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Ameritron AL-811 problems

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  #1  
Old 06-09-2005, 01:39 PM
KD7UGY
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Default Ameritron AL-811 problems


I have a Ameritron AL-811 that has been converted to use 572B tubes. When I got it it would only do 100 watts so I replaced the tubes, let it sit on stand by for a day and then it would do 800 watts for a while. But after a little while it would start dropping off on power so I let it sit on stand by for several days and everytime I checked it it would be doing less power and now it only does about 80 watts. So does it make sense that it could go through tubes just sitting on stand by? And why would it do that? Could it be that the power supply voltage is too high? I think the voltage sits around 1800 volts according to the meter, but I'd have to check to make sure.


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Old 06-09-2005, 03:24 PM
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The 572b tubes are a very durable transmitting tube. you have to be very mean to them to make them go south. 1800 volts is probably the minimum you would want to run them at, they run better at around 2500. Check your idling current on the plates. to do this key the amplifier with no carrier or RF input. You should see around 90 milliamps of current. Then check the current at full output. You should see about 500 milliamps, which is a reference which you should not exceed. Watch the high voltage at full out and make sure it is not dropping to low, 300-400 volts drop is about right depending on how stout the transformer is. These are all things to look for if your amp is working normally. Maybe some else will jump in with some other pointers too.
73
Ed
KC0PZE

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Old 06-09-2005, 05:24 PM
KD7UGY
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One other thing I forgot to mention is that it has been modified to not use a relay cable, so the relay operates by receiving RF. I have heard many times that 572B's are a really strong tube so I'm assuming something in the amp is wrong. It's just that the more I let it sit on stand by the lower the output drops until it gets to around 60-80 watts and I'm pretty sure the tubes are going bad. I'll check the current and voltage tonight and post what I find out.

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Old 06-09-2005, 06:43 PM
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Default Narrow the problem down

What would happen if you let it set cold for a day or two?

While it is setting on standby, are the plates of the 572B's turning orange (drawing current all the time) or are they gray (not drawing current until transmit)?

Also, do you have a dummy load so that you can eliminate the antenna as a problem and, also, what does it do on the other bands like 20 or 40 meters?

.

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Old 06-09-2005, 06:45 PM
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With 1800 volts and 500mA plate current those 572B's should show no color on the plates whatsoever under keydown conditions.In standby mode they should last a lifetime.As Ed said,check the bias.If it is too high the tubes will draw too much current but should still not be damaged.I would be suspicious of something in the power supply causing a drop in plate voltage but it must be awfully low to cause only 60-80 watts output.How many watts are you driving it with and is it being tuned and loaded properly?

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Old 06-09-2005, 07:18 PM
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Default AL-811

Quote:
Originally Posted by QRN
.....As Ed said,check the bias.If it is too high the tubes will draw too much current but should still not be damaged.......
The bias is supplied by diodes D-19 thru D-24

http://www.ameritron.com/man/pdf/AL-811.pdf

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Old 06-10-2005, 01:20 PM
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Okay, I checked it out last night and here are my results, everything was done into a dummy load with a bird 43, with 10 watts carrier in on am I got 40 watts out, plate current-75ma grid current-30ma. Voltage without a carrier is 1800 and voltage with 10 watts in, 40 out was down to 1700. increasing the drive to 25 watts got 60 watts out but the grid current won't go over 80ma and the plate current won't go over 35ma. I tried it on various bands from 10-160 meters with the same results or at least close to. This is about the same way it acted before with the old tubes, then when I replaced them everything worked the way it should for a while. Does this sound like the bias is off and needs adjustment? Thanks for all the help.

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Old 06-10-2005, 07:38 PM
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Default Ameritron 811A

The bias is established by the diodes D19 thru D24 in the B-minus lead (center-tap of the filament transformer). Check the diodes; they should all be the same.

Looking at the schematic, point W6 (B-minus) is connected to W5 when the relay RLY-1B closes. B-minus continues up thru S-4 (this switch could be flakey), thru the current meter, M1. R-10 is a 330 ohm resistor in series with the negative side of the meter (make sure itís good) and, finally you have a 0.6 ohm resistor R-9 between B-minus and chassis ground.

I am not sure how the conversion to 572Bís were done; are there any resistors on the tube sockets themselves (maybe 47 ohms at 2 watts?), if so check value of all and make sure that they all are connected.

I cannot believe that the amplifier is eating tube with 1800 volts and no color showing; something is stopping you from drawing current. Replace the DC Blocking capacitor, C-13.

.

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