CRT Testing - Low Filament Voltage

[bhegges]

I have a 21FBP22 CRT that I am trying to test with a Sencore CR70. Once connected I cannot get the filament voltage much above 3.5 volts. With the lights out I can see a very very dim glow but that is it. No shorts present. I was temped to let the CRT "cook" a bit but was worried that I might overheat the tester.

The tube is likely bad but out of curiosity what is pulling down the filament voltage? If the CRT lost its vacuum I would assume the filaments would have burnt up. The tester has worked on other tubes so I don't think that is the problem.

Thoughts?

[jr_tech]

CRT is likely gassy...the gas conducts the heat away from the heater, so it remains fairly cool (neck of CRT gets warm quickly)... resistance of the cool heater remains quite low, causing the tube tester power supply to current limit at a low voltage.
If you applied 6.3 volts to the heater from a supply that doesn’t current limit, it likely would blow the heater quickly.

jr

[old_coot88]

If the jugg has lost vacuum, the heaters are immersed in a heat-conductive fluid (air) which keeps them cold and therefore at low electrical resistance, drawing excessive current. That lugs the tester's transformer down which can quickly overheat it. And the CRT neck gets super hot super fast.

[Electronic M]

Gassy tube is likely, but before you close the book on it, do you know that the tester is working properly? Do you have a different known good picture tube you can test to verify the tester?

If you know the tester is good hooking the tube to a high power 6V heater transformer and observing opperation is another way to confirm gas. The Sencore manual may also have a section on gassy tubes...My B&K 466 gives pretty clear indication of gas on its cutoff test....It will even light up the gas!

[Yamamaya42]

you MAY be having the same problem that I had with my B & K 409B.

As in not enough amps to light the heaters of the tube and it is being loaded down.
http://manuals.repeater-builder.com/...0Operation.pdf

If what I see in that is right, it hardly has 1 amp of heater power, and the roundy needs almost 2A.

my 409B is foolishly the same way, in order to test a 21fjp22, or something like it, i have to use a separate filament transformer.

[Yamamaya42]

it's THIS line i find disturbing

"Meter accuracy +/- 5% calibrated at 6.3 volts 900 mA."

does it mean it only puts out .9 amps at 6.3v? :O

[Tom9589]

Not necessarily. It could be that the manufacturer just calibrated the meter accuracy under those exact conditions. 21" roundies certainly draw more current than 900 mA. Does the meter manual say that it will test 21" roundies? If so, I would expect it to put out enough current to light up the filaments.

[Yamamaya42]

Originally Posted by Tom9589

Not necessarily. It could be that the manufacturer just calibrated the meter accuracy under those exact conditions. 21" roundies certainly draw more current than 900 mA. Does the meter manual say that it will test 21" roundies? If so, I would expect it to put out enough current to light up the filaments.

I can't tell in the case of his tester, but it definitely WAS the problem with my B & K 409B, even though the 21fjp22 was on the general tube list for it, and a socket was even made for it, having only 1.2A available, it could not light the heaters and the voltage dropped to about 3v.

At this point, he has nothing to lose by trying a filament transformer.
If the tube is bad / gassy, there is really no hope for it anyway, but if the tester is struggling to light up the heaters like my 409B, it will also allow his to test it w/o straining it.


EDIT..

This was a test of the roundie in the GE CTC-15 clone i got. using 3a 6.3v transformer
https://i.imgur.com/58xIKOB.jpg