Substitute 12KP4 for a 12LP4
 

I recently picked up this 12KP4. It tests very good on my Sencore CR70 - plenty of emission and grid control. However, when I popped it into a worked set in place of it's 12LP4, I'm getting nothing :tears:

The specs look nearly identical on paper. The filament is glowing and I have ~10 KV at the anode. Maybe it's the lack of the DAG coating ? The previous owner removed it all to put the CRT on display. This set does have a doorknob cap on the HV so I figure it shouldn't make much difference.

Also, the 12KP4 takes slightly more focus current, but that shouldn't matter much - should it ?

Here are the specs: 12LP4 12KP4

As far as I can tell, a 12KP4 does not require an ion trap... might try it both ways however, as we have seen several tubes rebuilt with different guns than orig.
jr

It should work fine, the 12Kp4 is shorter and aluminum so it shouldn't need a trap.

The focus coil could be pulling the beam off center if it's too far out of adjustment but I would suspect the lack of Dag in this case, I had a set that wouldn't operate with an uncoated tube.

You could try an Ion trap, the tube may have been rebuilt at some time with a gun that requires one, if the neck is too short you might have a problem installing it though.

I thought it would be shorter too, but it's about the same length :scratch2: I covered about half the bell with some aluminum foil, made sure it was grounded and fired it up. No difference. Then I took a closer look at the gun. It sure looks like it's at an angle. I'll dig up a single and dual magnet and give it a try.

I don't see any signs of a rebuild. I also don't see any part#. I'm just going my the seller's word that it's really a 12KP4. All it says on the base is "CBS Palomar". On the other hand it is an aluminized 12" CRT.

That did it :banana: Of course it needs adjusting, but it's not destined to be used in this chassis. I tried both a single and dual ion trap. The results were pretty much the same.

Does that mean that I should go with just a single :scratch2: I have plenty to choose from :yes:

Do you think it might also be a 12LP4C?

http://tubedata.milbert.com/sheets/137/1/12LP4C.pdf

John

It could easily be a 12lp4. CBS Palomar was their rebuilt tube line in the late 50's and early 60's. I wouldn't be surprised if it took a single magnet ion trap. Sellers seem to think that if the tube is aluminized that it automatically is a 12kp4. It probably came with an instruction sheet to use a single magnet ion trap.

Thanks for the info guys. Yep, it's a rebuild. I pulled the CRT, felt along the neck and found a seam about 5" from the base. They did a good job - you can just barley see the seam.