CRT (Re)Building and ageing - details of processes

[Heatercathodeshort]

[Continued from How to treat a pre-war tube]

Hello,

A great deal of information regarding tube rebuilding has already been posted on this site some months back. I wont bore you with it all again.

When a crt has been evacuated and the getters fired, a voltage of around 90 has to be passed from cathode to grid via a 5watt lamp [with the heater activated] for some hours. At first the lamp does not light but after a while as the cathode starts to liven up the lamp begins to glow faintly as current passes. This process is called 'aging' and is identical to that carried out by the leading tube makers. [PLEASE DO NOT DO THIS WITH YOUR VINTAGE TUBES]

With old crt's that are passed their best the small getters fitted have a struggle to cope with stray gas particles that have been released from the gun assembly or tiny impurities within the 'vacuum'. As the tube warms up and current begins to flow, the tired getter livens up and absorbs much of this gas. [Its a TINY amount] It takes some time for this to be released back into the tube and with normal daily use the warm up period is very short. leaving the set for many months allows the gas to once again be released from the getter and it starts all over again! I worked very closely with one of the best rebuilders of all time during the 60's and 70's and learnt a great deal from him, some of which I have already posted. Hope this partly explains the long warm up query.

Regards John.

[Duke_Nukem]

This process is called 'aging' and is identical to that carried out by the leading tube makers.

Well, no, the process described is “rejuvinating”. The original manufacturing process varied somewhat between manufacturers but usually involved operating the cathode at different but carefully chosen heater voltages with and without the application of any electrode voltage. At least one stage would be performed whilst the CRT was still on the pump as the initial reaction of creating an oxide cathode involves the release of a fair amount of gas.

As far as I can tell, the process is first combining the constituants of the cathode to produce material which then, under the influence of an electric field, migrates to the surface.

... leaving the set for many months allows the gas to once again be released from the getter and it starts all over again!

As far as I can tell, the gettering process is a chemical reaction and unlikely to release gas once “adsorbed”.

Certainly the effect of long term non-use is mainly one of emission and not to do with gas. This can be demonstrated using a CRT tester where you can verify that there is no gas current but emission rises steadily.

One of the barriers to CRT's in very early TV was ion fact that they _needed_ gas in order to focus the beam ! Worked great when brightness didn't change (e.g. a scope) but lousy for telly purposes).

TTFN,
Jon

[Heatercathodeshort]

The rejuvenation process involves actually sparking off a small layer of the cathodes emitting surface and is not the same process as tube 'aging'. Heating the cathode without a very slight current flow does very little if anything at all. There are pictures of tubes with lamps connected being aged at the Mullard tube factory and this is how all the tube builders did this. To prove the point about the getter, the early versions of the Py32 rectifier valve suffered from internal gas after around a years use [1957] This resulted in the valve glowing a bright purple for around an hour until the getter mopped up most of the gas. The Ht slowly rose as the glow diminished only to be repeated the following evening. The PY33 replacement did not suffer this fault due to a larger getter. I worked with John Brown of Central Tubes since I was a very young lad. I purchased my first regun from him [MW53-20 ] when I was 16. He re gunned all types, EMI, metal cone, bonded faceplate, rimband and all the BBC's colour monitor tubes including the 27" schools Cathodeon's. John would give me tubes for test and I would visit his works and watch all the processes. It was truly fantastic. Strangely I spent the Bank Holiday weekend at the Rudgwick steam and Country show with a gas analyst who now works for BOC that worked with John 30 years ago. Regards John.

[Duke_Nukem]

The compounds initially deposited on a cathode during manufacture have zero emmisive properties. However suitably heated they react together to produce the elements that will form the emissive surface. Problem : these elements would just love to react with some oxygen. Thus the reaction must only be initiated in a vacuum. The reaction releases copious amounts of CO2, thus the process must be done whilst the CRT is still being pumped. This is a chemical reaction and no electric field is necessary.

The elements then need to diffuse to the surface. This is done by over-running the CRT heater so as to achieve a particular temperature and applying an electric field. This was often done at different temperatures and voltages for specific times that would vary between manufacturers but the net result is to produce an emitting surface whose active cross section is only a few atoms thick.

During operation, the surface does evaporate, albeit fairly slowly. However as this occurs, more of the emissive material diffuses to the surface since after all there is an electric field present.

The above description comes from various sources, including Spangberger (spelling?) and Telman.

What I wondered is after a long period of non-use the surface would begin to re-diffuse back from whence it came - very slowly of course, but it wouldn't take much for there to be a recoverable drop-off in emission.

Py32 rectifier valve suffered from internal gas after around a years use [1957] ... The Ht slowly rose as the glow diminished only to be repeated the following evening. The PY33 replacement did not suffer this fault due to a larger getter

Must admit not heard this one - come to think of it I've not seen a PY32 in action, always been 33's. I'll have a rummage in my box of valves as I never looked that close at the two. The only difference I'm aware of is that the PY33 used the new method of cathode construction originally applied to UHF small signal valves (!) - the cathode surface was much flatter, therefore the anode could be placed closer to the cathode, hence the much lower forward drop.

Phew, its warm, gonna have to take my anorak off now .

TTFN,
Jon

[Heatercathodeshort]

hello Jon,
I burnt my anorak many years ago but by the look of it I will have to buy a new one..with wooden toggles. I can only state how the aging process was carried out to full success. john's tubes lasted for ever, a bit like the AW43-88 Mullards. They were still showing full emission when the sets were scrapped some 8 years or so later.
At very busy times John would let me have an 'unaged' tube. When installed in the set it would show a very flat picture lacking mainly in contrast. Focus was always sharp. If I ran the set at full brightness thus creating a current flow from cathode to grid the emission would rise slowly to maximum after around around 4 hours. The best tubes were always the ones that had been under aged slightly allowing the final hours to brighten up in the set. A similar process was used by LUBECK and RE-VIEW, also very good rebuilders.
Regarding the PY32's. The original version was a twin anode style and shape very similar to the PZ30. These tended to be OK but they then resdesigned the valve, reducing it to the shape of the later PY33 and fitted it with a small getter. This was the disaster version and I can remember visiting customers homes where the set had been left on to warm up for over an hour! The wall at the back of the set was faintly lit with purple light from the gassy PY32. As the getter gasped at the task it eventually receded and the H.T. came up....[The Ferguson 306T comes to mind most] The later PY33's had a huge getter on the top of the electrode assembly and gave around 10v more H.T. They rarely failed. Regards John.

[FERNSEH]

As a follow up to HKshorts essay I can remember of a similar CRT aging technique employed by a CRT rebuilding firm which was located near Durham.
The CRT heater voltage was supplied at the correct voltage and then a potential of about 60 volts was applied between the cathode and grid via a limiting lamp.
The procedure took many hours until the lamp lit.

The PY32 rectifier valve in question with the slow warm up problem was the version with the tubular envelope and having an old style pinch to mount the electrodes.

DFWB.