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Author Topic: Flex5000 with AL-82 amp  (Read 2752 times)
K9FV
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Posts: 480




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« on: December 10, 2012, 07:19:41 AM »

Hello all, I posted this in the SDR forum, but decided this might be a better spot.

I'm playing with a Flex5000 - neat radio.  I have an issue that I don't understand with the Flex/amp. I have a 756Pro sitting on desk with coax going to AL-82, then thru LP-100A watt meter, 4-position antenna switch to different antennas and dummy load.  Been working good for quite a while.  With the Flex5000 added, I put a 2-position antenna switch with coax from AL-82 to switch, then position #1 to 756Pro with 2 ft of coax and position #2 with 12 ft of coax to Flex. 

My issue is as follows (all into dummy load): on 40 meters and higher freqs, everything seems to work good - no re-tuning required.  On 80 meters with 756Pro I load AL-82 to 1300 watts or so, then move antenna switch to Flex position and I have to re-tune AL-82 a bit - not much, but a bit to get max output (1300 watts or so).  On 160 meters, it takes a LOT of re-tuning to get max output.

I wondered if the 12 ft vs the 2 ft of coax had anything to do with it so I connected the 12 ft of Flex5000 coax to ant #2 on the 756Pro - nope, switch between 756Pro ant #1 and #2 (moving antenna switch to match) and everything is good.

Why would just connecting the coax to the Flex require re-tuning?  I still get the same output from AL-82 with Flex or 756Pro by re-tuning the AL-82.

Any ideas?  Comments?

73 de Ken H>
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N4CR
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Posts: 1694




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« Reply #1 on: December 10, 2012, 11:15:16 AM »

Just a guess...

The output impedance of the two rigs are different. This causes the input tuning circuit of the AL-82 amp to load into the tubes differently.

It's very possible for transmitters to not be exactly 50 ohms output. Most transistor or FET outputs are low impedance and this gets matched up towards 50 ohms with a transformer after the output stage. Hitting exactly 50 ohms can be a real challenge.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
W1QJ
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Posts: 1486




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« Reply #2 on: December 10, 2012, 11:33:17 AM »

Just a guess...

The output impedance of the two rigs are different. This causes the input tuning circuit of the AL-82 amp to load into the tubes differently.

It's very possible for transmitters to not be exactly 50 ohms output. Most transistor or FET outputs are low impedance and this gets matched up towards 50 ohms with a transformer after the output stage. Hitting exactly 50 ohms can be a real challenge.

I'd say that is a good explanation and probably is the case.
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K9FV
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Posts: 480




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« Reply #3 on: December 10, 2012, 01:33:21 PM »

thank you for the input - that does make sense.......  I'm surprised how far off the output impedance seems to be on 160 meters, but can easy see 80 meters as it's fairly close.  I do know how difficult it is to get 50 ohms when trying to match up the output (and input) for MOSFET amps.

Thank ya'll again for the help,

73 de Ken H>
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N4CR
Member

Posts: 1694




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« Reply #4 on: December 10, 2012, 04:19:39 PM »

You can prove this by tuning up the first radio, switching to the second radio and only turning the input tuning slug and see if the amp comes up to power.

The next experiment would be to tune all the input slugs to be between the two rigs so the difference is minimized. That may not be possible.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
K9FV
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Posts: 480




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« Reply #5 on: December 10, 2012, 06:57:24 PM »

Adjusting the input circuit would work - A bit more trouble that I wish just now.  As long as I had some idea what was happening it's ok - easy to adjust plate 'n load for different rigs on just those two bands.

73 de Ken H>
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N4CR
Member

Posts: 1694




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« Reply #6 on: December 11, 2012, 02:00:54 PM »

Adjusting the input circuit would work - A bit more trouble that I wish just now.  As long as I had some idea what was happening it's ok - easy to adjust plate 'n load for different rigs on just those two bands.

73 de Ken H>

Of course that would not be any kind of long term fix. Just proof that we guessed right. Then you find the middle and leaveritethere.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
KC4MOP
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Posts: 755




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« Reply #7 on: December 14, 2012, 07:25:02 AM »

I was going to comment on the different length coax but you did the check with same results. hmmm Hard to believe that the output Z between the two radios would be that different.
May have to do with some type of reactance that makes the tuning change in the amp.
There would be no way to stick an antenna analyzer on the outputs of the radios to see if it is a reactance of sorts, without some damage from RF WATTS. So, we have to guess what's happening.

Fred
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