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Author Topic: Can This Tube be Repaired? anode heat damage  (Read 2048 times)
N2EYE
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Posts: 85




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« on: November 21, 2008, 07:34:22 PM »

 I  damged a 572b quartet  soldering  directly to anode caps.  I know, I am wearing my dunce cap with great humility.  Can these otherwise good tubes (fairly new matched) be repaired?
 
 I see a hole on top  center of each tube cap where a rod  (graphite  anode?) pokes through, flush with top surface. That opening was a continuous surface connecting anode to tube cap before I melted it.  What can I use to re-solder this material to cap again?

I suspect I melted whatever substance connected the anode to the tube cap.   I searched web for info to no avail.  Any suggestions??
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W3LK
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« Reply #1 on: November 22, 2008, 06:44:02 AM »

<< I damged a 572b quartet soldering directly to anode caps.>>

Why would you do that? Plate caps are cheap.

73,

Lon - W3LK
Naugatuck, Connecticut
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W5RKL
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« Reply #2 on: November 22, 2008, 08:15:24 AM »

I've seen this happen to small beam power tubes such as 6146s. I had a pair of 6146As in an SB-400 transmitter that had the same problem. The caps came off exposing the anode's connection. Other than the anode cap coming off the tubes were fine.

As a test, I cleaned the anode cap then pressed the cap back onto the tube ensuring the anode tip protruded through the hole in the cap. I applied solder to the cap and anode wire which, making a good clean connection. I checked the cap to see then placed the tubes in the final.

Test show this procedure worked. The tubes produced the specified output without any adverse affects.

Having said that, I do not recommend this as a permanent fix or a money saver even though it worked! Just replace the tubes with new ones, it's safer and more reliable.

73
Mike
W5RKL
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W8JI
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« Reply #3 on: November 22, 2008, 08:26:51 AM »

You can solder the caps back on. That's all they did at the factory, besides some high temperature glue.

First, go find some very high temperature glue or glass bonding cement.

make sure you do that!

Next, take the caps and remove the solder in the top ends. You can even do that by heating them and dropping them on a hard surface to knock the molten solder off.

Make sure the wires sticking out of the seals are clean and shiney on the upper tips.

Put high temperature glue or cement on the tube seal and push the cap on so the wire protrudes, and let it set up a while. Make sure it is seated and the glue fully dries.

Wrap the bottom of the cap carefully with a damp strip of cloth and tape it in place. Use a very large iron to heat the tip of the cap quickly while flooding the cup in the top of the cap with solder.

It will be just like they did it at the factory when you are done.

I'd practice on a  few old tubes first.

73 Tom
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N2EYE
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« Reply #4 on: November 22, 2008, 10:32:42 AM »

Thank you gentlemen,  
I will try to re-solder the tube caps.

Lon: I was trying to decrease capacitance on the Tune side of the pi network in a homebrew amp. 572b's  high output capacitance is problematic   when using air variable caps in the pi network. The large surface area between  tube caps and the B+ connectors were 4 extra capactitors I did not need. It worked the first time. When I had to remove the wires a few weeks later, I used a large gun rather than my pen iron and ....

Tom, Mike: Your replies were encouraging. I miss my amp/QRO  as QTH is at the bottom of cliff.  
The caps are still firmly on the bottles. I have been conservative with the heat.
The trick seems to be  doing it with tubes upside down so gravity makes  solder bond to anode. The edge of the inner-hole in cap is sooo small surface area-wise.  Regular solder and flux did not bond to the anode rod.  My next attempt will be  silver solder with a drop of that acid flux I used on nichrome wire.

 Bad idea??
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G3RZP
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« Reply #5 on: November 22, 2008, 10:34:10 AM »

Solder as per Tom's instructions.

Adding to what Lou said, get some plate caps. They're cheap enough. Or make some from copper flashing and maybe a 4-40 screw and nut.

That has the small advantage giving a bit more heat dissipation round the tube plate cap.
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W8JI
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« Reply #6 on: November 22, 2008, 02:47:45 PM »

Use regular soft solder. That's what the manufacturer uses.
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W3LK
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« Reply #7 on: November 22, 2008, 04:52:08 PM »

<< Lon: I was trying to decrease capacitance on the Tune side of the pi network in a homebrew amp. 572b's high output capacitance is problematic when using air variable caps in the pi network. The large surface area between tube caps and the B+ connectors were 4 extra capactitors I did not need. >>

W8JI will correct me if I'm wrong, but I doubt soldering directly to the tubes had any effect, one way or the other. If capacitance in the connections was a problem, the amp manufacturers would not be using plate caps. Smiley

73,

Lon - W3LK
Naugatuck, Connecticut
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