[Amps] 160m mosfet linear amplifier problem with ferrite cores

[David Cutter]

I've looked again at the jpg and I think you have used trifilar windings, so, that's my first idea more or less gone.  Imbalance can be due to poor coupling between the windings themselves or imbalance in the driving currents.  With high voltage supplies, balance error should be small (compared to say a 12V amplifier where a small difference in voltages can show a large imbalance %).  If you could measure the current in each half of the primary that would tell you the whole story: you will need a HF current probe, preferably 2 probes and dual trace scope which will also tell you about cross-conduction.  

Another thought: you have 6 transistors in parallel in each half and emitter (sorry - source) degeneration.  Did you try matching the FETs for gain or phase delay?  If not, it's conceivable that one side is conducting harder or for longer than the other side and causing an imbalance.  I don't know if the source resistor is the optimum value for mis-matched FETs, but a higher value would increase negative feedback and help with this; you have oodles of gain to sacrifice.  Along the same lines, the input drive to each FET may need optimising for the same reason.  Check all your resistor values, one might be a dud.

This is all armchair speculation, I don't have enough experience for better detail.  Someone with modelling experience could probably do this quite quickly.  

Alex has probably got the answer regarding core material.  

David
G3UNA
  ----- Original 

        And how can i repair this problem???





          You might consider imbalance in each side of the primary which will cause a net dc to pass, saturation and over-heating.  This may be rubbish but it could get others thinking along a different path.

          David
          G3UNA


          > HI. i have constructed this linear amplifier 500w rms (2kw pep) (50ohm) with 12 mosfet irfp360, http://tzitzikas.webs.com/linear500w.jpg for 160m band.
          > When i gave 3watts of driving r.f power, it gave to output only 190w at 106VDC (6A current). Two radio amateurs who have construct this linear claim tha it gives 500w
          > r.f power at 110vdc.
          > But when i tried to give 4watts of driving r.f power the ferrite Cores (43 material)
          > of transformer T3 broken! Which do you think is the problem??
          >