Will a 21fjp22 work in place of a 21cyp4
 

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doubtfull you'll get a good gray scale without changing some resistors to compensate for more efficient phosphors. Service bulletins were published, I'll see if I can find one. I sure can't trust my memory on this one. If needed, you can try swapping drives../red & green for magenta, red & blue for yellow, etc.

Don't forget to be careful about new the anode connector you will install. Wrap layers of tape around the ring near the cavity on the tube to prevent arcing through the HV connector to the ring.
Any other suggestions on this?
Kevin

CRTs
 

Comparing the "Examples of use of Design Ranges" on the datasheets for the 21CY and 21FB, they will sub for each other.

This is a 100% retrofit and will work perfectly. The gun mounts are the same gun on both tubes and from the 21CY you don't use the 56K resistor with spring that bridges the two anode buttons. I don't know about changing any resistors as you will get better tracking with the 21FB/FJ and will make wonderful color. I did this many, many times in the '60-70's and never any problems. Don't worry about any resistors as nothing has to be changed except leaving off the double button anode resistor for the newer tube. Julian

I second this comment. I ran my CTC-10C with an FJP22 tube for years with no issues. I finally found a CYP22 to rebuild, and this tube has been in this set since about 1984. Thank you, Video Display Corp. of Atlanta. I wonder if VDC is still in business?

VDC is still in business, but not taking round color CRTs.

Is Empire Video still in business? Several websites have their rebuilt tubes - maybe they'll still rebuild a 21" round tube - they did in the late 80s.

HBF Electronics (No affiliation, just came up on a search) carries Empire tubes - mostly 19/23/25/25V stuff.

Possible resource...

Cheers,