo]

Hi guys,

I've just happened to buy 2 Eimac 304THs with a good price, but they have different plates! I did a quick search thru the net but only found 1 type of plate for this tube only, so until now I don't have any clue.
Can any of you show me any info about this?
And here is the photo of these 2 304THs, the datecodes on them show quite close manufacturing time (7224 & 7249).



Thank you,
Regards,
Duong

[HollowState]

This is a good question and something I wondered about years ago when I actively bought and sold industrial tubes. From what I have learned, the tube on the left is the commercial version. While the one on the right is the military version. That is what I was always told when I asked about it. If you bought a new one from Eimac, you would receive the commercial style in a yellow box.

Both plates are made of Tantalum and act as a getter when at operating temperature which is red-orange. Electrically they are the same and interchangable as far as I know. I'm pretty sure the military style came first because it's a World War II item. The very early types had the grids connected by an external band rather then internally as yours are.

The only other thing I know for sure is that they are very fragile. The filaments will break with the slightest bump, so handle them very carefully. It's also not uncommon that the older style have become gassy or lost vacuum. Yours look good because the metal pan inside is still shiny. If they've gone to air, you'll know it when you light the filaments because they will burn and smoke. If you have a return option, check them as soon as you can. Good luck.

[hoangduongo]

Thank you HollowState,
As you said, the left 304TH is commercial version while the right one is military version, but when I search around the net, it seems that I can only find the "military version". Personally I think the "commercial version" is supposed to be more popular?
I bought these tubes from a local guy whose father works at a radio station before, and according to him, the tubes have been tested for filaments. I also don't find anything abnormal with them, hope they will work well

[bear]

Very important to run them for 24hrs or more with the fils on full before attempting to put B+ on them... also be sure to use some sort of current limiting on the B+ when trying them out initially. this will help to assure that if gassy there will be no "fireworks" in the tube.

I have also read elsewhere something about applying HV AC on the plate to "getter" residual gas... a search online might find more info - especially ham radio sources/forums.

_-_-bear