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Author Topic: Instructions For Discharging Filter Capacitors in Ameritron AL-80A Amplifier  (Read 6666 times)
WI5O
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« on: October 13, 2010, 02:51:29 PM »

Does anyone have a set of instructions for discharging filter capacitors in an Ameritron AL-80A amplifier?  I am going to need to take the 3-500Z tube out for packing.  Thanks.  -Phillip 
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W1QJ
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« Reply #1 on: October 13, 2010, 03:37:03 PM »

The answer to this question is subject to much discussion.  We can get super technical or we can make it simple.  Assuming your amp is on at this very moment and the HV meter is reading 3000 volts and you turn the amp off and "in real time" watch your HV meter slowly return to 0 volts you can be pretty certain that the caps have been bled off.  To be absolutely safe you then pull the plug out of the wall.  The next morning you open up the amp and with a very good insulated screwdriver you short the HV at the tube anodes to ground.  You then should be safe to remove the anode cap and pull the tube.  Assuming you have the amp on and you do not read any HV on the meter, you are not certain by any means that the caps will bleed down after you turn the amp off.  At that point you would have to assume the caps have not bled down and would have to take a different approach.
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WI5O
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« Reply #2 on: October 13, 2010, 04:00:13 PM »

I suspect I can just connect a lead in with a 2 watt or greater resistor to the chassis on one end and to the anode of the tube with safety insulated alligator clip.  I've already made sure that there is no reading (zero volts on HV meter) when I select the HV position.  It bleeds off completely in about ten minutes or less most of the time.  I will leave the unit off a couple of days as a precaution and then discharge with the above method described by Ameritron and your answer.   I suppose I am a glutton for safety - better safe than dead.  Thanks for your answer W1QJ

The answer to this question is subject to much discussion.  We can get super technical or we can make it simple.  Assuming your amp is on at this very moment and the HV meter is reading 3000 volts and you turn the amp off and "in real time" watch your HV meter slowly return to 0 volts you can be pretty certain that the caps have been bled off.  To be absolutely safe you then pull the plug out of the wall.  The next morning you open up the amp and with a very good insulated screwdriver you short the HV at the tube anodes to ground.  You then should be safe to remove the anode cap and pull the tube.  Assuming you have the amp on and you do not read any HV on the meter, you are not certain by any means that the caps will bleed down after you turn the amp off.  At that point you would have to assume the caps have not bled down and would have to take a different approach.
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W8JI
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« Reply #3 on: October 14, 2010, 03:12:22 AM »

You are wasting time waiting for hours. After a couple minutes the HV will be nearly zero.

The only exception to this would be if you have an open bleeder/equalizing resistor. If one is open it will show as a slower sag than normal from about 500 volts down to zero. That voltage can linger for 20 minutes with an open bleeder.

Unplug the amp to be sure you do not turn it back on, and clip the anode to ground through a low value  watt resistor using an insulated system. The reason to NOT directly short the anode to ground is to NOT hurt the meters or meter protection diode from a surge if a filter capacitor is up, although that is a very minor worry.

Just be sure the meter registers HV when it is on, then watch the meter decay when off and unplugged. There is nothing that can be harmful once that meter is back at zero, provided it was up before shutting down.

73 Tom
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K4DPK
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« Reply #4 on: October 14, 2010, 09:33:01 PM »

I usually short from the BOTTOM of the plate RFC to ground, just in case there's enough energy stored somewhere to blow open the plate RFC.  The shorting stick is a twelve-inch insulated rod with a 10k, 10W ww resistor and a grounding pigtail on one end.  This avoids catastrophic energy dumps.

Phil C. Sr.
k4dpk
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AD4U
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« Reply #5 on: October 18, 2010, 06:17:45 AM »

Ditto on everything already posted.  But since you asked this question, I assume you are not at all comfortable working around HV.  What I am going to suggest is an old timer's trick and probably overkill, but it may make you a bit more comfortable going inside your amp. 

Make up a 12 - 15 inch (or so) length of #14, #16, #18 wire.  If you use insulated wire, remove about 1/2 inch of insulation from each end of the wire.  Attach an alligator clip to one end.  Drill a small hole in a 12 inch (or so) length of 1/2 inch PVC pipe and push the "free" end of the wire through this hole, leaving a total of about 1-2 inches of the wire sticking out of the PVC pipe.  Secure (twist?) the wire to the PVC pipe.

To make sure your HV caps are discharged:

1.  Unplug the amp
2.  Wait several minutes (or as long as you want to wait - wait a day if you want to)
3.  Insure the HV meter is reading zero
4.  Connect the alligator clip to the chassis of your amp
5.  Hold the 12 inch PVC pipe by the end AWAY from the end with the wire attached
36  Touch the bare end of the wire to the anode of the 3-500 tube

Nothing should happen.  If you get a loud POP and wet your pants, there was still HV present.

Like I said, this is gross overkill for someone who is comfortable working around amps and someone who understands how the HV system works.  However, since you asked the question, I assume you are not familiar with HV. Once you ground the anode of the 3-500 to the chassis, you can BE SURE there is no HV present.

Dick  AD4U



 
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AD4U
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« Reply #6 on: October 18, 2010, 06:33:39 AM »

Sorry Phil, I posted at the same time you did.

Dick  AD4U
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K4RVN
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« Reply #7 on: October 21, 2010, 06:33:21 PM »

WI50
Don't just leave the amp off  a couple of days, unplug it. I am sure you would do this but make sure. I would also do what Phil Sr said to short the capacitor to the case. I always do with my AL80A just to make sure no voltage is hiding somewhere. I have owned mine 22 years this month without getting a shock.
I must admit that I have always used a good insulated screwdriver to short the tube plate cap connector with no problems. I think I'll make me a shorting stick with a resistor next time.

Frank K4RVN
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