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Non destructive Tube Reactivation, Updated!

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Non destructive Tube Reactivation, Updated!

Post by Alastair E » Wed Feb 06, 2013 9:24 pm

I've been doing some experiments lately on increasing emission of mediocre/poor CRT's.

The set I've done this to is the 'unboostable' trinitron type, a KV1320.

This set was pretty low having poor tracking and focus, just a nasty flat, faded greenish tinted pic was the best I could get.

The idea was given to me by something I've read somewhere of setting up the heaters and drawing a very small current for a number of hours.

I elaborated on this, looking at the effective emissive area of the cathode, (Around 2-2.5mm) a 'good' one should easily pass around 5mA without damage using G1 as the 'anode', for a short period.
This was confirmed using one of those cheapo 6" B/W portables, popular a few years ago, that had remained unused, 4mA was drawn with the set up described below.

The set up was as follows-

A 6.3V heater Transformer and a 12V lighting transformer were used.

The 6.3V supplied the heaters of the CRT to be worked on and also supplied into the 12V winding of the lighting transformer, one rated for 50W that I had around.

The mains connections of the lighting transformer therefore derived around 115V which was fullwave-rectified and smoothed by a 47uF 450V cap.

A Series Current Limiting resistor of 33K was connected to the positive of this 'HT' supply derived across the 47uF cap, which after rectification, smoothing etc, read 145V. I wanted to keep the voltage fairly low so as not to cause any of the usual arcing that destroys the cathode material like often seen with mains derived and some commercial 'boosters'.

A Voltmeter was attached across the resistor to monitor current draw.
The current with the 33K is limited to a little over 4mA (4.4mA ish)

The heaters of the KV1320UB were connected and allowed to run for 10-15 mins to fully heat up.

The negative of the 145V was connected to a croc-clip to connect to the cathodes of the three guns independently. The positive via the 33K was attached to the common G1.

On application of the cathode connection, the initial voltage across the limiter R was 20V.
Very slowly this increased over a period of minutes to 80V odd, then started to go down.
At this point, I disconnected and went on to the next cathode. Same again, although the highest volts was 98V before it started to fall again. Last gun did much the same, with 78V before the volts went down.

The tube was then left with only heaters for 10 mins.

On application of the cathode connection to the first gun 133.5V was found and the current was drawn through that cathode for 1 minute before going on to the next. Again 133.3V or thereabouts was read and again held for 1 minute. The last gun read 134.3V and the current drawn for 1 minute.
This time, the voltage did not fall with any of the cathodes, it remained above 133V.

The tube was then left for another 10 minutes and re-tested by seeing how much volts derived over the current-limiter. All three cathodes drew 133.4-134.3V, which equates to around 4mA.

The tube was then left to cool without heaters or 'HT' applied.

Then tube re-heated (6.3V, no over-volts) and re-tested. The same 133 odd volts was derived straight away indicating an increase in emission, over what the initial reading was.

Now here's the rub! The 3AT2 EHT rectifier has died in this set (O/C Heater) so will not be able to confirm whether theres any improvement or not until I receive a replacement. I ordered one from the USA this morning.
...............................

The above was copied from a post I made on 'That Forum' where it gained apparently little interest apart from one reply.

Since then, Ive received the new EHT Rectifier valve, (3AT2) and fitted it earlier on. I tried out the set and cannot quite believe the picture quality.
Where before, it was faded, green tinted and somewhat de-focussed with a little red flaring, it appears transformed, since setting up.
The picture is I would say near what a new CRT would have shown. Bright, Sharp and grey-scale tracking perfectly.

Its a shame that the test is somewhat unequal, as the set also had a new EHT rec due to the original failing O/C heater.
--The set beforehand didnt show any signs however of an EHT rectifier issue like ballooning etc, the rec heater just quit ...

I feel this 'non-destructive' method is worth a try as it places little strain on the cathode's oxide coating and no spark erosion of material which leads to either a very short life or destruction of the cathode entirely, also the important clearance between cathode and G1 is maintained due to no erosion.....

As the above is a Trinitron type tube, the cathodes are very small. (About 1.5mm diameter) as illustrated above, I limited current to 4.5mA
A more usual CRT with its larger cathode surface area I'm estimating by size(About 2.5mm) should take around 8-10mA...

If anyone tries this out--Please let us know how it turns out.....

 
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Re: Non destructive Tube Reactivation, Updated!

Post by rob t » Wed Feb 06, 2013 9:37 pm

wish i had known this 20 years ago as we scrapped quite a few large screen sonys that would not reactivate useing a b+k
rob t

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Wed Feb 06, 2013 9:56 pm

Wish I had known too! Maybe I could have brought out a better tube booster!

I'm hoping that others have success with this, it would alleviate the strain on whatever NOS/good CRT's are still around and give life to an otherwise dead set....
--How effective this is on a Really poor/dead tube I've not tried. The Sony was pretty grim but not completely dead, and I have no other 'dead tubes' to play with!....

Hopefully it isnt just a flash in the pan The Sony has been running now for a few hours with no signs of issues. I did at one point temporary reduce the heater-volts to 5.5V to see what if any effects there were.
I didn't see any apparent picture issues at a reduced voltage...

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Terrykc » Wed Feb 06, 2013 10:31 pm

Alastair E wrote:... Hopefully it isnt just a flash in the pan ...

I thought that was what you were trying to avoid in the first place ...! :=D

Excellent result!

Let's see how Trevor gets on with his ...

 
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Re: Non destructive Tube Reactivation, Updated!

Post by sideband » Wed Feb 06, 2013 11:01 pm

We've had a chat in the staffroom and have agreed that this thread should be made sticky. Appropriate glue applied!


Rich

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Michael Watterson » Wed Feb 06, 2013 11:28 pm

I reactivated a ZERO emission EBL31 in an AVO VCM161 by the theory of making heater hot and only taking a tiny current. (Needles didn't even move)

Initially I kept G1 near cut of and increased heater voltage till some current flowed and then reduced back to 7V and set current to about 1% of spec by g1.

After some hrs at 7V it was 20% approx of spec when tested properly at 6.3V heater
For this model of radio a friend was restoring for someone
http://www.radiomuseum.org/r/pye_p28p_2.html

During 1st hour I would reduce the current as it rose by reducing g2 and making g1 more negative. I think the key is just to have the cathode very hot to "reactivate" it and only a tiny current to avoid damaging the surface.

Though 20% sounds very low, it just limits the maximum volume to "normal listening level". The set is fine (now that all the dodgy caps done).

This EBL31 was missing top cap and wire too. I soldered a bent wire with a little hoop on and put epoxy around it. Then a few hours later fresh epoxy to hold a new top cap and soldered it.

Conclusion
I think your method does make sense. A lot of the "reactivators" are too "violent" for some kinds of emission issues. The cooking at low emission current is I think the key factor

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Thu Feb 07, 2013 12:23 am

The established theory of cathode emission-loss over time is the barium and thorium metal deposited on its surface evaporates, leaving the surface devoid of emissive components.

The heating to a higher temp than normal is one way to drive the thorium and barium metals to the surface--and it stands to reason? that a high current flow at high temps would strip/evaporate this new material away....

The idea to draw only a very low current at say,20% overheat therefore would allow the emissive material once at the surface to remain there and accumulate.....

I have a theory that CRT's which always seem to operate at very low currents--only 100 odd uA form a type of 'cathode-interface', a known issue that first appeared during the days of early computing where some valves remained at cut-off for some considerable time. Special cathodes were developed to combat this for the 'SQ' type valves used in later years....

--This sort of seems to be confirmed by the fact that worn tubes seem to fall off in the frequency --light to dark response as seen on a test-card grating for instance...

My idea was to draw a comparitively high (in relation to normal beam-current) through the cathode, but not so high that would cause it any perminent damage, as is the case of a normal tube reactivator.

It may well be found a combination of both high heat of cathodes and low/medium current through cathodes will be beneficial for some tubes. Time and experimentation will tell.....

I'm also a firm believer in reducing the heater voltage of a CRT somewhat to extend its life. Those worried about 'cathode-poisoning' under this condition need not worry, that only occurs when the electron cloud is insufficient to keep at bay any Ions present, --which at such low beam-currents tubes run at, and good cathodes able to supply many 10's of the current of the beam, this effect doesnt present itself....

The Sony sets for instance often drive the heaters too hot--the voltage of the 1320UB for instance is/was 6.7V (derived from a mains Tx). Ive placed a series-resistor to lower this to just 6.0v.

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Michael Watterson » Thu Feb 07, 2013 12:48 am

'cathode-poisoning' is more of a risk with dull emitter Thorated filaments than Oxides coated indirect cathode, they have a much narrower operational range.

Most 6.3V valves are happy even at 5V. Worth remembering if you have a 6.3V higher power rectifier pin compatible with 5V rectifier socket on a radio. It will work fine. It depends on application.

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Thu Feb 07, 2013 1:19 am

Michael Watterson wrote:'cathode-poisoning' is more of a risk with dull emitter Thorated filaments than Oxides coated indirect cathode, they have a much narrower operational range.

Most 6.3V valves are happy even at 5V. Worth remembering if you have a 6.3V higher power rectifier pin compatible with 5V rectifier socket on a radio. It will work fine. It depends on application.


Yes--Now you come to mention it, Some phono pre-amp circuits in the old tuner-amps used to run very low heater volts on valves like ECC83's to make them more 'linear' 4.8V seems to ring a bell, but may be wrong...

--It was not a known issue for these to fail prematurely that I know of. In operation, the cathodes are barely glowing in them--very odd to see!

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Michael Watterson » Thu Feb 07, 2013 11:53 am

Yes, if a valve at 6.3V can drive a 22k Ohm load and you are using a 220K Ohm load, then you are able to run at very much reduced heater voltage. It will likely last for ever.

How often does an EF80 used as IF amp need replaced compared to UL41 or EL41 :)

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Terrykc » Thu Feb 07, 2013 12:23 pm

Michael Watterson wrote:... How often does an EF80 used as IF amp need replaced compared to UL41 or EL41 :)

Come to that, has anybody ever had to change an EF80 in an IF strip ....?

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Thu Feb 07, 2013 4:42 pm

Hmm--Very interesting!

Not sure why your neck got overly hot--assuming you had the official heater-volts applied...

The 'dissipation' at 2mA and 175V (although thats not the voltage across the tube--some is dissipated in the load-resistor) would have only been 0.35W total circuit P.diss.

--As the volts C-G would be only a part of the full 175V the dissipation would be much less than that--Maybe there's some unknown forces at work!--who knows....

To be honest, I didn't see any huge difference in the neck heat--But then again must admit I didn't grab ahold of it during or afterwards!

TBH--I shouldn't worry too much about the heat causing a crack. Years ago, I saw a A56-120X running off 25V it was giving more light out the back of the set than the front!--The tube-base had carbonised too!
--Had to larf at that one!

Sometimes, a break to cool may be an idea--we are on new ground here so experimentation is the name of the game!

Glad you got some usable emissions from the cathode as it is. Sony has been on probably a total of 6 hours and appears unchanged.
Be interesting to see how your CME17xx holds up...

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Thu Feb 07, 2013 5:22 pm

Definitely a tube by tube basis for current....

Start off low--If that does it, stop there, but if not try a little more being careful on the type/size of the cathode/s

I think the very most important bit is not ever to allow the cathode to arc, I can think this can only be done by keeping the voltage below say--200V, again depending on tube type. Big old mono guns with large clearances between C-G could well tolerate much more, but tiny delicate things say--a Rigonda 6" with a pin-head cathode I doubt would tolerate much more than 100V and 1 mA

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Brianc » Sun Feb 10, 2013 8:23 pm

Bring on the CRM121s :D

I feel a little box coming up with a small transformer & rectifier, a resistor (perhaps a 5 watt pot) and a milliameter or a couple of terminals. Hmmm!

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Sun Feb 10, 2013 9:42 pm

Ah--Great stuff!

Ive not run the Sony for a few days, Ive been messing with the Bush....

I did think of getting hold of a tube-booster from ebay, summit like a B-K and modding it up.

If I can get a cheapo one, I might just do that. It would be nice to see the improvement in the same 'scale' as an official tester has....

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Mon Feb 11, 2013 12:41 pm

Hi Trevor-- Had you tested the emission before you tickled the tube with the method above...?

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alistair D » Tue Feb 12, 2013 12:00 am

Back in the mid 80s Television magazine produced a design for a tube re-activator that had the option of applying 1 second on, one second off pulses between the cathode and g1. The design rectified and smoothed the mains then fed it to the grid via a 15 watt pygmy bulb as a current limiter. The current through the tube was around 65mA when the bulb filament was hot and much higher before the filament heated up. I used one of these for many years but was never impressed with the success rate(there was often sparking inside the tube so maybe with hindsight I should not have been surprised) What I am wondering is that if the pulsed idea was applied to this low current design maybe greater success could be obtained. It should certainly help reduce the temperature rise on the tube neck.

Al

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Tue Feb 12, 2013 12:40 am

Hi Alistair--From another Alastair...

Well--Certainly. Its all rather experimental, maybe a pulsing would be better than constant current. The heating effect noticed by Murphy310 I cannot explain, the circuit full P-Diss is less than a watt or so...

I have no idea why that happened, I didn't notice it myself...

The old 15W bulb bopper was a popular device, Used them myself many times, but always thought them very vicious--All that arcing and banging about in the tube--Cant be good for it!
--When it arcs like that, its removing both good and bad stuff I'm guessing so could--and did sometimes--make things a whole lot worse! :=D

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Vintage Rob » Sun Feb 17, 2013 12:46 pm

I have a selection of rather old (poor) 12 and 15" mono crts.

I am very interested in trying to bring these back to some form of life.

I was very interested in this post but my small brain is getting stuck on this quote:-


On application of the cathode connection to the first gun 133.5V was found and the current was drawn through that cathode for 1 minute before going on to the next. Again 133.3V or thereabouts was read and again held for 1 minute. The last gun read 134.3V and the current drawn for 1 minute.


Does this mean I need to connect the cathode and the gun together for this reading?

I would be very happy to try this out and post back my results.


Cheers
Rob

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Sun Feb 17, 2013 9:10 pm

The only connections I used were the heaters, The Cathode and The First Grid.

My reference to 'gun' was for the Colour CRT I was messing round with. ie, moving onto the 'next-gun' meant that I was then working on a different Cathode and Grid of the same CRT..

I think I started with the 'Blue' moved on to the Green, then the Red.

For your Mono CRT's you'll only have the single Cathode-Grid to worry about.... :thumbr:

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Vintage Rob » Sun Feb 17, 2013 9:16 pm

Magic...thanks for that!!

My poor old brain is tired at the moment!

Cheers :thumbl:

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Vintage Rob » Mon Feb 18, 2013 3:30 pm

Afternoon all.

Monday...Shop shut today....time to experiment!

Right I am now getting set up to test some VERY low old 9" & 12" round tubes. Nothing to lose! They all measure naff all on the CRT tester

1 x Mullard 31-18
1 x Cossor 121K
1 x Mazda CRM92

I'll report back tonight with the results. :thumbl:

Cheers
Rob

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Vintage Rob » Mon Feb 18, 2013 8:34 pm

Here are the finding of todays fiddling
DSC_0212.JPG
Here is the tube emission before I worked on it.
DSC_0213.JPG
Here is a picture of the emission after some work. It was set up with 175v through a 10k resistor. It slowly went up to 5mA and stopped.
DSC_0217.JPG
I thought I would try increasing the voltage to 250v, carefully watching the cathode grid gap to make sure there was no arcing, still no change is current drawn so I slowly increased the voltage but pulsed the HT in one second bursts, still watching the K-G gap the current all of a sudden went up to around 16mA when the volts got to around 310v. at this point I switched off and just ran the heaters for 10 mins. Connected back to the tube tester.....
This is the result!!


I am now the proud owner of a great 12" Mullard tube.
It had been bopped years ago (little card on side of tube confirmed this). Popped in the box and marked knackered by the old TV shop they came from.

Sadly the Cossor didn't fair so well but the emission increased from nothing to half way into the red.
The mazda was the same but increased a little more.

So. It would seem that the Mullard tube seemed to fair best.

These tubes were knackered so had nothing to lose. I kept them just in case I needed to send on to be re-gunned!

I hope i have made sense in what I have written. Im naff at explaining things!! :lol:

 
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Re: Non destructive Tube Reactivation, Updated!

Post by mark pirate » Mon Feb 18, 2013 9:29 pm

I would like to give this ago with my CRM 92, but I really don't want to kill it off, as I have just restored the set that it's in :O

The set has a separate heater transformer, so I can use that, I have a variable HT supply that goes from 0-360v at 300ma, but I really need to be careful to limit the current, any advise?
Cheers
Mark

 
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Re: Non destructive Tube Reactivation, Updated!

Post by Alastair E » Mon Feb 18, 2013 9:43 pm

Vintage-Rob--

Thats a good result on the Mullard!--You mention that particular tube had been 'bopped' in the usual way before too?
--I have an old Hitachi CSP680 thats been done a few times in the conventional way, maybe I'll give that one a try, Its Really very bad--Seriously doubt there's any cathode left in it though...

Your other two,--maybe increasing the heater a little at a time to start things off may help--Time it seems helps some tubes, just leave them with say, 7V heaters (assuming 6.3V normal) and 'HT' applied for a while and monitor them....



Pppenguin,

I would certainly consider it. Its a fairly low-risk process, there's no destructive arcing going on, it may take some time for anything to happen, The Sony tube I did originally took a little while to get going....

Start off low, say max current, 5mA (33K, 150V) try that, then increase if no apparent improvement....
--I wouldnt go much above 15mA though, the cathode in any--even big old mono tubes --is quite small....

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