I thought I would pull out a few of my as of now-unrestored 21" B&W tabletops to check the CRTs and assess their restorability, and I've come across something concerning with a Philco from 1953. The CRT has absolutely no cutoff, but it tests like a brand new tube with the cutoff control maxed. There is a blue glow visible in the neck of the tube which varies in intensity with the adjustment of the cutoff - is this a gassy CRT? There are conflicting reports online, so I am unsure if I want to part the set out or not yet. The picture tube is a 20DP4A, which doesn't seem to be very common.

Any input would be appreciated.

Probably is gassy.
Purple neck glow usually equals doom.
On my B&K 466 tested cutoff is checked by adjusting the cutoff knob for 2 division increase of the needle. If adjusting the knob causes it to shoot past the 2 division mark and there is no setting that yields that fine adjustment then the tube is a dud...That is often accompanied by purple glow in the neck. Gassy tubes usually test quite strong. If you power the CRT with a working chassis there will probably be internal arcing between the gun and the HV, accompanied by the HV system shreaking in distress.

IF the glow originates from the glass surfaces, such as the inside of the neck glass or the glass rods holding the metal gun parts, it could be caused by stray electrons hitting the glass, like is seen in some audio output tubes. Not an indication of gas.

jr

IF the glow originates from the glass surfaces, such as the inside of the neck glass or the glass rods holding the metal gun parts, it could be caused by stray electrons hitting the glass, like is seen in some audio output tubes. Not an indication of gas.

jr
It does appear to originate from the two glass rods visible in the photo - are those the same rods you're referring to? I am tempted to carefully take the tube out and put it in my 21" Zenith since the tubes seem to be virtually identical as far as voltages and the pinout go. Just not sure I want to chance disturbing the disintegrating yoke cover..

It does appear to originate from the two glass rods visible in the photo - are those the same rods you're referring to? I am tempted to carefully take the tube out and put it in my 21" Zenith since the tubes seem to be virtually identical as far as voltages and the pinout go. Just not sure I want to chance disturbing the disintegrating yoke cover..

Quick and dirty way to do it would be to keep the tubes in both sets, place both sets back to back connect the socket and HV from the good chassis to your suspect tube, turn the brightness on your good chassis to minimum and power it up...if you get a tiny bright spot in the middle of the screen of the suspect tube then it is fine.

I did the above once when I was in a move and couldn't find my tester. It got the job done.

It does appear to originate from the two glass rods visible in the photo - are those the same rods you're referring to?

Yes, that is rod glow/rod charge, but do I also see a pencil line thin blue beam in the gap between the two electrodes? That may indicate a fairly high gas level in the tube.

jr