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Author Topic: AL-80A and the 3-500Z  (Read 1325 times)
N9HSB
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Posts: 12




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« on: August 17, 2004, 08:32:42 AM »

 I have recently purchased an Ameritron AL-80A.  I have noticed arcing in the EIMAC 3-500Z when in QSO.  I switched over to the dummy load and found that, with the relay engaged and no drive from the exciter, within 30 second or so the fins in the 3-500Z would glow orange and arcing would occur from the bottom of the tube.  At least I think it is the bottom of the tube.  It happens real fast.  The AL-80A shows a plate current of 80ma and a grid current of 35ma with a HV of 2800v in this idle position with relay keyed and no drive.  If I do drive it to operation range the heating arcing rate is about the same time.  The amp seems to load to the rated output on 80 and 40, but does drop off some on the higher frequencies.  Sure could use some help.  Might be ok like this for contest but not for rag chewing.  HI  HI.

Cliff
N9HSB
   
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W8JI
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« Reply #1 on: August 17, 2004, 07:47:23 PM »

A red color is normal and desirable, arcing is not.

As a matter of fact if you never turn a 3-500Z red it will go bad.

The grid current is not normal. I suspect you have a bad meter protection diode. That diode is on the outside edge of the PS board where all the electrolytic capacitors are mounted. It is a single diode loacated near where the red/white and brown wires leave the PS board.

One way to tell if that diode is bad is if you see grid current when there is no RF drive but the amplifer is keyed. You also will notice the grid meter exactly tracks the plate current meter no matter what you do with tuning and loading.

The NORMAL behavior of the grid and plate meters is the grid meter will peak when the plate meter diops when you adjust the plate tuning. If it does not do this, that diode is bad.    

That diode can be replaced with and 1 amp or larger rectifier diode. A 1N4001-1N4007 will work as will a 1N5400-5408 series diode.

As for the arcing, do you see an actual spark? Are you sure it is inside the tube, or is it a reflection in the glass?

When you have the amplifier UNPLUGGED, have the cover off, and after you are SURE the HV is zero you can check the diode I mentioned. You should also look carefully at the 3-500Z anode connector. Make sure it is tight. Look carefully for loose wires around the tube and plate choke.

73 Tom
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N9HSB
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Posts: 12




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« Reply #2 on: August 18, 2004, 02:05:09 PM »

Well tom you hit it right on the head with the diode.  There was a dead short across it.  Thanks very much.  I replaced it with a 1N4005.  That is what was in there.  I did a quick tune up on the dummy load.  Look good.  Then I did the key the relay with no load.  The grid current was now at 0ma.  I held it in for the 30 seconds or so, then flash.  Oh yes I can say it's in the tube.  I had the lid off this time. WOW! After that the new diode is now dead short.  Also the T/R relay chatered when this happened.  Well we know how the first diode blew.  Thanks for your help Tom.  Any further comments would be great.

73's
Cliff
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W8JI
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« Reply #3 on: August 18, 2004, 07:23:01 PM »

Cliff,

You have a gassy tube. I think I have some stuff on my web site about that.

Now here's what you can do.

When the AL80 was designed 3-500Z's were very reliable. We elected to NOT install a surge limiting resistor in the anode because there was already 10 ohms or more of stray resistance and tube arcs were extremely rare.

That isn't the case now, so I'd probably add 20 ohms of a high energy absorbing resistance between the anode supply end of the RF choke and the electrolytic caps. You'll also have to add a bit more shunt capacitance from the cold side of the choke to the chassis. Maybe an additional .003uF or so. Three more .001 7.5kV caps will do.

Then when the tube flashes surge current will be 3000/30ohms=100 amps instead of 3000V/10ohms= 300 amps like now. If you use a 1N5408 diode and a surge limiting resistance it will survive occasional 100A arcs without the irritation of removing the cover and changing the diode.

You also may have wiped out the zener diode in the bias.

By the way, be sure the HV is at the factory spec or lower. If your amp has the buck-boost winding be sure it is configured to provode the lowest HV.

It also helps to always load the amp as heavy as possible. This keeps peak anode voltage down.  

You'll almost certainly have to change the existing 3-500Z out, but 3-500Z tubes are not very reliable now compared to how they used to be. Sometime you will do OK, sometime not. You might wind up going through a couple tubes before you find a good one that lasts.

I've had batches of new 3-500Z's where the entire lot of 50 were bad. That used to be unheard of.

Be sure whatever tube you buy has a good warranty and the company who sells it is reliable.

73 Tom
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N9HSB
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Posts: 12




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« Reply #4 on: August 20, 2004, 11:44:37 PM »

Yes Tom the zener was bad as well.  I am going to replace it with NTE5182A.  I found a 20 ohm 10 watt cement wirewound to put between the rf choke and the capacitors.  I cant find the .001 uf at 7.5 kv caps.  The best I could find was .0015 uf at 7.5 kv.  Well Tom let me thank you again.  You really have been a big help.  The Gentleman I got this from told me he had not used it for some time.  That explains the gassy tube.   Hey I looked at your web site.  Very nicely done.  Just gobs of good info.  

73’s
Cliff
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KZ1X
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Posts: 3229




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« Reply #5 on: August 22, 2004, 11:36:49 PM »

It sure helps to have the amp designer around on eHam, doesn't it?

;-)

Now, if I could only find a site like "eCar" or something, where I can chat with the guy who designed my wife's minivan ...
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K2QPN
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Posts: 70




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« Reply #6 on: August 29, 2005, 03:39:12 PM »

I am having the same exact problem as Cliff. Cliff what was the final fix? Did you also replace the tube? Where did you get the parts? History: I am trying to fix an AL-80A for a friend. First I had to replace the antenna relay (bind) and the zener diode. Then the QSK board proved to be bad and I disconnected it. The amp then worked until it arced and I had to replace the meter diode. Worked until it arced again. With a new diode, key down for 30 second and the plate glows red and then it will arc.

I will try adding the resistor, caps and a heavier diode.
Any other ideas???
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N9HSB
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Posts: 12




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« Reply #7 on: September 06, 2005, 04:04:15 PM »

THE MAIN CAUSE OF FAILURE WAS THE TUBE. I DID PUT IN A STOUT DIODE AND REPLACED THE TUBE. I HAVE NOT HAD ANY TROUBLE SINCE.

73'S
CLIFF
N9HSB
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