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Author Topic: How to getter a 3-500 tube  (Read 58935 times)
W8JI
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« Reply #15 on: January 04, 2011, 07:37:49 AM »

OK, if I install a tube that shows negative grid current with no HV on the plate, the tube has a short and is junk, Right?  Throw the tube away so you don't come back 5 years later and sell it or install it in an amp. Right?

Right. If it shows current with little or no HV in a good amplifier the tube is junk.

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If a tube does not show negative grid current with HV at any level up to full operating HV, AND shows no blue gas color at HV idle, then you begin to drive the tube with RF and it arcs blue, What is the fate of this tube?  Can this one go in the getter jig?


I've sucessfully recovered tubes that arc or flash over at 2000 volts, but the odds of them staying good or even recovering are a lot less than tubes that only arc or flash at 4000 volts or more. I never ran statistics because I've only looked at 20-30 tubes and the sample is too small, but I can't recall ever being sucessful with one that failed below a few thousand volts.
Remember it is the PEAK voltage that is the problem, not the dc voltage. When RF is applied peak voltage can be much higher than dc. Look at the voltages here:

http://www.w8ji.com/demonstation.htm

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I am trying to determine the parameters of a "junk" tube and a tube that might be able to be saved. 


I've had tubes go right back to being bad, even those that initially high potted to 4kV or more, but I have a lot less luck with tubes below 2-3 kV.


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Question:  I would think that a tube that shows negative grid current with no HV with just a small amount of filament voltage (bringing up with a variac) would have a pretty hefty short that should be able to be measured on an ohmeter.  One tube I did measure a short indeed, the other I did not.  What is conducting the current in that tube between filament and grid that is not detectable?

Heat moves the filament slightly. I test for grid-cathode with reverse bias current on a hot tube. My bias supply is 1000 volts maximum and current limited to 20 mA. An ohmmeter not only has too low of a voltage, it also can't safely be used with full filament voltage for some meters. This is because there might be half of filament voltage put across the meter.

I also bang on the tube while watching for faults.

The test fixture I built for 8877's back when Eimac was having problems with H-K shorts ran the filament voltage up and down every few minutes, and had a fairly high voltage negative bias on the grids through a relay. The relay would latch if a tube ever conducted. We ran tubes for at 24 hours that way to cull out the bad ones. The tubes required a fan even with just filament power. It tested several tubes at once. :-)
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W0BTU
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« Reply #16 on: January 04, 2011, 07:51:44 AM »

... I am trying to determine the parameters of a "junk" tube and a tube that might be able to be saved.  ...

From reading this post and dozens of others like it, I'm starting to come to the conclusion that the parameters of a junk tube is, if it has "3-500Z" printed on the glass. Geez-o-petes. What did Eimac do wrong? All glass power amp tubes aren't that bad, are they?
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AA4HA
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« Reply #17 on: January 04, 2011, 08:17:27 AM »

From reading this post and dozens of others like it, I'm starting to come to the conclusion that the parameters of a junk tube is, if it has "3-500Z" printed on the glass. Geez-o-petes. What did Eimac do wrong? All glass power amp tubes aren't that bad, are they?

I think you jest, the 3-500's are just a very old design that has been out of manufacturing for quite a few years. The folks who are reviving 3-500's are trying to extend out the life of amplifiers that are based around that particular tube. As quantities of recoverable tubes continue to decline then the measures that folks will go through to revive the near dead will get more extreme. Eventually there will be one ham who owns the last 3-500's who relearns the art of glassblowing so they can rebuild tubes from the inside. (or really tiny people in space suits who can replace a burnt out filament).

Since it is glass there is always a minute amount of gas that will leak in through the glass itself or around tube seals. (glass really is a super high viscosity liquid, not a solid).

I would go so far to say that the two markets that have continued to keep ceramic tubes alive are in industrial induction heating applications and MRI machines. If Eimac was just making tubes for hams they could have five employees working out of a barn. Industrial/Medical customers are their bread and butter.

Tisha Hayes
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WB2WIK
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« Reply #18 on: January 04, 2011, 08:58:12 AM »

... I am trying to determine the parameters of a "junk" tube and a tube that might be able to be saved.  ...

From reading this post and dozens of others like it, I'm starting to come to the conclusion that the parameters of a junk tube is, if it has "3-500Z" printed on the glass. Geez-o-petes. What did Eimac do wrong? All glass power amp tubes aren't that bad, are they?

Eimac hasn't made this tube in a very long time.  Others still do.

I think it's a great tube, if you buy a new one and use it a lot.  They can last 10+ years in daily service, yielding an annual cost of $17.50 for a tube that can produce a kilowatt output on HF.  Not a bad deal at all.

The "problems" are mostly with storage life.
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W1QJ
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« Reply #19 on: January 04, 2011, 09:42:32 AM »

You Know what guys?  I think you talked me out of the getter jig.  It seems like the success rate is low and the problem most likely will return again in time.  Problem is I have bought several hundred SB-220 amps over the past years and unfortunately the sellers have not been very honest.  You pay a price that should reflect a working set of tubes and you get one or both that are bad.  I could suck this up if I was buying one amp and needed to buy a set of tubes that would last me 10 years, but with what I do, if I get a bad tube or tubes, I am out.  I can't tell you how many of the amps I bought that were "AOK" had probelms!  More than I care to imagine.  Thankfully I can fix any problem that comes down the pike except for the bad tubes.  Not to mention how many of these amps had bad band switches.  Thank God that does not affect me, but I fear for the poor guy who is looking for a good working HF amp and buys it.  After several hundred SB-220's in out of here I estimate 7 out of 10 amps have a burned band switch contact.  usually 15 or 10 or both, but I have seen eveyone along the way.  So, got a bad SB-220 you want to unload, let me know I'll take it off your hands.  Thanks Tom and the rest for the good info on this topic.  Lou
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KH6AQ
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« Reply #20 on: January 04, 2011, 09:48:45 AM »

Transistors never need to be gettered.
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W1QJ
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« Reply #21 on: January 04, 2011, 11:36:37 AM »

I guess some day I'll just have to give in to solid state amps like I did with transceivers.  It may be sooner than I think. Grin
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G3RZP
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« Reply #22 on: January 04, 2011, 02:23:19 PM »

>Transistors never need to be gettered.<

Doesn't mean that they are good, or necessarily, that they will stay good.

30 + years in the semiconductor industry doesn't convince me....even having done space grade stuff.
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W8JX
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« Reply #23 on: January 04, 2011, 03:12:10 PM »

Transistors never need to be gettered.

Tubes generally never need to be either if they are run properly.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W0BTU
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« Reply #24 on: January 04, 2011, 05:27:06 PM »

I think it's a great tube, if you buy a new one and use it a lot.  They can last 10+ years in daily service, yielding an annual cost of $17.50 for a tube that can produce a kilowatt output on HF.  Not a bad deal at all.

The "problems" are mostly with storage life.

I agree 100%. I wasn't very clear when I said what I did. However, not everyone can avoid storing them, and that's the big "if" here.

My question is whether tubes like the 3-1000Z, 4-1000A, 4-400A, 833A, 833C, and other glass tubes have a storage problem like that -- a pin seal leakage problem -- whether made by Eimac or somebody else.
« Last Edit: January 04, 2011, 05:30:13 PM by Mike Waters » Logged

K6AER
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« Reply #25 on: January 04, 2011, 05:43:39 PM »

Three factories in China are making 3-500Z and ZG's. The need for these tubes are fed primary by the amateur radio market. Most manufactures of industrial equipment use ceramic tubes or have gone solid state.

MRI's stopped using tubes 6-7 years ago. The days of cheep pulls are long gone.

Aside from rotating glass tubes each year so they can be gettered, the proper use of the tubes has nothing to do with the glass seal leaking or not. That is a function of manufacturing luck and some skill. I have scene EIMAC's fail that were just manufactured and Chi Com tubes run forever.

The most important decision is to buy the tube from a supplier with a warranty.
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W8JX
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« Reply #26 on: January 04, 2011, 05:52:00 PM »

the proper use of the tubes has nothing to do with the glass seal leaking or not.

While you cannot prevent leaks (and all tubes "leak" to some degree) you can mitigate the effect of it by making sure tubes get to proper operating temperatures the absorb what gasses may be present.
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W0BTU
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« Reply #27 on: January 04, 2011, 06:20:04 PM »

Let me ask this question again, in another way.

MANY commercial transmitters in the USA use 4-400As. This is not an uncommon tube for amateurs to use. Do 4-400A's (or similar tubes in the same power range as the 3-500Z) have the same storage problem as the 3-500Z or not?


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KM3F
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« Reply #28 on: January 04, 2011, 09:49:54 PM »

Doing this is a good learning tool and experience but to expect to recover any tubes with any degree of certainty for others is a bit of over expectation and risky for friendship and reputation.
You must be awhere of max grid 1 dissipation or you will melt or distort the grid structure.
The plate anode needs at least 400 watts of dissipation to even begin to show color for gettering action on the 3-400/500 tubes.
You don't have to draw grid current unless there is some special reason or you want to see if grid control is proper.
About the only special time you may need to attempt to draw grid current is to burn off any contamination that causes the grid to heat and begin to lose negative bias control over plate current and causing a run-away condition.
At zero bias or grounded grid, any combination of voltage and current that will cause the anode to dissipate 400 watts or more is enough.
Then the time and activation cycling has to be played with for each tube in order to see if a recovery is becoming worthwhile.
Good luck.
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W8JI
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« Reply #29 on: January 05, 2011, 07:26:56 AM »

Let me ask this question again, in another way.

MANY commercial transmitters in the USA use 4-400As. This is not an uncommon tube for amateurs to use. Do 4-400A's (or similar tubes in the same power range as the 3-500Z) have the same storage problem as the 3-500Z or not?




Yes, they all do. Some problems come from the kovar used to seal the pin to the glass. It is subject to deterioration from moisture.

I remember back in the prime of Eimac. Their engineers actually visited larger OEM's. Conversations during visits were worth their weight in gold. Jim Aurand and the senior engineer from Eimac Salt Lake, where the glass tubes and 8877 were made, told me how RCA overkilled the dissipation of a driver in a BC transmitter and was having life problems with tubes. They said they dropped back from a 4-400 to something smaller (perhaps a 4-250, I can't recall) and the life problems went away. This is because the anode started to show slight color.

We also had deep conversations about the grids, filaments, and other things. This was because I wanted to know exactly how hard we could run the 3-500Z in the SB1000/AL80 amp, and what life or damage we could expect.

While grid current in the gold plated grids of 3CX800 and 8877 tubes is critical, their senior engineer was very clear that grid current in the 3-500 was not a major concern except for proper tuning. He said the grid dissipation limit was set in tests by secondary emission, and not by heat failure or life.  He explained the test method.

Since this came from the actual tube design engineers, I go by what they told me. (I also saw they had hand-drawn cartoons about nichrome parasitic suppressors on the walls in engineering, so they paid attention to the outside world. )

73 Tom

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