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Author Topic: New to linear amplifiers, Question  (Read 571 times)
KG5QHF
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Posts: 1




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« on: November 06, 2017, 06:18:46 AM »

I recently purchased an Ameritron AL-811h from a SK’s estate. The amp powers up and shows normal plate voltage, but upon keying, there is no plate or grid current. Upon investigation, I see that one of the four tubes doesn’t light. Moving that tube to another position, it still doesn’t light. My question is, can one bad tube cause this kind of problem, or is something else going on? Can I remove this bad tube and try to run it on 3 as a test? Physically, the tubes look okay, at least to my untrained eye.

One thing I thought strange. When I got the amp, I discovered all four tubes were just sitting loose in their sockets. It seemed strange that they would have come unseated on their own. Perhaps the last person to trouble shoot just left them that way. Besides that, the amplifier is very clean inside and out, and appears to have been well taken care of. Date codes show it was produced in 2011.

Thanks!
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K8AXW
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Posts: 6312




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« Reply #1 on: November 06, 2017, 09:51:18 AM »

Pull out the bad tube and see what happens. It will still operate.

Do you have it integrated properly with your exciter? (Although it's un-American to read the instruction manual before operating, I can't suggest you do so strongly enough!  It's too nice of an amp to screw up just "fiddling around with it.")

Can you hear the amp relay kick in?  Are you applying drive in the exciter CW mode?

Those are the basic questions that need answering before you go any further.

Tips:  If you have the amp keying line hooked up and the exciter RF output connected to the amp RF input, when you push the mic button in the SSB mode or press the key in CW mode, the amp should key up and show idling plate current. (Assuming the amp is not in the "bypass" mode)

As you speak the grid current should increase. Switch to the CW mode and depress the key.  If you show grid current, you tune the amp to resonance.

Stay within the grid/current parameters, remembering that the plate current will top out less because you're running just the 3 tubes.


Consider the possibility that all 4 tubes are kaput!  I'd give the band switch, the variable caps and the RF choke very close scrutiny for any burn/arc evidence.
 
It can become more complicated than this so come back with your findings.
« Last Edit: November 06, 2017, 09:55:43 AM by K8AXW » Logged
N1UK
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Posts: 2081




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« Reply #2 on: November 06, 2017, 11:25:21 AM »

I just fixed an AL811H and only one of the two relays was switching in due to a blown switching transistor.

When you key the amplifier with no rf input are you seeing a no drive Plate current of 75mA ?  If not then you probably have a problem with the bias diode string or the relay drive circuit.


Mark N1UK


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