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Author Topic: degassing 3-500z  (Read 5338 times)
W4VKU
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Posts: 347




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« on: November 18, 2012, 08:31:42 PM »

I have a pair that has been on the shelf for a decade. Would like to see if i can resurrect it to keep it alive,
for what it is worth. Have a TL922 here. Also have a short pigtail to connect to the 120v/20A outlet and can
limit the current drawn, by introducing 40/60/150w lamps in series. Don't have an auto-transformer.

Thanks for any references to degass and make the 3-500z useable.
Krish
w4vku
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K8AXW
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« Reply #1 on: November 18, 2012, 08:51:34 PM »

That subject has been discussed at length in this forum.  Suggest you start back through to locate this information.  It will be worth you time.  Very interesting and informative.

I don't think your thoughts or idea will work.  You will need to get the filament running at full voltage and bring the HV up in increments and then let the tube(s) draw current in order to "getter" the gas out.  The exact procedure and specifics will be found in earlier posts here in the Amplifier Forum.
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K4RVN
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« Reply #2 on: November 18, 2012, 09:11:06 PM »

Take a look at this link to past forum discussion. You may get some ideas from WX7G and others who posted specifics. W8JI
also posted and you can pretty well bank on what he said if you understand it.
I personally have used an old 3-500 which was good when I pulled it, but set up for years. I did let the filament heat up for a day just to get the tube warm. The plate actually does the gettering from what I read, but I wanted to warm the tube although those who supposed to know said I wasted my time doing that. Mine was stored in a basement. Take a look at the past forum and see if you can find something to help you.http://www.eham.net/ehamforum/smf/index.php/topic,72672.0.html

Frank
« Last Edit: November 18, 2012, 09:14:43 PM by K4RVN » Logged
K1ZJH
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Posts: 978




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« Reply #3 on: November 18, 2012, 09:12:32 PM »

Is there a problem with the set of tubes, or are you assuming that have gone bad since they've been in storage for a long time? I've fired up ancient Eimac 3-500Z tubes that were in storage for decades and they've worked. You can't always assume they will have gone gassy.

Pete
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KC4MOP
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Posts: 733




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« Reply #4 on: November 19, 2012, 03:31:47 AM »

Is there a problem with the set of tubes, or are you assuming that have gone bad since they've been in storage for a long time? I've fired up ancient Eimac 3-500Z tubes that were in storage for decades and they've worked. You can't always assume they will have gone gassy.

Pete
Hi Pete
I think there is a procedure to ease the tube back into service after a long idle period. Possbile damage to the tube to be shocked into operation and in a gassy condition and the HV.

Fred
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WX7G
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« Reply #5 on: November 19, 2012, 05:13:57 AM »

With only the TL922 for equipment your only option is to install the tubes and fire up the amp.
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W1QJ
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« Reply #6 on: November 19, 2012, 05:18:09 AM »

You don't have to de-gas a tube that is not gassy.  You should first determine if the tube is gassy.  You can do this by using a variac on the primary of the amplifier. A cheap $20.00 flea market unit will work for this test.   You slowly bring up the line voltage as you observe for any gas in the tube.  If the tube is severly gassy or has a G-F short you will quickly see negative grid current with hardly any line voltage.  Monitor grid current, plate current and the tube itself visually as you bring up the line voltage.  If all looks good keep the line voltage increasing.  If the tube is acting normally up to the CW voltage then you can usually unplug the Variac and use the tube in the CW voltage position.  You can then apply some drive to the tube and watch for any gas with RF applied.  If that is good you can then run a small amount of rf for an extended time to get the plate hot and leave it run like that for a while if it makes you feel good.  I usually keep the line voltage increasing until full HV is reached if I do not see any sign of gas I will key the amp and see if I see any gas then.  Usually a tube that shows no sign of gas with full HV could show a little when keyed.  But I have noticed that a tube that is severly gassy will show up as the HV is increased.  The bottom line is the tube is either gassy or it's not and you should determine this first off.  It takes all of 10 minutes to determine if you need to take any measures to de-gas the tube.  Done this way you can safely check the tube for gas.  
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W4VKU
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« Reply #7 on: November 19, 2012, 05:27:30 AM »

Thank you all for the tips.
These are old Eimacs that were given to me, when i purchased the amp.
So i am assuming that they have gassed up over the years to err on the side of caution.

73s
krish
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N4CR
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Posts: 1666




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« Reply #8 on: November 21, 2012, 02:16:02 PM »

I've been under the impression for many years that any arc consumes some of the gas if a tube is gassy.

Is that true?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
GM3SEK
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Posts: 47




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« Reply #9 on: November 22, 2012, 01:06:27 AM »

I've been under the impression for many years that any arc consumes some of the gas if a tube is gassy.

Is that true?

No. An arc creates intense localized heating of the metal surfaces, so mostly it releases gas. Metal atoms will quickly 'plate out' onto any surface that they strike, but the lighter oxygen, nitrogen and hydrogen molecules (which are present as trace impurities in the metal) will be released into the vacuum space.

Cleanup of gas molecules will typically require several seconds - the better the vacuum and the larger the volume, the longer it will take. A typical arc will only last a small fraction of a second before the amplifier shuts down, so it doesn't have enough time to be involved in any cleanup operations.

But if a transmitting tube arcs while it's hot, the chemical getters within the tube will still remain active for several seconds before the tube cools down. This is enough time to mop up the stray gas molecules, so the getters are the main reason why arcs often seem to have 'cured themselves' afterwards.


73 from Ian GM3SEK
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KD0REQ
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« Reply #10 on: November 22, 2012, 09:33:32 AM »

however, if the arc happens to take out something important, like for instance the grid, or blows up the biasing, or perhaps blows through and takes out the driving system, then there is no recovery without a healthy credit card as backup.

which is why all the leading engineers caution that safety shutdown circuits are a must in a RF power amp, and you see them in commercial and broadcast equipment but rarely in the amateur market.  the present implementations easily double the cost of the amp.  back in the thyratron and plate relay days, not so much, but lots of false triggers.
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GM3SEK
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Posts: 47




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« Reply #11 on: November 22, 2012, 12:08:58 PM »

however, if the arc happens to take out something important, like for instance the grid, or blows up the biasing, or perhaps blows through and takes out the driving system, then there is no recovery without a healthy credit card as backup.

which is why all the leading engineers caution that safety shutdown circuits are a must in a RF power amp, and you see them in commercial and broadcast equipment but rarely in the amateur market. 

Shameless plug! www.ifwtech.co.uk/g3sek/boards

Quote
the present implementations easily double the cost of the amp.  back in the thyratron and plate relay days, not so much, but lots of false triggers.

For amateur-sized amplifiers, it is usually enough to have a surge limiting resistor in the B+ supply, backed up by a shutdown circuit that disconnects the transformer primary. Correctly configured, such a system will shut down the amplifier with a quiet 'tick', often without even blowing a mains fuse.


73 from Ian GM3SEK


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N4CR
Member

Posts: 1666




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« Reply #12 on: November 22, 2012, 03:47:16 PM »

I've been under the impression for many years that any arc consumes some of the gas if a tube is gassy.

Is that true?

No. An arc creates intense localized heating of the metal surfaces, so mostly it releases gas. Metal atoms will quickly 'plate out' onto any surface that they strike, but the lighter oxygen, nitrogen and hydrogen molecules (which are present as trace impurities in the metal) will be released into the vacuum space.

Cleanup of gas molecules will typically require several seconds - the better the vacuum and the larger the volume, the longer it will take. A typical arc will only last a small fraction of a second before the amplifier shuts down, so it doesn't have enough time to be involved in any cleanup operations.

But if a transmitting tube arcs while it's hot, the chemical getters within the tube will still remain active for several seconds before the tube cools down. This is enough time to mop up the stray gas molecules, so the getters are the main reason why arcs often seem to have 'cured themselves' afterwards.


73 from Ian GM3SEK


You are assuming I was referring to a destructive arc caused by the transmitter at full power and voltage.

If a non-destructive (and that means no burnt metal anywhere) arc happens in a gassy tube, of sufficient voltage and highly limited current, such as from a Tesla coil, can the gas be reduced?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
GM3SEK
Member

Posts: 47




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« Reply #13 on: November 23, 2012, 01:44:34 AM »

I've been under the impression for many years that any arc consumes some of the gas if a tube is gassy.

Is that true?

No. An arc creates intense localized heating of the metal surfaces, so mostly it releases gas. Metal atoms will quickly 'plate out' onto any surface that they strike, but the lighter oxygen, nitrogen and hydrogen molecules (which are present as trace impurities in the metal) will be released into the vacuum space.

Cleanup of gas molecules will typically require several seconds - the better the vacuum and the larger the volume, the longer it will take. A typical arc will only last a small fraction of a second before the amplifier shuts down, so it doesn't have enough time to be involved in any cleanup operations.

But if a transmitting tube arcs while it's hot, the chemical getters within the tube will still remain active for several seconds before the tube cools down. This is enough time to mop up the stray gas molecules, so the getters are the main reason why arcs often seem to have 'cured themselves' afterwards.


73 from Ian GM3SEK


You are assuming I was referring to a destructive arc caused by the transmitter at full power and voltage.

Well, you did say "any arc" (see above :-)

Quote
If a non-destructive (and that means no burnt metal anywhere) arc happens in a gassy tube, of sufficient voltage and highly limited current, such as from a Tesla coil, can the gas be reduced?

Sorry, I don't have any information about that very specific kind of arc.

But going back to your original question, an arc does not ever "consume" gas - not in any normal sense of that word. The only way for gas molecules to be removed from the vacuum space is to anchor them onto a solid surface. An electrical discharge through the gas is not doing that.


73 from Ian GM3SEK

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N4CR
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Posts: 1666




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« Reply #14 on: December 06, 2012, 03:23:36 PM »

I came across this from some research.

http://lists.contesting.com/_amps/2007-05/msg00237.html

Tom seems to believe that an arc will degass.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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