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Ameritron AL-811 problems

Discussion in 'Amplifiers' started by KD7UGY, Jun 9, 2005.

  1. KD7UGY

    KD7UGY Guest

    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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  2. fasteddy

    fasteddy Active Member

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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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  3. KD7UGY

    KD7UGY Guest

    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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  4. paws264

    paws264 Active Member

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    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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  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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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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  6. paws264

    paws264 Active Member

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    AL-811

    The bias is supplied by diodes D-19 thru D-24

    http://www.ameritron.com/man/pdf/AL-811.pdf
     
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  7. KD7UGY

    KD7UGY Guest

    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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  8. paws264

    paws264 Active Member

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    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 it’s 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 572B’s 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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  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    That amp should not require any mods to convert to 572B's.572B's are like 811A's on steroids.Electrically the same but with 160 watts plate dissapation instead of the 65 watts for an 811A.

    I'm confused by a couple of your statments:
    First you said:

    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.


    Then you said:

    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.



    Are you sure that is right because more power output should result in more plate current,not less.Either way something is wrong with the bias because the plate meter should read around 110mA +/- 25 % (according to the manual) with the amp switched inline and with no drive applied.

    Try reducing the loading control somewhat and see if that raises the grid current.It may be loaded too heavily.

    Check R12 and R13 in the ALC circuit as well as the setting of R14 which is the ALC adjust pot.If it is set wrong it will limit the amount of drive from the radio regardless of what the radio is set at.That is of course if you have the ALC line connected to your driver radio.

    After that :?: :?:
     
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  10. KD7UGY

    KD7UGY Guest

    Sorry for the confusion, I meant that the plate current won't go over 80ma and the grid current won't go over 35ma. I'm not sure on the ALC adjustment, when I bought it the guy told me that the ALC had been defeated when they changed the relay to be keyed by receiving RF instead of being operated by a relay cable, so I don't have the ALC hooked up. Does that sound right? I'll look inside of it tonight and see what I can find. Thanks again for all the input.
     
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  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    I forgot about the amp being modded to accept RF keying.If the ALC circuit was defeated then that is not a factor.If an amp is loaded too heavy it will prevent it from drawing much grid current.That amp should see about 550mA plate current and 150mA of grid current with +/- 1600 volts on the plate.These values are with the max of 70 watts drive.What radio are you driving it with and what is the input SWR going into the amp?
     
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  12. nomadradio

    nomadradio Active Member

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    Uh, you're not running this from a 120-Volt 15-Amp outlet, are you?

    With adequate filament voltage on that tube, a 572B should last more or less forever in that box. It's like running a box meant for the 572B on "low" side all the time.

    If you're running it on 120 Volts, and the outlet circuit isn't stout enough, you'll see the color of the filaments inside, dim noticeably with full drive.

    This would indicate that the line voltage was falling under load, causing the 6.3 Volts on the filaments to fall, as well. This dimming of the yellow-white filament color indicates that the filament temperature is falling.

    Running that filament at too low a temperature can and will cause the tube to go "soft", and appear to be worn out and weak.

    If this is in fact the root of the trouble, you won't see those filaments dim when you key it now. If you remember seeing that happen when they were 'fresh', that would be a clue.

    And if you're running it from a stout 240-Volt circuit, this is probably not the cause.

    73
     
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  13. KD7UGY

    KD7UGY Guest

    Thanks for all the help guys, I think nomadradio figured out my problem though. I am running it on a 120-volt 15 amp circuit and I didn't see the filaments dim but that sounds like what I probably did. So if I want to run it could I get by with running it on a better 120-volt circuit like say 20 amps or should I run the amp on 240 or should I put 811 tubes back into it?
     
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  14. paws264

    paws264 Active Member

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    I'm glad we have a handle on the situation however, I have a problem with this conversion; there are circuit differences in the amps constructed to run 572B's and the amps constructed to run 811's

    572B amps

    Heathkit SB-200. 572 B amp

    Yaesu 2100B w 572B's

    All the 572B amps have a grid leak resistor in the control grid circuit to provide "self bias' for the tube.

    811A Amps

    Ameritron 811

    The 811 circuit has the control grid grounded; if a 572B tube is put in that circuit without the proper modifications done for the grid leak resistors then that tube (the 572B) IS gonna draw more current than it should and it's gonna draw current when it should n't.

    When the RF sensing keyer was put in there I wonder what was done to the center point of the filament circuit; I wonder if it is still switched thru a relay or is it hard grounded.

    I wonder why it was only doing 100 watts when it was sold to him?

    .
     
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  15. nomadradio

    nomadradio Active Member

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    I predict that the damage has been done to that set of tubes. Just changing the line voltage will NOT undo the damage. Might have prevented it, had you done so in the first place.

    You can run 120 Volts, so long as you hold the drive down to about half-throttle. If you draw less current from the wall, the voltage drop won't be that bad.

    A 20-Amp 120-Volt circuit will be wired with #12 wire. A 15-Amp circuit with #14, usually. The fatter wire will have a smaller voltage drop at the same current draw. The reason the problem is worse on a 120-Volt circuit is that you need TWICE the amperage on a 120-Volt circuit as you need on the 240. This causes twice the voltage drop in the same size wire, for the same output wattage on the linear. Turn the drive down, and the current demand drops with it. So does the voltage drop on the outlet wiring.

    If the 811A tubes haven't had this same (mis)treatment yet, try them on a 240-Volt circuit, if you can. Or hold back to about half of full power. Either strategy will work.

    The old textbooks describe a procedure to "rejuvenate" the thoriated-tungsten cathode in a tube like that. I have had pretty poor results on the ones I have tried to bring back to life. Mostly it involves turning up the filament voltage on a test rig. More often than not, the filament just burns out.

    You DO have to blow air on them when trying this trick. The heat from the filament alone gets out of hand when you turn up the filament voltage, even for 15 or 20 minutes.

    If you placed larger wires in the wall, and fed your 120-Volt outlet with #8 or #6 wire, the problem would be partly fixed. Trouble is, most building codes forbid using wire larger than #12 for a 120-Volt circuit. Might void your homeowner's insurance if they discovered it after a house fire.

    Besides, wire that large won't fit under the screws on a 120-Volt outlet.

    Here's hoping the 811A tubes haven't been "flattened" yet.

    73
     
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