Tube reawakening

[Ian - G4JQT]

A couple of years ago I was lucky enough to acquire a NOS MW31-74 for my rebuilt TV24.

I haven't anything to compare it to so I might be mistaken, but I think the tube is a bit on the dim side (although that may be normal for a non-aluminised tube. The EHT seems about normal). Would it benefit from some gentle 'treatment'? As its had virtually no use I don't expect any aggressive rejuvenation is necessary.

I understand just leaving it on for a day or so will help, but does that mean the brightness is set to normal, dim, or off? Would just activating the heater and nothing else be sufficient? If so for how long? And the voltage - 6.3 AC or something slightly different?

Thanks.

Ian

[mark pirate]

I would use the set for several hours, In many of the sets I have restored the initial emission was not good, I tend to wake the CRT up by running it on a CRT tester before attempting to restore a set.

Some rise in emission quite quickly, some can take some time.
Generally, Mullard tubes always seem to wake up and give a decent picture.
I would not attempt to rejuvenate a tube unless it was really poor, however, my B&K tester has a very gentle "Clean & balance" mode, this is often all that is needed.


Mark

[Panrock]

Originally Posted by mark pirate

I tend to wake the CRT up by running it on a CRT tester before attempting to restore a set.

I am now wary of doing this and keep testing times to a minimum, having once observed slight phosphor damage after a lengthy test (with a 'Radar' Tester). I haven't a clue why this should have been and am happy to be over-ruled on this by someone more knowledgeable.

Another thing. Aren't we supposed to remove any ion trap magnet before testing? Why should this be?

Steve

[mark pirate]

I am now wary of doing this and keep testing times to a minimum

Maybe something to do with your tester? I have never had a problem with my tester.
If it is delivering the correct heater voltage, and you are only switching to emission test at intervals to check progress, I can't see why it could cause any damage to the phosphor?


Mark

[Heatercathodeshort]

Please don't attempt any form of 'testing' or 'wakening up' The MW31-74 should rise to full emission within an hours use. It should give an excellent picture. I presume it is a new Philips/Mullard and not a regun.

I totally agree with Steve's advice. Most tube testers can damage the cathode material no matter how gentle they claim to be.

If you MUST test a tube, carry out the actual test procedure [with grid to cathode voltage applied] for no more than 2 seconds, enough to see the meter reading.

In practise it is rarely necessary to test a tube. The classic symptoms of low emission are well known ie Flat picture lacking highlights that any increase in contrast or brightness gives the test card a shiny satin appearance, the overall result being a dim display.

Other faults in the receiver can produce a similar display such as changed value anode load resistors to video amplifiers but a flat tube is immediately recognized by even the workshop apprentice and would not need the application of a tester to prove the point.

I cannot stress this enough. Don't ruin or shorten the life of these old fragile tubes by over exuberant use of testers or even worse reactivators of any type. Just my opinion.
Regards, John.

[Ian - G4JQT]

Thanks for the suggestions. No, I wasn't suggesting it needed more than switching on. But it will have been off for about four months next time I have access.

It does come to life after an hour or so, but I want it like that after about 15 minutes after I've switched it on and to stay like that next time it's used.

So, for clarity, does one leave it on for this 'treatment' at normal or minimum brightness and for how long? Can this be achieved with just the heater on for a day or two? (Does the possibility of cathode poisoning apply here?)

I don't really want to keep the whole set on continuously for days to do this if it's not necessary.

Thanks.

Ian

[mark pirate]

It does come to life after an hour or so, but I want it like that after about 15 minutes after I've switched it on and to stay like that next time it's used.

If the CRT is good, it should produce a decent picture as soon as the set has warmed up.

Initially the CRT will improve over the first few hours of use, it should stay that way as long as the set is not left sitting for months on end.

If not, there may be a problem with the set itself, I have never had a CRT that drops back to the point it takes over an hour to produce a picture once initially woken up.

Check the heater voltage at the CRT after the set has warmed up, then check it at say 10 min intervals. It should be close to 6.3 volts. it is possible that the CRT has an internal fault, but highly unlikely.


Mark

[Nickthedentist]

As someone who hasn't got any vintage TVs, this was all a bit theoretical to me.

But I found an early 1980s 3V29 VHS video dumped by the WEEE bin last week, which looked like it had come from a loft clear-out. I plugged it in, and it sort-of worked, but the fluorescent display was so dim that it could barely be seen in a dark room. 5 mins later it was just legible. And 10 mins later, it was like new. Same effect, presumably.

[Heatercathodeshort]

[Dried up capacitors in the display power supply that have reformed. J.]

[Heatercathodeshort]

Give it about 30 mins a day for about a week. It should improve but I have known some tubes just to be slow heating. It is the best part of 70 years old and if NOS may require 'ageing' simply used on a regular basis for a while at normal brightness level. This is a very precise process carried out just before the tube is finally passed for dispatch.
J.

[Panrock]

There was once another thread about this, on the Golborne Forum.

http://golbornevintageradio.co.uk/fo...d.php?tid=8211

I am trying to make up lists of features of low emission and soft tubes, to clarify the difference between them. Which effect is generally dominant in a decades-old tube that's just been switched on: low emission or soft?

Are there errors and omissions in the following lists? There could well be!


Low emission/flat

Dim, low contrast picture (otherwise known as 'a picture as weak as pss', as a long-serving BBC Wood Norton lab tech used to put it).

Clipping on highlights.

Incorrect gamma.


Soft/gassy (A tube that showed all these effects would be really bad!)

Impossibility of obtaining a sharp focus.

Dim raster.

Flair (flattening and defocussing on highlights). Also often seen on triode gun tubes.

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 at the grid of the tube.

Picture can turn negative on advancing the brightness.

Milky getter.

Purple glow in gun assembly.

Differential gas focussing effect, causing astigmatism.


Steve

[beery]

Hi Ian,
A couple of points.
1 I assume you have an ion trap magnet fitted. After refitting the tube it needs to be adjusted. When the set has warmed up, it needs to be moved around and backwards and forwards a little until you get both the brightest raster and no corner shadows.

2 Check the A1 (g2) voltage with a digital meter (an AVO will load it too much). Measure it after 5 minutes and again when you found the tube has got brighter. It should be about 300v dc on this set.
Note that if you have to fix an A1 supply fault after refitting the tube you will find that the ion trap magnet needs a tweak, because the A1 voltage will affect how much the magnet deflects the beam.

Cheers
Andy

[beery]

Hi again,
Another note about the ion trap magnet is that the approximate position of the trap magnet (the starting point if you like) is with the arrow on the magnet (opposite side to the retaining screw) pointing towards the anode cap. Start with the magnet about 1/2" along the neck from where the base and neck meet.

Cheers
Andy

[toshiba tony]

I was in the trade in the day of thermionic devices and surface mounts chips but what was the reason for waking tubes? I remember certain sets had the heaters lit all the time (no warm up), but cathode poisoning was a problem for some sets. I remember a Thorn chassis (950mk11?) that we used to mod to remove heaters when powered down. Some PA high power valve amps had a standby switch on that left the heaters simmering but I can't remember if they suffered with heater poisoning, sorry, off topic but I have dementia now so I struggle, was in the trade nearly 50 years but I still swot up old gear when I can. Thanks anyone.

[Ian - G4JQT]

Originally Posted by beery

Hi again,
Another note about the ion trap magnet is that the approximate position of the trap magnet (the starting point if you like) is with the arrow on the magnet (opposite side to the retaining screw) pointing towards the anode cap. Start with the magnet about 1/2" along the neck from where the base and neck meet.

Cheers
Andy

Thanks Andy.

Yes, the ion trap is set for maximum brightness and from memory on testing when I finished rebuilding the set (literally - stripped down to nuts and bolts level, thorough cleaning and rebuilding with mostly new components and a gentle realignment where necessary) all voltages were within spec.

Unfortunately the set is in Norfolk. I'm locked down in Reading having tested +ve for Covid in early December. I recovered OK at home - although it was pretty horrid. Now getting cabin fever and I am getting desperate to return to my workshop in Fakenham and doing stuff!

However, I feel obliged to follow the rules and stay put until we can move about the country again. In the great scheme of things, a small price to pay, although Dry January really hasn't helped!

Ian