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How to safely "wake up" new (old stock) Eimac 3-500Z's?




 
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Author Topic: How to safely "wake up" new (old stock) Eimac 3-500Z's?  (Read 15025 times)
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WA2SQQ
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« on: March 30, 2012, 02:34:17 PM »

I have two brand new Eimac 3-500Zís that were purchased about 20 years ago. Theyíve been sitting in their original boxes on the shelf. Itís time to do some much-needed maintenance on my SB-220, so Iíve decided to replace the original tubes which are definitely soft. Iím aware that I donít want to just install the new tubes and start using them. According to Eimacís recommendations (below) they suggest ďrunning them for several hours with filament voltage only appliedĒ

Contrary to Eimacís recommendations, many people are saying that the filament voltage alone isnít hot enough to do the degassing.

Iím sure among the AM boat anchor community that many of you have been faced with this predicament. Whatís the safest way to proceed?

Bob, WA2SQQ

Eimac says:
http://www.cpii.com/library.cfm/9
"Degassing Tubes that may have gassed up can be partially degassed by putting them in the equipment and running them for several hours with filament voltage only applied.
After the initial filament-only degassing; operation for an hour or so at reduced plate and screen voltages is desirable. This allows the getter to soak up and hold
any residual gasses. In directly-heated filamentary tubes, the getters are generally zirconium-bearing materials, which depend on heat to activate the gettering action."

Others say,
ďYou can let the filament run til the cows come home and it will not help. The gettering agent in the 3-500Z is zirconium, and it is coated on the anode, NOT the filament. If you don't want to take my word for it you can look on Internet, but zirconium catalyzes most common gasses optimally at 1000 degrees C or slightly hotter. 1000 degrees C takes about 300 watts of anode dissipation, and would be a bright red color on the anode. The anode has to be at least 500C for the zirconium to have any effect, and that is a faint black red color. Running the filaments is a waste of time. Anode heat is what absorbs gas.Ē
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K1ZJH
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« Reply #1 on: March 31, 2012, 11:41:32 AM »

Eimac's last sentence applies for the 3-500, which is a directly heated cathode.
In directly-heated filamentary tubes, the getters are generally zirconium-bearing materials, which depend on heat to activate the gettering action."
You need to get the plates red to activate the gettering.

BTW, I just tried three tubes that I had in storage for almost 40 years, a well used
Eimac 3-500Z and a pair of PL-175 tubes. They all went right to work in a SB-220.
I expected a light show, but they survived Smiley

Pete
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WD5JKO
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WD5JKO


« Reply #2 on: March 31, 2012, 11:48:30 AM »



So how does one get 3-500Z's which are Hi-Mu triodes RED hot at reduced plate voltage? Apply positive grid bias? Doesn't sound easy in the SB-220, and more likely requires a test fixture.

Jim
WD5JKO
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K1ZJH
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« Reply #3 on: March 31, 2012, 12:18:09 PM »

Run them in a test jig, and yes, positive grid voltage. Or, just use them on the
air in a SB-220 in the SSB position and get on the air and put some hours on the
plates.

Pete
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W2VW
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« Reply #4 on: March 31, 2012, 01:13:11 PM »



So how does one get 3-500Z's which are Hi-Mu triodes RED hot at reduced plate voltage? Apply positive grid bias? Doesn't sound easy in the SB-220, and more likely requires a test fixture.

Jim
WD5JKO

Test fixture or short across the zener in an old amp with a CW set of taps on the plate supply.

Throw the output tank out of resonance enough to get the tubes glowing. Use cation not to exceed rated grid current.

Late 80s 3-500Zs can be a problem due to QC issues.

Any other dates seem to be OK unless they were used by hams who thought the orange glow was bad for the tubes.

Lots of monkey hear monkey repeat on the WWW by hams who insist these tubes must be rotated into service every 6 months or they will go bad. Not in my experience. Same thing with the 6146 "expert" who insists certain ones won't in a Valiant.

A few have retired to their corner promising to come back with further info on the subject. Haven't seen it yet..............
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KM1H
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« Reply #5 on: March 31, 2012, 01:33:21 PM »

If the outgassing is minor a SB-220 in the CW position will work but even at zero bias wont get the anodes at much color at only 75ma or so.

The goal is to run them at a voltage that wont sustain an arc and Ive found a high percentage of late Eimacs, after the late 80's Salt Lake City fiasco, to be outgassing.

The SB-220 could be turned into a jig by running the filament/bias transformer normally and rewiring the plate transformer to a Variac. Then wire in an external bias supply which can be just about any shop type adjustable LV supply. You can even reverse the bias diode and filter in the 220 supply and run it thru a pot.

Carl
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W2VW
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« Reply #6 on: March 31, 2012, 02:23:15 PM »

If the outgassing is minor a SB-220 in the CW position will work but even at zero bias wont get the anodes at much color at only 75ma or so.


Carl


No kidding? Probably why I mention throwing the tank out of resonance. That implies drive to the careful reader.
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KM1H
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« Reply #7 on: March 31, 2012, 05:19:13 PM »

Nobody should do this procedure to an unknown or gassy tube with any RF drive; thats just asking for a big bang since the RF voltage is added to the DC voltage:o
Some live lucky.

Maybe they didnt teach that when you bought your Extra.
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W2VW
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« Reply #8 on: March 31, 2012, 06:52:22 PM »

Nobody should do this procedure to an unknown or gassy tube with any RF drive; thats just asking for a big bang since the RF voltage is added to the DC voltage:o
Some live lucky.

Maybe they didnt teach that when you bought your Extra.

Slow day trolling the other forums?
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DMOD
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« Reply #9 on: March 31, 2012, 07:50:31 PM »

For my Henry 2K-4, I simly put them in and run them for an hour on filaments alone and then shoot the juice to it; 10 Watts drive, tune to resonance, and then let it Continuous carrier for 15 minute intervals. I then input about 20 watts and do the same thing, then crank up the drive until the grid current is about 125mA and let the plates cherry for about 30 seconds.

Never exceed 125 mA grid current and 700 mA plate current.

Phil
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"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
KM1H
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« Reply #10 on: March 31, 2012, 08:14:26 PM »

The best thing for any amp that uses gas prone tubes is to keep it exercised by getting on the air and running some good color. Doing it as an AM linear is the most fun and sure beats doing it into a dummy load; I try to exercise the LK-500ZC about once a month as busting CW pileups is too quick to do any good.

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