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Author Topic: REGLUING AND RESOLDERING PLATE CAPS IN OLD TUBES (811A TYPE)?  (Read 2017 times)
N6QWP
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« on: September 08, 2018, 02:17:40 PM »

Just received some very old 811A tubes and 2 of them have loose plate caps and one has the plate cap completely off.  The loose caps seem to have too much space between them and the glass.

Seeking the correct way to resolder those caps back flush and tight with the glass and what type of glue (epoxy?) to use.

Could I just get away with using some hi temp epoxy (JB Weld even?), or do they require special gluing to avoid "hot spots" on the glass?  How much to apply?

I heard that even fingerprints on the glass envelope can lead to problems at high temp, so wondering about the correct gluing procedure.  There are some spots of crud on the glass but I am sure that I can remove that stuff.

I am hoping that they are still sealed against leakage after all these years, but need to secure the plate caps before I can install them in my test platform to test for being gassy.
« Last Edit: September 08, 2018, 02:33:21 PM by N6QWP » Logged
KC4ZGP
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« Reply #1 on: September 08, 2018, 05:00:10 PM »

QWP,

Yes, the skin oil on the glass can cause uneven cooling of the glass envelope. Handle the tubes with paper towel, soft cloth.

Maybe hold the tube upside down, heat the cap and let solder flow downwards. Calipillary action will suck in solder.

Or remove the caps. Hope there's enough wire to rig your own plate connection system. This is ham radio. We overcome.

Are the tubes even worth the effort. 811 aren't that expensive.

Be safe. Lethal voltages within.

Kraus
« Last Edit: September 08, 2018, 05:02:35 PM by KC4ZGP » Logged
N6QWP
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« Reply #2 on: September 08, 2018, 05:52:14 PM »

Thanx Kraus--Am trying to "save" these old RCA 811A tubes for my 30L-1 amp which uses these tubes mounted horizontally.  From everything I have read, finding a few of the "good old ones" is worth the hunt?

It's a gamble all the way, but supposedly worth that effort ....if successful.  Because of that issue, I am spending waaaay more trying to find some of these relics (that are good), than I would on "new" tubes, but since they are horizontally mounted, they are supposed to be what will last the longest--if one can find "good" ones.

if it is all a "wild goose chase", then all I have wasted is some time and money.  I am looking to find enough of these to keep my old 30L-1 running, without experiencing the flashovers that I have in the past.

Still looking for input on gluing and soldering the plate caps.

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KC4ZGP
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« Reply #3 on: September 08, 2018, 06:56:18 PM »


Alrighty.

Good luck man and be careful.

Kraus
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N6QWP
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« Reply #4 on: September 08, 2018, 09:38:26 PM »

I'm starting to wonder if "glue" really is  required to hold the plate caps on Huh  Might it be that a "tight solder job" is all that is needed?

Can I accomplish a tight enough "bond" so that glue of any kind is not needed?  Will "regular" solder hold up to the high temperatures of one of these tubes without letting go?

Any OT's around that have encountered this issue before?

I have a pretty tidy investment in trying to solve this issue with these and other old tubes and would appreciate knowing if I am just wasting my time (and money)?
« Last Edit: September 08, 2018, 09:47:51 PM by N6QWP » Logged
KC4ZGP
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« Reply #5 on: September 09, 2018, 05:32:01 AM »


QWP,

If the caps are still on, they're probably still connected. Just be careful handling them.

Or heat the caps, the solder melts, the caps fall right off. Rework them.

Or replace the old 811 with Brand! New! 572s.

Kraus



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KM1H
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« Reply #6 on: September 09, 2018, 06:59:30 AM »

The closest you will come to the original these days is furnace cement.
https://tinyurl.com/y9n3o3dx

Follow instructions carefully and then:
Clean the glass area to be covered, lightly moisten and apply the cement. It helps to use heat to cure; I place the tube in an operating amp or jig with a modest weight to keep pressure on the cap. Dont disturb for 12-24 hours depending on the cement instructions. In a 30L1 you will have to get creative or cobble up a jig.

Super glue and epoxies have been known to crack the glass when shrunk under heat.

Carl
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N6QWP
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« Reply #7 on: September 09, 2018, 07:54:34 AM »

Thanx for that info on glue.  Still wondering about whether to unsolder and "tighten down" the cap as I am gluing....or just to fill the gap with the furnace glue.  I guess putting a weight on top while it is setting up will "sort of accomplish" the same thing?

I am perplexed at the obvious gap in two of the tubes (between the cap and the glass).  I suppose that is from someone pulling off the plate cap connectors, while in an amp prior to my getting them, after the glue had already failed?

I'd just like to do a proper fix, that will avoid putting too much strain on the glue after they are installed in an amp (when removing them in the future).  I do not notice gaps in other 811A's....the caps appear to touch the glass (no visible glue between them).

One related question:  Will the heat from running the filaments alone be enough to cure the glue?  It will be much easier to rig up a vertical jig (for gluing) if i do not have to deal with HV.

Is the glue actually needed???
« Last Edit: September 09, 2018, 08:15:48 AM by N6QWP » Logged
AC5UP
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« Reply #8 on: September 09, 2018, 11:07:48 AM »

I am perplexed at the obvious gap in two of the tubes (between the cap and the glass).  I suppose that is from someone pulling off the plate cap connectors, while in an amp prior to my getting them, after the glue had already failed?

Gluing to glass is a challenge as the smooth surface doesn't offer much in the way of nooks & crannies to bond with...  If you've ever read the instructions for Gorilla Glue you learned that it's a polyurethane which reacts to water by expanding.  When the pieces being glued are clamped securely this expansion forces the glue deep into the surface pores for a superior bond.  https://www.gorillatough.com/product/original-gorilla-glue/

I'd recommend it except for the service temperature limit of 200 degrees F and the challenge of clamping the plate cap during cure.

Gorilla Weld (epoxy) has a temperature limit of 300 degrees F and might be worth considering.  https://www.gorillatough.com/product/gorillaweld/

At times I'm impressed by how rare it is to see a plate cap come loose, considering the thermal cycling.  As for the separation, likely caused by pulling the cap off.  The wire used is intentionally soft as a brittle wire would be more prone to break flush with the envelope.
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N6QWP
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« Reply #9 on: September 09, 2018, 11:34:08 AM »

While I am looking at these 811A's, I notice that one of them has no silvering inside the glass....only a white, cloudy residue.

I'm thinking that this might be a clear indication of a "bad tube"?  Anyone familiar with these enough to  have thoughts one way or the other?

I thought that the silvering was what gettering uses to get rid of gas.  If there is none left AND it looks like it might have been replaced by the white cloudy area, is that a sign of either a previous flashover or of a bad (gassy) tube?

PS--Upon further examination, I notice that another of these tubes has the beginnings of white residue opposite where the "getter" is located.....and there is a faint misty ring  of silvering "pushed about a third of the way up around the envelope".


Another indication of a bad tube?   Looks like I may not even have to test these two?
« Last Edit: September 09, 2018, 11:48:14 AM by N6QWP » Logged
AC5UP
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« Reply #10 on: September 09, 2018, 12:27:03 PM »

While I am looking at these 811A's, I notice that one of them has no silvering inside the glass....only a white, cloudy residue.

Not a good sign.......

The silvering inside the glass is the 'getter', a mercury compound that reacts with gasses when heated.  In the case of oxygen this forms mercury oxide with a rusty red color and is a tell-tale for 'slightly gassy'.  Cloudy white usually means there's a hairline crack in the envelope and it's far too gassy to be rescued.  Slightly gassy tubes can be revived by running them for a few days on filament only, but my experience has been that's often a waste of time...  Better to replace the tube than limp along with a part well beyond its prime.
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G3RZP
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« Reply #11 on: September 10, 2018, 01:38:54 AM »

I think it's magnesium, not mercury. But if the silvering is white, the most useful thing you can do is wrap the tube in newspaper, hit it with a hammer to break the glass, dig the glass out of the base and unsolder the wires from the pins. Dump the broken glass and tube insides.  Then you have at least got a 4 pin base that can be used for things like making plug in coil forms.....
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AC5UP
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« Reply #12 on: September 10, 2018, 09:23:57 AM »

Late last night I got the bug to do a little web crawling regarding this subject.....

For years I've called the silver splotch inside the glass the 'getter'.  Wrong.  The getter is the metal ring opposite the splotch, the splotch itself is the 'keeper' and forms from material migrating off the getter ring.  It's condensation.

Click here for more:  https://en.wikipedia.org/wiki/Getter

As for the composition of the getter ring, this web page  http://k5jxh.com/odds-ends/gas.html  and Wikipedia agree that barium is the most common in consumer grade tubes.  Larger power tubes that run hotter may use a mix of metals like aluminium, magnesium, calcium, sodium, strontium, caesium and phosphorus.  This doesn't explain the silver splotches we've all seen with a rust red outer ring, but it is what it is.  I assumed mercury and mercury oxide 'cuz it damn sure looked like it.

I won't vouch for the accuracy of every statement by K5JXH on the second web page quoted, but it's worth a click for some very cool pictures...  Look at this one verrrry carefully:



This one caught my eye as well:



BTW:  K5JXH also mentions the importance of restoring the mercury to the base of a mercury-vapor rectifier tube after being moved.  Put the tube in a sturdy sock with the pins facing the toe.  Swing the sock in a circle like an impromptu centrifuge for a few moments.  Mission Accomplished.
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N6QWP
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« Reply #13 on: September 10, 2018, 11:25:05 AM »

Great reading AC5UP--Thanx for sharing that source....lots of good info.  Answers a lot of questions about gassy tubes.

Blue glow is OK....

Purple is NOT

Where the gas is glowing also important......(did not know that)
« Last Edit: September 10, 2018, 11:39:48 AM by N6QWP » Logged
KM1H
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« Reply #14 on: September 10, 2018, 04:09:37 PM »

Gettering is the process of removing any
Quote
gasses
left over inside the tube from the manufacturing process. Over time and often abuse those gasses will be released and requiring regettering.

The other "gas" is outside air and is completely different and rather than getting into 20 pages of bickering over air content lets just keep them completely separate.

Air ingress destroys a tube.

Anode outgassing can be successfully removed from the vacuum and a tube restored to use. No guarantees as the amount of gas to be dealt with varies considerably.

Since gas also occurs in a ceramic metal tube there is no visable color to see, just the blown fuse and other components from an internal HV arc. The regettering process is also different. Air ingress still results in a paperweight but some tubes can be rebuilt.

Carl

Carl
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