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Author Topic: "clicking" and low power (<200W) on AL-811  (Read 2115 times)
W4PAH
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« on: August 09, 2013, 08:37:32 AM »

Hi all,
I have been using my AL-811 (not new, but new to me) successfully at 500W for the past few weeks. Recently (actually, the second time this has happened), my fuses both blew and I replaced them. Since I replaced them, behavior has not been typical. I can't get anything over 200W (most is in the 175W range). I use a tuner after the amp, and I know that the antenna is matched quite well to the amp, but my SWR meter on my rig (K3) is showing a high value.

When I tune up at 20-25W I can get about 100W out from the amp. When I move to 50-65W I get < 200W. And there is clicking. I can't tell if it's a relay or if it's something else.

So, the main issues are
1. High SWR from the rig to the amp (when the antenna is < 2:1 SWR to the amp)
2. Low power from the amp to the antenna

Happy to provide other information if folks have ideas on where to look.

Thanks.
-john W4PAH
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N3QE
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« Reply #1 on: August 09, 2013, 09:21:09 AM »

The amplifiers internal HV meter should prove useful, both key up and key down.

The clicking seems very likely to be the relays and indicates a droop in the amps LV DC control voltages that may or may not track a droop in HV. The same LV DC control voltage drives e.g. the meter lamps.

If the tubes are drawing so much current that the voltages droop and fuses blow, diagnosis goes one way. If the tubes aren't drawing much current but voltages are still drooping and fuses blow, diagnosis goes a different way.

The high SWR on all bands, I would check alignment of the band switch. Does the amp "peak up" at same knob positions as before?
« Last Edit: August 09, 2013, 09:23:38 AM by N3QE » Logged
W4PAH
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« Reply #2 on: August 09, 2013, 09:24:41 AM »

Thanks for your reply N3QE.

The amp does peak up at the same positions as before.

Ameritron/MFJ support (I called them after I posted this to the forum) suggested bad tubes as well as a bad D16 (this is the diode involved with metering functions). I may have blown my fuses before because the meter was reading incorrectly. Something to check.

I suppose new tubes are definitely in my future. Is RF Parts the best place to get these? I'm going for 811A tubes, I believe, not 572B's.
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N3QE
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« Reply #3 on: August 09, 2013, 09:34:45 AM »

Those diodes in the metering circuit and power supply board seem to fail under many fault conditions. That's true for a lot of amps. (I own an 811H but have worked on a many brands of amps over the years).

They're like a 5 cent part, and in theory easy to test and replace, but I have never figured out why they fail so often. In the metering circuits I'm most likely to find them failed "shorted".

If I don't trust the meter I look in the ventilation slots of the amp to make sure the tube plates show some color under extended full current, but are not growing bright orange. There's other colors too, the beam power amp tubes (e.g. 6146B's and sweep tubes) often show this bright blue color when they fail. I haven't seen that color on a triode. My "look at the tube and make sure it's the right color" diagnosis method probably sounds incredibly naive to those who adjust their filament current and plate voltage to a fraction of a  percent :-)
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N6AJR
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« Reply #4 on: August 09, 2013, 09:38:49 AM »

Ameritron also sells 811 tubes.  I agree with your decision, 811 tubes are much cheaper, and will last about as long, and the amp is designed for them.  go to mfjenterprizes.com ( spelling) and search on 811 tubes. should be around 65 bucks for 3 of them, matched.  keep your old ones that are good as spares.
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W4PAH
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« Reply #5 on: August 09, 2013, 09:48:30 AM »

W8JI's page seems to point to bad/shorted tubes.

http://www.w8ji.com/ameritron_811h_572_amplifier_trouble_shooting.htm

Quote
3.) Amplifier blows fuses when power switch is turned on (always use proper 250V fast-blow fuses), blows step-start in other amplifiers

    Check for shorted tubes
        Remove the plug from outlet, remove the cover, and remove the top white anode connectors from all tubes. Do not let the white ceramic plate caps touch anything; lay them carefully on the tube glass. Replace the fuses and cover, plug the amplifier in, and turn the power on. If the amplifier powers up with tube plate caps disconnected, a tube is shorted
        You can inspect the tubes for a silver area on the plate (sign of overheating), or reconnect tubes one at a time until the problem reoccurs to find the bad tube. Never connect the amplifier to power lines with the cover removed
        If the problem is not tubes, defer service to someone experienced with high voltage circuits

5.) Low Power Output

    This is normally weak tubes, ALC, or improper tuning (see Tuning Supplement Sheet)
        Normal RF power gain is about 12 times input power. If drive power is 40 watts, the AL811H should produce 400-500 watts into the antenna and 600-700 mA plate current
        Weak tubes also often cause high input SWR on all or most bands
        Grid current should normally be 1/3 of plate current when properly tuned near full output
    Be sure your RF output meter is good (see Tuning Supplement Sheet)
        Check your radio power through meter with amplifier off. Radio should be 100 watts using high power scale of meter

I'll also check D16 using this (also from W8JI's page)

Quote
1.) Test on SSB using MOX or push-to-talk with no mic gain. With no RF drive power, but the amplifier keyed, the amplifier’s grid current meter deflects about the same as the plate current meter. This is a common problem

    D16 shorted (811), D117 in AL80 series mainframe, including AL572. This will also cause the plate and grid current meter to track each other, and indicate false grid current
        This problem is almost always from a defective tube or tubes that have flashed over internally, generally from excessive internal gas
        This problem occurs after HV has been shorted, arced, or discharged to chassis
        In older AL811H and AL572 amplifiers, grid circuits using resistors should be modified to current production
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