Hans-Albrecht Haffa schrieb:
Dear Peter,
I have had similar problems in constructing PAs.
First: One reason for the ringing might be the transistors. Put a capacitor
2n7/ 3kV ceramic from
drain to source at each PA-transistor. This is likeliy if the pa is operated
with an quiescent current.
sec.: If You dont have any quiescent current at all, than there is no damping
resistor at the transformer for the winding that is not driven . Any
winding of a transformer that carries no current will ring. This effect has
been never described in amateur radio . The only thing You can do against is
adding an quiescent current or using an topology where all windings carry
allways a current, that is only two windings, pri and sec.
More upon request.
Hans-Albrecht Haffa
DK 8 ND
CBA, ++49-(0)7161 - 87847
Hi Hans,
the problem Peter has with the T2 of the G0MRF PA (here with four MOSFETs)
I have had too.
1.no current in the quiescent state.
2.all windings carry current during transmission.
3.if I operate the PA in up to 200W mode there is no ringing (no vibration).
4.if I operate the PA in 600W mode there is no ringing in the first
two minutes of operation. after two minutes the ringing begins accompanied
with reduction of output power (permeability loss depending on temperature
I guess).
All problems have been solved by cooling the T2.
btw. the output transformer of my other PA (up to 800W) consists of four
toroids Philips RCC36.6/15.6-4C 65, Al3000, color grey. absolutely no
ringing problems or losses when getting hot!
best regards
Uwe/dj8wx
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