12" EMIscope 3/31 rejuvenation?
 

Has anyone here tried to rejuvenate an Emiscope 3/31 CRT with a Sencore CR-70? The one I have puts out a picture but is completely washed out when room lights are on. I should think the process of rejuvenating it might be pretty much the same as any triode CRT, but the question is what American tube would be a good model or what settings would be right? Any thoughts?

What base does it have? Assuming it uses a common socket you might try testing it using a common tube setting and see how it reads. If it'll test then it should rejuvenate using the same settings.

Thanks for commenting, Eric. Actually, the tube has no base. The pins come out like an octal version of a 12AU7, but with the same diameter as an octal tube. The tube is truly a triode: two pins for filament, one for grid, one for cathode. No plate, save for the HV (8.5 KV). It's a very simple thing! I don't think I've seen anything like it in American product.

I'll mention this thread over at VRAT where there should be more experience with British CRTs.

http://www.forum.radios-tv.co.uk/vie...php?f=5&t=6432

IIRC, those GE 8" metal CRT's used in the 8" Arvins, were triodes as well.

Hi.
I'm not familiar with the Sencore CR-70, can anyone enlighten me with its operation? Over here there are numerous other CRT Boosters or conditioners that are more familiar.

This thread over at VRAT has a lot of stuff on current best practice for rejuvenating CRTs.

http://www.forum.radios-tv.co.uk/vie...php?f=5&t=4171

Weren't a LOT of those early CRTs pretty dim, anyway ?

I would use a Beltron if you want to try a rejuv. But, as mentioned earlier, very early tubes were not stellar performers. Please don't think I knock them at all, but my 7 inch motorola electrostatic pictures are dim compared to some of the late production 5 and 7 inch solid state black and white TV's. Also, I would test it before whacking it. That has got to be a very rare tube!

Not all early CRTs were dim. Part of the problem is that many of the ones we see are now old and well down on their original performance. Using the Emiscope 6/6 as an example. This is a 12" magnetic deflection, electrostatic focus CRT from 1936. It runs at 5kV EHT. The ones I've seen range from dim to very dim to totally unusable. The one that I had rebuilt in 1986 gave an astonishingly bright and sharp picture. Perhaps not bright by modern standards but a lot better than any originals.

Let's hope that Ebeam can rebuild these CRTs successfully so that we can enjoy these sets as they were intended.

When I read the original post, I was thinking of that crazy Philco Neotron CRT, that was made only one model year. It had an end on it like a 12AU7, about the same diameter.
I don't remember of anyone rebuilding that CRT.
I also wonder how long Philco made replacements.

I wouldn't think any pre-war CRT would have ever been bright enough to watch in a fully lit room. This was the norm even for many American non-aluminized CRTs through the 1940s into the early 50s. If it has a useable picture I wouldn't risk ruining it with a rejuvination; though if you're going to try the CR-70 is one of the most capable machines for the job.

I've wondered if rejuvenation would work on other kinds of tubes, as the cathodes I think are similar. Probably not worth the trouble, though. I think one issue is that CRT grids are more rugged than those in regular receiving tubes.

It'd be interesting to try it, though. I'd bet the fine wire grid structures will light up white hot.:eek: Very low thermal mass when under heavy electron bombardment (think steel wool over a gas flame).