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Author Topic: AL-811 amp question.  (Read 3293 times)
KC2RDC
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Posts: 12




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« on: November 05, 2010, 06:51:05 AM »

Hello to all on the forum,I have an Ameritron 811 amp that I just picked up,it has 3 new Taylor 811 tubes installed,my question is the amp tunes up fine on all bands but 10 meters,it has the 10 meter mod done and the input has been tuned for a low swr ,the grid current meter goes past 200 ma. when tuning the up on 10 meters going all the way to the end of the ma. scale,it does the same into any of my antennas and two different legal limit dummy loads,all of the rest of the bands are fine as far as grid current,anyone have any ideas as to why this is happening or what I can look for would be appreciated.

Thanks
Chris
KC2RDC
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WG7X
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Posts: 350




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« Reply #1 on: November 05, 2010, 07:03:35 AM »

Chris,

I can't give you a specific answer for your amp, but I can say that on my AL-82, I have to slowly bring up the drive while watching the grid current on ten meters. If I try to tune quickly, I get into the same scenario as you describe, only not that bad.

Have you tried slowly increasing drive while watching the grid current, or does it immediately go high even with low drive?

73 Gary
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KC2RDC
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Posts: 12




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« Reply #2 on: November 05, 2010, 07:20:27 AM »

Hi Gary,thanks for the reply,the grid current goes high as soon as you put 20-30 watts into the amp,the output is around 200-250 watts,also I am tuning the amp with a pulse tuner (pecker),I dont try to many different things because of the meter being pinned past 200 ma.and the tubes being new,I dont have an antenna for 160 but 80,40,20,15,17 the grid current is the same on those bands being around 150 ma..

Thanks again

Chris
KC2RDC
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WB2WIK
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Posts: 20612




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« Reply #3 on: November 05, 2010, 08:59:06 AM »

Turn the LOAD control more clockwise (higher number) and try again.
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KC2RDC
Member

Posts: 12




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« Reply #4 on: November 05, 2010, 10:53:59 AM »

no difference in changing the load control setting,grid current is still very high,I tuned the amp on 12 meters and it tuned up fine,I thought maybe it was the bandswitch but 10-12 use the aux setting so would that rule out the bandswitch as being a problem.


thanks
Chris
KC2RDC
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K0ZN
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Posts: 1553




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« Reply #5 on: November 06, 2010, 10:46:28 PM »

Hi.

I have an AL-811 and will try to relate to your situation.  First, based on what I see with my AL-811, 30 watts is much drive for tune up. The grids
on 811A's are quite fragile and there is not much grid dissipation available. Start tune up with about 15 to 20 watts and increase the power in steps.  
DO NOT exceed the maximum listed grid current in the Owner's Manual....you are guaranteed to destroy the tubes if the grid current gets too high.
HEED the warnings in the Owner's Manual. They are serious (and correct) about the grid current limits!

Can you find a "dip" in the plate current?  FYI: tuning is very critical on 10 M....a small amount of control change can make a big difference.

Have you tried tuning up in to a good quality 50 Ohm dummy load?....that would tell you if there is a problem external to the amp.

On my amp, on 10 M,  LOAD = 2.75  and the PLATE = 9.2   The tuning is VERY sensitive/critical on 10 M. i.e. the tuning is very sharp; a little change in the control knob goes a long way.

My amp will not reach the "published" output on 10 M.... it has been a while, but I was thinking it only put out about 350, but it will run full
output plus on the lower bands....just the nature of the beast.  811A's are not efficient at 28 Mhz.

Try tuning it into a good dummy load....that will tell you a lot.

Are up absolutely dead positive the 10 M mod is correctly done?

check  ALL ...and I mean EVERY nut and bolt and especially those that connect grounds and chassis grounds for tightness. Ameritron/MFJ is absolutely notorious
for having loose screws. I had a critical grounding lug on mine that had a very loose screw. Make sure the coax connectors external to the amp are good.

Good luck.

FYI: treat those 811A's kindly. They are nowhere near as "tough" as Ameritron advertises them to be!

73,  K0ZN

« Last Edit: November 06, 2010, 10:57:34 PM by Jon M. Pollock » Logged
KC2RDC
Member

Posts: 12




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« Reply #6 on: November 07, 2010, 03:07:44 AM »

Thank you Jon,I have tuned up into 2 different legal limit dummy loads with the same results,I did tune it with the starting setting you use and it seems to tune about the same way,I tuned it with 20 watts and it had a good grid current also increasing the drive to 30 watts the grid current was around 160ma.and the amp was putting out 400 watts with a tone,I'll leave well enough alone,also you are correct on critical when making adjustments it is very touchy on 10 meters,thanks again Jon and to everyonr that replied to this post.

Thanks again to everyone.

Chris
KC2RDC
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WX7G
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Posts: 6136




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« Reply #7 on: November 07, 2010, 05:56:02 AM »

I retubed my AL-811 with Taylor tubes about a year ago and the grid current is higher with these tubes.
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K0ZN
Member

Posts: 1553




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« Reply #8 on: November 07, 2010, 07:03:59 PM »


Hi, Chris. 

Glad you got it going OK. There is no sense in trying to squeeze the last watt out an amp; being a slightly conservative in the drive and output will make absolutely
NO difference in your signal to somebody 1,000 miles away....but the tubes should last longer and you are less likely to have problems.

I STRONGLY suggest you read the excellent info on W8JI's website about the proper way to tune an amplifier. This article also
explains WHY you need to tune an amplifier in the correct manner and what happens if you don't.  GOOD STUFF!

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

73.   K0ZN
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