The 'soft' tube

[Panrock]

Okay fellas and fellettes, it's time for me to own up. I have to admit I've long been woolly about what are the signs on the screen of a soft or gassy cathode ray tube. Can the experts here put me right on this?

I tentatively offer some possible signs of a soft tube below. Are they right or wrong? 

Impossibility of obtaining a sharp focus.

Dim raster (otherwise known as 'a picture as weak as p***', as one BBC Wood Norton lab tech used to put it).

Flair (flattening and defocussing) on high-lights.

Effect like that produced by a tired EHT rectifier, caused by excessive beam current load on the EHT.

Poor low frequency performance on test card, smearing on blacks, caused by the gas reducing the input impedance of the tube at the grid.

Any more symptoms?

I'm posting here rather than UKVRR, because there are fewer of you, you are all kindly souls and won't let on about my ignorance, will you? 

Steve

[Panrock]

To which I would add:

Most 1950s-and-before sets seem to have soft tubes at first light after years of inactivity. However, these tubes can generally be 'woken up' with 6 or more hours of continuous use (not necessarily in one session), as the getter 'gets' to work.

Most, not all... look at Trevor's latest acquisition.

Steve

[Murphyv310]

Hi Steve.
Thanks for the compliment.
You have summed it up nicely.
One other thing that is noticeable is on advancing the brightness the whites on the picture can turn negative.

[Freya]

You didn't mention the milky getter but I'm sure you know as it would be the 1st thing on the list.

[Panrock]

What is the chemistry behind the changes in getter appearance?

[Murphyv310]

Hi.
Of course when the tube is down to air you get a beautiful purple glow in the gun assembly.
I recently got a 9 inch CRT that when fitted it was decidedly flat. The ubiquitous run for a few hours made no difference. Emission on the tester was way into the green. Ion trap adjustment was odd with a big range of adjustment that made little difference.
For some reason I decided to increase the heater volts. At 7 things improved and at 8.2v the picture was perfect and the ion trap behaved correctly. The odd thing is emission is spot on even at 6.3v on the heaters. Is the cathode off centre and the higher heat is doing something? Who knows, it a regun of some kind a bit odd looking gun and a odd phosphor kind of green looking.

[peter scott]

I think gassy tubes tend to suffer from astigmatism but like you mention Steve they can improve with use.

Peter

[Murphyv310]

Hi Peter.
Yes I've noticed the issue with astigmatism. Why do you think that would happen?

[Panrock]

Could the astigmatism be an effect related to unwanted 'gas focussing' being added to the magnetic or electric standard focussing? Weren't some early tubes gas focussed by design?? No, I don't know how that works...

Steve

[Kalee20]

I've actually heard of gas being used to IMPROVE focus.

High-energy electrons, travelling fast towards the screen, knock electrons out of the gas atoms in the path. This leaves a pencil-like 'core' of positive ions at the centre of the beam.

This 'core' of positive ions, then attracts wandering electrons back towards the centre of the beam (electrons, being negatively charged, repelled each other so an electron beam, unlike a ray of light, naturally diverges anyway).

I can quite believe that the undesirable other effects of gas easily outweigh the focussing effect, but - does this happen, and how much pressure is needed?