[Amps] Dentron MLA2500 SWR Question

[Adrian Rees]

Hi all
 
I have a Dentron MLA2500 linear, which mostly works fine, and I'm getting lots of RF out.
 
However, on 3.5MHz (no other band)  it is difficult to tune and was wondering if someone could explain to me what is going on, and how I may cure the the problem ? So here goes, remember this is just on 3.5MHz (80 M)
 
Prime mover is IC740 
 
I tune the Dentron with about 10w from the IC740 into a Dummy load. When I have peaked the linear, the SWR seen by the IC740 is around 1.8:1, as I increase the drive, the SWR seen by the IC740 goes up to as much as 5:1 or more, thereby reducing drive power. This slow cycling between lots of drive, but high SWR, then the IC740 reducing drive (to protect the finals) reduces the SWR, the power drops, and the whole thing starts again.
 
I have got a small ATU installed between the Linear and the Radio, but this is not really curing the problem. My other HF radio (756ProII) with its Auto ATU is a little better, but again it cycles.
 
Both Prime movers get to about 25 - 30 watts of drive and the cycling starts. (Which means I only get about 250 - 300 watts out).
 
I know the Dentron doesn't have a tuned input, hence the small ATU between it and the Prime mover. 
 
I guess the amp is going into some form of hysteresis ? Perhaps I am wrong, but some insight into whats happening would be good, and any suggestions as to how to fix this annoying action would be great.
 
Thanks
 
 
Adrian MW1LCR 
Wales