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Author Topic: Loading 3CX800A& amps  (Read 1117 times)
KA5ROW
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« on: June 05, 2017, 12:27:27 PM »

Loading amp using 3-500z I load by adjusting for max output on my watt meter and have done this for 30 years.
I am considering getting a new amp like the AL-800H uses a pair of 3CX800A7. Can I just load it for max power instead? Adjusting the grid? I would not want to run such expansive tubes. When I do adjust my AL-80B for max it ends up with the recommended current settings anyway.
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W8JX
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« Reply #1 on: June 05, 2017, 12:36:44 PM »

They 3cx800 is a great tube but it has a somewhat fragile grid that is easy to damage. You need to watch it closely when tuning. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KM1H
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« Reply #2 on: June 05, 2017, 01:42:27 PM »

Since that AL-800 and 800H has a very fast grid overload circuit it will take some serious work to try and hurt them.
See page 5 in the manual
http://www.ameritron.com/support.php?productid=AL-800H

All that tube needs is about 10 ma of grid current (per tube) for full output when tuned properly and with a full output tube. Trying for the maximum rated 60 ma is asking for trouble.

Carl
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W8JX
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« Reply #3 on: June 05, 2017, 04:13:22 PM »

Since that AL-800 and 800H has a very fast grid overload circuit it will take some serious work to try and hurt them.
See page 5 in the manual
http://www.ameritron.com/support.php?productid=AL-800H

All that tube needs is about 10 ma of grid current (per tube) for full output when tuned properly and with a full output tube. Trying for the maximum rated 60 ma is asking for trouble.

Carl

I agree it has grid protection but it is best to not leave it to chance. Like you said not much grid current is needed. With only 4 watts of max rated grid dissipation vs 20 watts for a 3-500 so you want to load grid lightly.
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N3QE
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« Reply #4 on: June 05, 2017, 07:37:12 PM »

Loading amp using 3-500z I load by adjusting for max output on my watt meter and have done this for 30 years.
I am considering getting a new amp like the AL-800H uses a pair of 3CX800A7. Can I just load it for max power instead? Adjusting the grid? I would not want to run such expansive tubes. When I do adjust my AL-80B for max it ends up with the recommended current settings anyway.

Yes, you load up for max power, watching the grid current. You don't "adjust the grid" exactly but you do tweak the load knob and typically at full power out you are drawing reasonable grid current.

The 3-500Z, it is nigh impossible to damage the tube with grid current and the AL-80B does not have a grid current trip.

The AL-800H amp is a very similar mainframe, but it does have the addition of a grid current trip so if you make some big mistake and grid current goes too far, it will kick the amp into standby before any tube damage.

After the first couple times you tune it up, you will know where to point the load control for reasonable grid current on each band.

When I got my first "watch the grid current amp" I would activate the grid current trip quite often. Now it rarely happens - I might have ten thousand QSO's between grid current trips. You'll be exactly the same way and do fine.
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W1QJ
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« Reply #5 on: June 06, 2017, 03:38:51 AM »

Yes, that is correct, the grid is fragile but it does have a trip circuit to protect.  At full power the grid current will be pretty low with good tubes.  Generally speaking the grid current will usually null or "dip down" when the power peaks up.  However at any given level of drive, if you increase the drive level significantly the grid current will climb VERY quickly.  That means when loading up the amp you will either have to take small increments of drive at a time OR advance the loading a tiny bit before you key down after adding a lot of drive power.  The grid current increases a lot more with this tube than a 3-500 does, but a small bit of loading drops the grid current very quickly.  Once you get the hang of it you will find it is a bit different but still easy.
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