SE topology
On Oct 10, 8:12*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 10/10/10 01:04, Patrick Turner so witilly quipped:
One would think I might have noticed misbehaving core materials but
afaik all core materials never become permanently magntized in one
direction.
I'd actually read about this happening before, if I remember correctly
(it was a long time ago) that a big problem with class A circuits is
slow magnetization of the core. *My solution was to reverse the current
flow rather than replace them (which happened to work), and being in
High School at the time I didn't have much cash to refurbish it
properly. *The transformers themselves were relatively small, so I'm
guessing that they were made of cheap or easily magnetized materials, in
lieu of ferrite or high quality iron.
Reversing the DC flow direction should not hurt anything with regard
to core performance. Wikipedia has some info about "anisotropic" core
behaviour which deals with cores that have a preferred magnetizing
direction of magnetization but apart from mention of this I cold not
find any other info after Googling "permanent magnetization
transformer cores". I didn't see any specific mention of slow/fast
core permability reduction over time in class A SE OPTs. I would have
thought the iron molecules relax fully each time the set is turned off
when there is no DC present.
But there is one slight issue of reversing of anode-B+ connections
because the active end of the primary may go to the primary end at the
largest turn length rather than the smallest turn length so the shunt
capacitance may change to perhaps 30% more which may affect the
stability if there is NFB.
In cheap old radio OPTs there is often only 1 section of P and one
section of S, often with P wound on first and the anode connection
made at the bottom of the bobbin and closest to the core so that the
shunt C is quite low because the bobbin base is 2+ mm thick. The other
end of P is at a steady B+ potential and may be close to S winding and
C does not matter.
In such a case when reversing the anode and B+ connection there is a
big increase in shunt C from anode to 0V, much more than 30%.
Patrick Turner.
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