Flex5000 with AL-82 amp

[K9FV]

Hello all, I'm playing with a Flex5000 - neat radio, and I know there are MANY nay-sayers on this forum for Flex rigs.  I have an issue that I don't understand with the 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 that to ant #2 on the 756Pro - nope, switch between 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.

Any ideas?  Comments?

73 de Ken H>

[KX0O]

Could it be a ground issue?  Maybe the amp is seeing a different ground when connected to the flex.

Doc
KX0O

[K9FV]

Ground issue?  Good point as the RF ground seen on 160 would certainly be different than on 40 meters - but since this is loading into a dummy load I wouldn't think RF ground issue, but am open to comments.

73 de Ken H>

[KX0O]

I would make sure the amp, the flex and the 706 were very tightly grounded... I am talking earth ground... You checked the coax but there is another ground path other than the coax shield correct?

Doc

[K9FV]

Sure is another ground path - the nice green wire from each rig to single point ground.

It was suggested on another forum that the rigs might not have a 50 ohm output, and this could affect input loading.

http://www.eham.net/ehamforum/smf/index.php/topic,86860.0.html

Seems sensible - maybe that's the issue?

73 de Ken H>