Pre-war CRT in Dr Röntgen's rays

[ppppenguin]

In 1986 I called upon resources that are no longer available. There are no CRT rebuilders doing commercial work. H-K assemblies may no longer be available. I doubt there are people around who know how to dismantle and rebuild a complex electron gun.

Most of the 6/6 and related CRTs I have seen have very poor emission. The number of good CRTs is very limited, almost certainly fewer than the number of sets that could use them.

The 6/6 electron gun is a very specialised design, probably unique. In particular it has a very high slope (gm) which allowed pre-war sets such as the HMV 900 to drive it directly from the vision detector. A wideband video output stage would have been difficult with the valves of that time.

Interestingly the subject was revisited in the late 1950s when companies such as Mullard/Philips were working on fully transistorised TVs. The video output stage needed a combination of high voltage and wide bandwidth that wasn't possible with transistors of that period. Proposed solutions included a high slope CRT. In practice, transistor design advanced rapidly with parts such as the AF118 and then the BF178 which could drive standard CRTs.

[Murphyv310]

Wow.
I've just seen this thread and It's totally fascinating. I shall be studying the images in great detail this evening.

[DrStrangelove]

I'd never heard of a heptode CRT: full contrast on a 15V swing. Impressive.

[Murphyv310]

There were a few CRTs Prewar that had much higher sensitivity so a Video output valve was unnecessary.

[ppppenguin]

It also meant that the amount of RF going into the detector made the final stage into something approaching a small transmitter. Needed really good screening.

[LENINGRAD T2]

So the 6/6 regeneration project still makes sense. I wonder if the deterioration in the functioning of the 6/6 CRT and similar results from the gradual loss of vacuum or cathode wear? This may have an impact on the scenario of rebuilding the picture tube. I am thinking very seriously about this project. At the moment I have collected about 200 picture tubes manufactured until the 60s. I estimate that 70-80% are functional. I have about 120 new electron guns and one pumping system with a turbomolecular pump. A jeweler's welder is on the way to work on the electron gun. Now I have to find some time. And this is probably the worst thing about all this. My goal is to regenerate the picture tube, not to recreate the regeneration process. I assume the use of new technologies that were not available in the past. I will never gain the experience that people who dealt with this issue had long ago. Any information on this subject is very valuable. The project will include several stages: removing the vacuum and restoring the vacuum in a working CRT, replacing the electron gun, replacing the parts of the gun. Unfortunately, today old CRTs have museum value. Using them for experiments with the risk of damage is very controversial. If the CRT is part of a TV set, it should definitely stay in it. However, if the CRT is in a closet, it can .......... 
It is interesting that some old sets did not require a video amplifier. I suppose that replacing the heater/cathode system in an old CRT with a slightly newer one should not affect the operation of the entire system.

[ppppenguin]

Most CRTs are soft glass. Working on borosilicate glass (Pyrex) as used in some pre-war CRTs is much more difficult. When I arranged that 6/6 rebuild in 1986 the glassblower at Display Electronics was an old-timer who had actually worked on these tubes before WW2. He knew how to make a graded seal using 2 or 3 different grades of glass to match the original borosilicate to the new soft glass pinch. The pinch is the part that has the glass to metal seals. It would have almost impossible to save the old pinch. Making a graded seal is a very skilled job indeed. Making good glass to metal seals is a whole specialist field of its own. There's a lot about it in this book: "Barry Vyse & George Robert Jessop, The Saga of Marconi-Osram Valve: A History of Valve-making at One of the World's Foremost Enterprises, Vyse Ltd, 2000 ISBN 0-9539127-0-1"

Have a look at Glasslinger's Youtube videos. He (she?) has done some very impressive work. https://www.youtube.com/watch?v=GAMRHcbE3g0

[LENINGRAD T2]

The glass master who made the film, as far as I can tell, is still working today. From his comments, it seems that he doesn't want to deal with CRT. Pyrex glass tubes and Pyrex glass are still available. So you can experiment with this glass today before you touch an old CRT. I think the main problem today is finding a suitable electron gun to replace - which is unlikely, or a working H/C assembly to replace from another CRT - which is more likely. The question arises whether an air-filled cathode from a good CRT will work in a new CRT. How long can it take from removing a good activated cathode from one CRT to inserting it into another and obtaining a vacuum again. Does such a time exist? In the era when CRT regeneration consisted of replacing the electron gun with the same one with a pumping tube, this was not a problem. Today, you can't reach for such a gun on the shelf. Glass connections can be obtained today by various methods, without using high temperatures.

[ppppenguin]

AFAIK, you cannot remove an HK assembly from a working CRT and re-use it. The cathode is activated in a vacuum and is spoiled immediately as it's exposed to air. If the correct HK assemblies are still available, which is not very likely, they have a relatively short shelf life unless sealed in a vacuum.

As for working with Pyrex, if a highly experienced glassblower said he'd rather make a graded seal to a soft glass pinch, rather than try to re-use the old Pyrex pinch he must have had a good reason.

"Glass connections can be obtained today by various methods, without using high temperatures. "
I doubt whether any such seal would withstand temperature variations and still remain airtight. Remember that the gun has to be heated to almost red heat using an induction heater to drive out unwanted gases. Ideally the whole tube is baked at high temperature while being pumped. This is not 100% essential as we proved with that 6/6 in 1986.

[Mike Watterson]

Can a pure nitrogen atmosphere be used for the work, with expert breathing via airlines?