What picture tube do I have and why does it not get hot at the neck.

[fumplet]

I think this is a 10bp4 but not too sure as the filament sure doesnt get very bright and the neck of the tube doesnt get hot either. zero emissions as well as zero cutoff. Do I have a 10bp4 or is it something else that needs more than 6.3 volts a/c to work? I cranked up the sencore cr70 volts to 8+ volts and the neck started to get warm as well as the usuall cr70 high pitched whine it makes when a normal crt filament load is applied to it. cr70 needle went just into the green at 8 plus volts. Gone to air, tube base pins need resoldered or maybe taken apart? I have tried several times to resolder the pins. The neck of the tube doesnt have the burnt filament shield from high hours of use. At normal 6.3 volts it doesnt get the neck of the tube hot at all. Is there a tube that takes like 12 or less volts but above 6.3?

[Penthode]

You should read my post on the 10BP4 I posted a few months ago. The cathode lead was blown open. No longer a cathode connection. I think the previous owner tried to rejuvinate it and the fine wire to the cathode opened like a fuse.

If you carefully examine in the neck you might see it. I did in mine! See the photo after I gave up and smashed the tube to look more closely inside.

My guess is that in manufacture the very fine wire used has a tendency to melt, especially if you try and rejuvenate. I guess that is why I got the set that contained this tube cheap!

https://videokarma.org/showthread.php?t=276674

[fumplet]

have no idea what Im looking at in my picture tube. maybe a flashlight might help so I guess I have a very dead tube

[Penthode]

Look at the end of the tube where internally the wire attach to the CRT gun. Look particularly closely to the cathode wire. See on my bad 10BP4 the gap?

[Electronic M]

+1

In the late 40s early 50s the 6.3V IIRC 600mA heater was the only game in town for CRTs. It wasn't until around the mid 50s that higher and lower voltage heaters with lower and higher respective currents became available to help series string designs. By then small round CRTs were fully obsolete.

[fumplet]

I will have to get a super bright light on the neck to see if mine is like that. All I know is it measures 3.6 ohms. So a bad blown open cathode wire will cause the heater to glow dimmer? I can get my element to start heating up the glass but thats like 8 volts. Almost seems as if somebody could cut a hole in glass......repair it.....glass weld up hole.....Looking its as if I need to remove the base to even look because I cannot see up over the filament shield.

[Penthode]

Originally Posted by fumplet

I will have to get a super bright light on the neck to see if mine is like that. All I know is it measures 3.6 ohms. So a bad blown open cathode wire will cause the heater to glow dimmer? I can get my element to start heating up the glass but thats like 8 volts. Almost seems as if somebody could cut a hole in glass......repair it.....glass weld up hole.....Looking its as if I need to remove the base to even look because I cannot see up over the filament shield.

I do not thing the filament is dimmer. It is heating normally. I am only saying if you have zero emissions even after raising the filament voltage to 8 volts, it is likely that the cathode connection is open circuit. Could be a solder connection but if you have addressed that, it is likely the connection within the tube.

I think this a warning to use rejuvenation with caution.

[bandersen]

He said emissions are just into the green at 8 volts.
There were 8.4 volt @ 450mA CRT filaments starting in the mid 50s as Tom mentioned. Possibly it was rebuilt with one? Seems unlikely but it's possible.

Check the CRT filament current at 6.3 volts. If it's around 0.6A, you can be sure it's a 6.3 volt filament.
Easy to do with your setup. Put a DMM in current mode between one of the CR70 filament clip leads and the CRT pin.

[Penthode]

Originally Posted by bandersen

He said emissions are just into the green at 8 volts.
There were 8.4 volt @ 450mA CRT filaments starting in the mid 50s as Tom mentioned. Possibly it was rebuilt with one? Seems unlikely but it's possible.

Check the CRT filament current at 6.3 volts. If it's around 0.6A, you can be sure it's a 6.3 volt filament.
Easy to do with your setup. Put a DMM in current mode between one of the CR70 filament clip leads and the CRT pin.

Sorry I missed that and misinterpreted the needle going into the green. Green means good.

So at 6.3 vac, is it not going into the green?