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

   Home   Help Search  
Pages: [1] 2 3 4 Next   Go Down
  Print  
Author Topic: How to getter a 3-500 tube  (Read 29116 times)
W1QJ
Member

Posts: 1521




Ignore
« on: January 03, 2011, 09:28:21 AM »

I am wondering if anyone that has encountered gassy 3-500z tubes has successfully managed to getter the tube and dissipate the gas and restore the tube to good use.  I have looked on the web for information on a tried and true method of gettering the gas in a tube but unable to find any information on going about doing it.  Of course there is plenty of information on "gassy tubes", but I am looking at possibly building a jig to getter the gas in these tubes I run across or even to supply the service to someone who may have a problem tube.  I think I have all the available parts to make such a jig, but i would need to know the particulars and the procedure to follow to successfully accomplish the task.  Any help out there?  Lou
Logged
W0BTU
Member

Posts: 1853


WWW

Ignore
« Reply #1 on: January 03, 2011, 10:11:18 AM »

Lots of info on qrz.com. Check out http://forums.qrz.com/showpost.php?p=1991563&postcount=9 and the link http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm.

"Under some conditions a glass tube can be restored to operation by running low anode voltages and positive bias on the grid. This will sometimes allow full operating anode temperatures to be reached, and the tube can be "cooked" for several hours. I've had about a 50% success rate restoring old 3-500Z's that have sat for years without use. Even though they initially arced severely at full voltage, by cooking them at low voltage and positive grid bias to show anode color vacuum was restored."
Logged

W1QJ
Member

Posts: 1521




Ignore
« Reply #2 on: January 03, 2011, 11:16:52 AM »

Thanks Mike, I have read that on the W8JI site.  I am just wondering if someone has more specifics.  Like what would be an appropriate "lower anode voltage"?  2kv?  How much bias at the specified "low anode voltage"?  Cook the tube  how long?  I am looking to make up a getter jig if you will.  I suppose I would need a glitch resistor and perhaps a fuse.  Tom claims he had 50% success, that's not too bad. 
Logged
W8JX
Member

Posts: 6686




Ignore
« Reply #3 on: January 03, 2011, 11:41:20 AM »

Basically you need to get tube hot. You do not want to baby 3-500's as they need to glow dull to medium red when running. The Getter function of tube works best when tube is hot. BTW, this is not a problem at all for this tube and is "normal" operation for its graphic anode design. The coats on it need to get real hot to work as a getter.
Logged

--------------------------------------
You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
W0BTU
Member

Posts: 1853


WWW

Ignore
« Reply #4 on: January 03, 2011, 12:40:56 PM »

... I am just wondering if someone has more specifics.  Like what would be an appropriate "lower anode voltage"?...

The first link to qrz.com explains that.
Logged

WX7G
Member

Posts: 6331




Ignore
« Reply #5 on: January 03, 2011, 12:54:39 PM »

3-500Z gettering parameters:

Plate at 500 volts
Grid at +60 volts
Plate current will be about 700 mA
Grid current will be about 200 mA

How long to getter the tube? If you have a hipotter you might hipot the tube Plate-to-grid periodically and getter it until it holds off 10 kV. It will not harm the tube to getter it for days.
« Last Edit: January 03, 2011, 12:56:57 PM by DAVE CUTHBERT » Logged
W8JX
Member

Posts: 6686




Ignore
« Reply #6 on: January 03, 2011, 01:46:50 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.   
Logged

--------------------------------------
You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
W1QJ
Member

Posts: 1521




Ignore
« Reply #7 on: January 03, 2011, 03:43:53 PM »

The info provided by WX7G is what I was looking for.  The plate voltage at 500 volts isn't enough to cause the gas to arc.  So I assume the tube plate will get real hot, getter the gas and then when put back into service it should not arc.  Is this a fair assumption?  Thanks for the information Dave, that was what I wanted to know.  Lou
Logged
WX7G
Member

Posts: 6331




Ignore
« Reply #8 on: January 03, 2011, 04:47:50 PM »

Yes adjust the grid drive and/or plate voltage so the plate glows a dull red. It's not critical. A fan blowing some air on the tube is probably a good idea. I would hesitate to check an old tube in some amps that can be damaged by a tube arc.

This datasheet will show you what the tube needs: http://www.umich.edu/~umarc/station/docs/3-500z.pdf
Logged
KM3F
Member

Posts: 526




Ignore
« Reply #9 on: January 03, 2011, 11:28:18 PM »

As additional info, you need to know that gas levels in a tube can get so high that the gas ionizes and begins to conduct like a neon and will arc with very high voltages applied.
When this happens the number 1 grid has no control of electron flow so a run-away condition devlopes in a hurry. (arc)
Tubes that gas up to this extent usually cannot be recovered.
Also you may see some tubes that glow with a vivid blue on the inter glass surface.
This usually is not gas related but some contamination that glows when electrons strike the outer walls.  The tube may work fine with no ill effects otherwise..
Logged
W0BTU
Member

Posts: 1853


WWW

Ignore
« Reply #10 on: January 04, 2011, 02:21:57 AM »

... it is quite normal for a 3-500z to glow dull to medium red and promotes a long life.

Correct. I would add, for additional emphasis, that it is not only normal, but ESSENTIAL. :-)

I was surprised that just 500 volts would work, since W8JI mentioned 1 to 1.5 kV in the link I provided above.

Something in the back of my mind tells me that after that filament is lit, you would want to get the plate good and red just as fast as possible. It seems to me that with air in the tube, that filament is living on borrowed time until the gettering is nearly complete.
Logged

W1QJ
Member

Posts: 1521




Ignore
« Reply #11 on: January 04, 2011, 05:39:29 AM »

I have decided to build up a "getter jig", I will be able to adjust the HV from 0 to 1.5KV and I will have an adjustable dc supply for bias up to about 100vdc and of course filament voltage.  Someone mentioned a tube having too much gas and can not be restored.  Case in point, while checking the 3-500's I have, I very carefully installed them in an SB-220 amp with a variac on the line voltage.  I installed a known good tube in one hole and an unknown tube in the other.  Put the meter in the grid position without the HV hooked up.  Turn the amp switch on and brought the line voltage up slowly.  Without too much line voltage I noticed the grid meter going negative.  I knew there was problems right away!  BTW this is how most grid meters in SB-220 blow out!!  Although looking at the grid meter as I did indicated a bad tube right away, the small amount of current needed to make me know there was problems would not harm the meter.  Assuming you bought a used SB-220, installed the tubes, had the meter in grid position, turned it on with a filament to grid short or very gassy tube , POP, there goes your meter! (most likely).  I recommend turning the amp on in the HV position, if the tube is bad, you will know it!! And you will save your meter.  This tube turned out to have a short and I was able to read it with an ohm meter.  I hooked it up to a car battery and I saw a flash of light inside and then the short was cleared.  But when full HV was applied it appeared OK, no blue color.  I thought the tube was saved.  Then once the antenna relay was engaged and drive was applied, there was small arcs of blue gas.  SO this tube has some gas in it.  Interestingly several other tubes had gas at different levels.  Another tube had so much gas that about 800vdc ignited the gas and blew the grid grounding resistor I use like a fuse instead of the choke.  That tube most have major gas!!  I doubt that one can be rehabilitated?  Anyway, I recommend to anyone who has an amp to get a variac.  It is your best friend!  Not so sure when I will find time to build the jig, but it is something I want to do.  Tube arcs ruin lots of things in amps, meters, zener bias diodes, and grid shunt resistors.  With grids lifted above ground on some amps, grid caps and grid chokes burn up too.  Firing up an amp on a variac can save the "big bang" which does lots of damage.  Also, the variac will tell you if you have a shorted diode or cap in the PS right away.  The HV will not come up quick and there will be some humming of the variac pretty quickly.  You can almost diagnose any problem in the power supply and bad tubes within seconds of seeing what happens as the HV begins to come up on the variac, sparing the big bangs.  Since the variac is only for testing, a 5A variac for such use is OK and they can be gotten very cheap.  $20.00 at a hamfest is common for a 5a  variac.  Happy New Year
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #12 on: January 04, 2011, 05:40:49 AM »

You are looking for a cookie cutter answer when there is none. There is no set voltage, except the voltage has to be lower than the ionization voltage of the gas. This typically is a few thousand volts, anode to grid, in a recoverable tube.

While low voltages can be used, low voltage requires more grid current (perhaps a few hundred mA) and a higher current anode supply. There is nothing wrong with that if you have a 40-80 volt 1/2 amp supply for the grid and a 600-800 mA 400-600 volt HV supply handy.

I use higher voltage because things are easier for me to manage. The AL80B has a tap on the transformer secondary that allows reduced voltage, and it is easier for me to come up with a variable supply that can be run up to 30 volts at 100 mA than it is something higher.

Plus the tube absolutely MUST have good forced air, so a higher speed fan in an AL80B chassis makes a nice fixture. Remember the tube filament is producing 75 watts of heat along with the 300 watts or so in the anode, and unlike amateur service this is continuous.

Asking for a set of operating conditions is counterproductive, because the best operating conditions would depend on what you use for a fixture. The only important thing is tube ratings are not exceeded, and the supplies you have will handle the load. As I said, and it is worth repeating, if a tube has a breakdown below a few thousand volts it almost certainly will not be recoverable.

A good 3-500Z, at minimum, should hold off three to four times the expected dc plate voltage. This is because in normal properly loaded operation peak anode voltage would be about twice the dc anode voltage. If the amp is mistuned or an excessive drive peak comes along, it is easy to go well beyond that 2 times value.

73 Tom
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #13 on: January 04, 2011, 05:52:37 AM »

I have decided to build up a "getter jig", I will be able to adjust the HV from 0 to 1.5KV and I will have an adjustable dc supply for bias up to about 100vdc and of course filament voltage.  Someone mentioned a tube having too much gas and can not be restored.  Case in point, while checking the 3-500's I have, I very carefully installed them in an SB-220 amp with a variac on the line voltage.  I installed a known good tube in one hole and an unknown tube in the other.  Put the meter in the grid position without the HV hooked up.  Turn the amp switch on and brought the line voltage up slowly.  Without too much line voltage I noticed the grid meter going negative.  I knew there was problems right away! 

If you see stable negative grid current right away on a slow ramp up, you have a grid-to-cathode short and the tube is junk.

A gassy tube that is recoverable will have an uncontrolled sudden avalanche of current that occurs at some voltage generally up above a few kilovolts.

To protect the meters, a single diode is needed to clamp the negative power supply rail to the chassis.

A filament to grid short will not hurt the meters in an unmodified SB220. About all a filament to grid short can do is make the tube draw current when the amp relay is not closed. If the relay is closed some voltage less than 2.5 volts is applied to the path from grid to chassis, up through the grid meter, through the zener, and back to the filament CT. The very low voltage (always less than 2.5 volts) and the total resistance of that path (plus threshold of the zener) is virtually always enough to limit current to safe values.

Meter destruction comes from an anode to grid arc. This is generally either the anode falling over into the grid from bad welds, or gas in the tube going plasma.

73 Tom


Logged
W1QJ
Member

Posts: 1521




Ignore
« Reply #14 on: January 04, 2011, 06:15:59 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?
 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 am trying to determine the parameters of a "junk" tube and a tube that might be able to be saved. 

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?
Logged
Pages: [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!