Substitute 12KP4 for a 12LP4 - Videokarma.org TV - Video

bandersen · #1 08-04-2010, 07:56 PM

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

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

jr_tech · #2 08-04-2010, 08:37 PM

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

***Eric H*** · #3 08-04-2010, 08:48 PM

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.

***Eric H*** · #4 08-04-2010, 08:50 PM

Quote:

Originally Posted by Eric H

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.

bandersen · #5 08-04-2010, 09:33 PM

I thought it would be shorter too, but it's about the same length

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.

bandersen · #6 08-04-2010, 11:01 PM

That did it

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

I have plenty to choose from

jeyurkon · #7 08-05-2010, 11:09 AM

Do you think it might also be a 12LP4C?

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

John

dieseljeep · #8 08-05-2010, 12:47 PM

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.

bandersen · #9 08-05-2010, 12:56 PM

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.