Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 2 [3] 4 Next   Go Down
  Print  
Author Topic: How to getter a 3-500 tube  (Read 58881 times)
W0BTU
Member

Posts: 2241


WWW

Ignore
« Reply #30 on: January 05, 2011, 10:00:45 AM »

So that's it. Thanks, Tom! :-)

I thought 833A's and a few other glass tubes didn't have that problem. I stand corrected.

That's funny about Measures' suppressors. I'd have liked to have seen those cartoons.
Logged

KH6AQ
Member

Posts: 7761




Ignore
« Reply #31 on: January 05, 2011, 04:38:29 PM »

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.

With zero volts on the grid a 3-500 needs 3 kV on the anode to dissipate 400 watts. This voltage might be too high for a gassy tube.
« Last Edit: January 05, 2011, 05:02:28 PM by DAVE CUTHBERT » Logged
KM3F
Member

Posts: 908




Ignore
« Reply #32 on: January 05, 2011, 05:24:18 PM »

Here are more things to keep in mind.
If the grid has an issue, it likely is due to being over driven such that the grid structure has sagged out of place and ended up very close to the filament.
The effects of this condition can make the tube sensitive to vibration and temperature changes during operation.
Be awhere that switching a tube's filament on creates a physical shock to the filament that can make it actually move. This could result in a short with the grid that has been abused.
The basis for this movement is the filament resistance is very low when cold such that high inrush current occurs before the filament heats and it's resistance increases.
As well, a test of plate current (Ib) VS  negative Grid bias (Eg) may not track a known good tube because the grid or it's turns are out of place physically or even one or more grid turns had been burned out and lay at the bottom of the tube structure.
Leakage should be rare in this type structure due to how the elements are mounted to the stem unlike a small receiving tube that often suffers from leakage between the elements having a leakage path accross the mica structure that hold all the elements in there precision alignment.
All of these items are the basis of pass or rejection at the time of mfgering.
Good luck.
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #33 on: January 06, 2011, 05:22:03 AM »

Here are more things to keep in mind.
If the grid has an issue, it likely is due to being over driven such that the grid structure has sagged out of place and ended up very close to the filament.
The effects of this condition can make the tube sensitive to vibration and temperature changes during operation.

Not likely in the 3-500Z because of grid materials and construction. Most grid-to-filament shorts are the result of manufacturing issues or physical shock. The grid is very rugged and pretty much immune to damage from exciter power. Not so with grids in oxide cathode tubes like the  8877 or 3CX800A7, but in thoriated tungsten tubes are different.

Quote
Be awhere that switching a tube's filament on creates a physical shock to the filament that can make it actually move. This could result in a short with the grid that has been abused.
The basis for this movement is the filament resistance is very low when cold such that high inrush current occurs before the filament heats and it's resistance increases.



This is largely another non-issue with the 3-500. We'd be hard pressed to find an amplifier that stresses the filament to the point of moving it. The real dangers seem to be repeated heating and cooling as tube dissipation moves from a cold tube to a few hundred watts and back again over and over and over, not the turn-on shock.

The only amp I ever saw that probably had turn-on shock failure was one that had the filaments turned off and on every single transmission. In an effort to extend tube life someone actually wired the filament to his TR relay system with a little time hang delay system. Series filaments can be bad news too.

I had long discussions with Eimac's senior engineer about filament inrush. He told me when they evaluated that in the lab, they strapped the filament pins of 3-500Z's directly to 200 ampere transformers and after hundreds of hard cycles could not crate a problem. He pretty much dismissed the idea that inrush was a concern.

More than that, if we measure the inrush of amplifiers, we are very hard pressed to find any that go much beyond a 2:1 current ratio on start up. Heath took a hard hit back in the 70's for inrush in amps. This was part of hysteria started by people who just couldn't take the fact that tubes go bad, and were assigning blame to inrush. Articles were written and published about correcting inrush without actually measuring inrush. The SB220 barely makes 2x running current on the first few cycles of start, and then current rolls off rapidly.

The worse tubes and systems I see for inrush are the slow heating oxide cathode tubes, and even they generally are not too bad unless the filaments are in series or the filament transformer is far too large in current capacity and the filament wiring is heavy.

One person attributes filament movement to magnetic fields, but if forces from magnetic flux are calculated they are on the order of only a few grams in smaller tubes like the 3-500 or 3-1000!!!

Quote
As well, a test of plate current (Ib) VS  negative Grid bias (Eg) may not track a known good tube because the grid or it's turns are out of place physically or even one or more grid turns had been burned out and lay at the bottom of the tube structure.

True.

Quote
Leakage should be rare in this type structure due to how the elements are mounted to the stem unlike a small receiving tube that often suffers from leakage between the elements having a leakage path across the mica structure that hold all the elements in there precision alignment.

Ceramic also, not just mica. Chinese tubes light up the ceramic from impurities:

http://www.w8ji.com/572b_problems.htm

73 Tom
Logged
W5WSS
Member

Posts: 2283




Ignore
« Reply #34 on: January 07, 2011, 07:51:06 AM »

My old amplifier uses two Eimac 4-400a. Due to circumstances beyond my control, it was shut down and stored. The Johnson Invader 2000 revamped into an amplifier was shut off for 12 years. The separate power supply is in original condition.uses mercury vapor rectifiers! I admit I expected immediate problems but was amazed after just "turning it on" everything started up. Amplified properly.
Logged
K9PU
Member

Posts: 67




Ignore
« Reply #35 on: January 07, 2011, 02:20:11 PM »

Consider placing the tube in a hard vacuum for an extended period of time (likely months.)  This, in theory, should allow gas to exit the tube.  High temperature inside the tube should allow for a faster exit of gas within the tube.  A hard vacuum is hard to get though, more than just a vacuum pump. 

If you have a means of measuring the internal vacuum of the tube you can monitor its improvement.

If the tube has a leak this may be for naught.  Of course you could leave it in the vacuum.

Good luck with this.

Scott
Logged
W0BTU
Member

Posts: 2241


WWW

Ignore
« Reply #36 on: January 07, 2011, 11:25:08 PM »


What about using externally applied RF induction heating to get the plate red hot? Practical or not, what are the chances?

I don't fully understand the physics of the gettering process. I don't have a problem with the concept that the oxygen in the tube is gettered; I think of this as an oxidation process. However, nitrogen is an inert gas. How can it combine with the gettering material? It must, though, because tubes that don't sit idle for many years don't have the problem.
Logged

W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #37 on: January 08, 2011, 02:44:51 AM »


What about using externally applied RF induction heating to get the plate red hot? Practical or not, what are the chances?

I don't fully understand the physics of the gettering process. I don't have a problem with the concept that the oxygen in the tube is gettered; I think of this as an oxidation process. However, nitrogen is an inert gas. How can it combine with the gettering material? It must, though, because tubes that don't sit idle for many years don't have the problem.


If the tube has so much air it can't even be lit up normally, it is almost certainly done for anyway. Apply a reasonable but low anode voltage, some positive grid voltage, have good cooling in place, and heat the tube.

Many tubes can be gettered by running the HV lower than full (like the SB220 on CW, or the AL80B on the lower voltage HV tap) and drive the amp with light drive jus enough to show color with the anode out of resonance. The only thing you need to watch is tube seal temperature, because the SB 220 only really can cool about 200-300 watts of dissipation per tube with the stock fan.

Anything more than that is not worth it. It's not a $3000 tube....yet.

73 Tom
Logged
W0BTU
Member

Posts: 2241


WWW

Ignore
« Reply #38 on: January 08, 2011, 11:39:50 PM »

... the SB 220 only really can cool about 200-300 watts of dissipation per tube with the stock fan.

Oh really? Is that the reason for the lower anode voltage "CW" switch position, or is the transformer the limiting factor?
Logged

W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #39 on: January 09, 2011, 12:24:44 AM »

... the SB 220 only really can cool about 200-300 watts of dissipation per tube with the stock fan.

Oh really? Is that the reason for the lower anode voltage "CW" switch position, or is the transformer the limiting factor?

The SB220 was designed when the legal amateur power limit was 1000 watts DC input minus the driver stage power. That's about 600 watts output carrier.

As such, there was no need to cool the tubes or amp much beyond that. That's why people unsolder the pins in the tubes when they try to run a kilowatt output.

The tap on the transformer was to allow tuning and operating the amp at legal CW power with about 70% anode voltage, and then when flipping to SSB tuning remains the same because plate voltage and plate current come up at the same ratio. This approximately doubles input power on peaks while average power remains well below 1 kW. (It would be illegal to have the plate current meter ever swing past the amount that allows 1000 watts on the highest voice peak.)

It was a entirely different world before the 80's.


Logged
W0BTU
Member

Posts: 2241


WWW

Ignore
« Reply #40 on: January 09, 2011, 03:36:36 AM »

...It was a entirely different world before the 80's.

Oh, yeah! Why didn't I remember that? :-)

Thanks, Tom.
Logged

KM1H
Member

Posts: 5041




Ignore
« Reply #41 on: January 14, 2011, 02:33:55 PM »

I almost sprayed my afternoon coffee while reading Toms comment about Eimacs nichrome cartoons! When I called Eimac after that infamous QST article I was told that xxx was responsible for the destruction of more 8877's than all other causes combined. He is still in parasitic mode on forums.

As far as Eimac 3-500Z production problems Im aware of the late 80's seals problem where tubes went to air just sitting NIB for 2 years. I believe this was after production moved to SLC. Another SLC adventure was when they tried to save money and just spot welded the top support to the anode. These broke loose, singly or all, causing the anode to either lean over or actually contact the grid.  More rumors sold many more uneeded inrush limiters. In the SB-220 family they do protect the AC mains switch; turning on in the CW position does about the same.

Ive been using a much modified SB-220 chasssis rescued from a CBer to getter tantalum plate 3-400/500's, 4-250/400's, and PL-175's since the late 90's (as well as old school tubes such as 250TH, 304TL, etc on another jig). HV is around 900V and grid voltage is variable; Ive changed things around since the first version I described on the AMPS forum and elsewhere back then. Ive done a few hundred 3-500Z's since then as every amp I convert to 6M automatically gets the treatment even before I test them on the Hi-pot.  The success rate is higher than Tom as Im obviously doing it to many perfectly good tubes but its a habit I got into and it takes no real effort on my part and it gives me that warm fuzzy feeling Roll Eyes Some test beyond 12KV which makes for good old Class C DSB AM candidates.


Now this brings up another question: Can you degas a graphite plate by bringing it to a low color?  I understand that getting them into the orange range actually releases gas. The few used Chinese 3-500's Ive done with a before/after hi-pot have shown an improvement but I didnt go beyond a dull red for a couple of hours.

Carl
KM1H

Logged
AA4HA
Member

Posts: 2630




Ignore
« Reply #42 on: January 15, 2011, 09:11:23 AM »

What about using externally applied RF induction heating to get the plate red hot? Practical or not, what are the chances?

I don't fully understand the physics of the gettering process. I don't have a problem with the concept that the oxygen in the tube is gettered; I think of this as an oxidation process. However, nitrogen is an inert gas. How can it combine with the gettering material? It must, though, because tubes that don't sit idle for many years don't have the problem.

Gettering is pretty simple when you understand the chemistry of the materials involved, the process of heating a getter for the first time and the conversion that occurs during the gettering process. You will also see the limitations of re-gettering a vacuum system.

Gettering materials are usually reactive, alkali metals in their pure form (no oxides present). The gettering component may be a coating on the inside of the tube, a little halo like metal structure or other odd looking components between the tube structure and the glass wall. Some tubes may make the anode out of a gettering material as well.

When a tube is manufactured they try to draw down the internal pressure as close as they can to being a hard vacuum. Sometimes a little bit of heat is added to encourage structures to outgas while the vacuum pump is still running. Once they get to a certain point the glass tube is heated, melted and twisted off. Some tubes have the characteristic glass dimple on the top of the glass envelope, others it may be in the base (like an ordinary light bulb).

Now you have a tube that has a pretty good vacuum but not quite perfect. For tubes with the little halo of metal (zirconium, barium, etc...) (pretty much any metal that has a name that ends in "ium") they may be put in an induction machine to heat that metallic component up to extraordinary temperatures. As the metal then cools, it sucks up oxygen molecules to form barium oxide, zirconium oxide, xxxium oxide...) For most tubes the gettering metal is a one time process at the point of manufacturing but some tubes can be re-gettered by heating them up again and letting the alkali earth metal reabsorb the trace gases in the tube.

You can see that there is a practical limit to this. The amount of alkali earth metal in the tube that can be converted into an oxide may be limited or sometimes by reheating a tube you can cause outgassing that ends up plating somewhere else. Also, gettering metal compounds may be mixed to draw down certain elements (oxygen, hydrogen, etc...) but may leave other gasses alone in the tube.

If you have a tube that is genuinely leaky and is losing vacuum you may get one or two shots of extending the life of a tube by re-gettering but evenutally the gettering will just give out completely as it is overwhelmed.

Some tubes may use a different process that is similar to gettering. This is similar to what is used in ultra-high vacuum systems and is called an "ion pump". (sputtering pump). It involves a cathode that is porous to gas and heat. I do not remember if the 3-500 is a conventioal alkali earth getter or if the anode may also be working as a sputtering pump when heated.

This is all +25 year old information from university physics classes and from doing some internship time at a few national laboratories.

Tisha Hayes
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W0BTU
Member

Posts: 2241


WWW

Ignore
« Reply #43 on: August 12, 2011, 08:21:04 PM »

Nice photo on page 152 in the September QST by W8JI, which powerfully illustrates why 3-500Zs need to be periodically gettered.
Logged

AG6K
Member

Posts: 1




Ignore
« Reply #44 on: October 04, 2011, 03:34:18 PM »

I think what is being misconstrued here is that there is really 2 getter functions of a tube. There is the one primarily used when tube is first built and the secondary in the tube in question is were anode/plate is usually coated with a zirconium alloy of sorts that oxidies when it is heated up and in order to oxidizes it needs air to do it. (it gives it a grayish color) If a 3-500z is run properly and not babied it will heat anode enough that it will keep tube gas free barring a major leak. As long as tubes are not arcing over there is no need to try to do a primary or OEM get to tube as was done when it was built. Some get nervous when they see plates glow on a tube (and they should if it is a sweep tube or a 811) but it is quite normal for a 3-500z to glow dull to medium red and promotes a long life.   

  good advice.  Congrats.
Logged
Pages: Prev 1 2 [3] 4 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!