This is an epilog to the recent discussion of filament inrush current
reduction by means of controlled flux leakage in the filament transformer. I
attempted to show in my comments that this was a viable concept
theoretically, and some simple circuit analysis verified that. I also
expressed some doubt about the practical possibility of accomplishing this
without going to a different (and more expensive) transformer design. In
order to fully resolve it in my own mind, I have done some additional
research on the subject, consisting mainly of speaking with several
well-qualified transformer designers. They all concurred with my opinion that
the desired result is NOT possible with a conventional transformer. In
particular, the primary and secondary have to be physically separated by a
certain space to achive the controlled leakage. None had ever seen a
successful design based upon a single, overlapped bobbin.
One of my 'experts' has considerable experience with the SB200/SB220 amps and
he confirmed my judgement that it has a conventional filament/bias
transformer. The only current limiting will be that normally due to circuit
resistance, including the fuse and AC linecord!
As far as using the filament choke for this purpose; I don't think the crux
of that issue was properly revealed, so here it is. Obviously, the choke
isn't going to work (for this purpose) without at least some modification.
Technically, it would be possible to simpy rewind the choke to produce the
necessary amount of L(common mode) at RF, and a much greater amount of
L(differential) at 60Hz. That would involve adding a winding with several
hundred times the number of turns it starts out with (the required inductance
is about 10(+4) times the RF inductance required for cathode drive
decoupling.) If somehow all of these physical stretches of the imagination
could be realized, the thing would still fail before getting 10% of the way
into the first cycle of the AC line because:.............The core of the
typical filament choke would never handle the flux produced by this large
inductance. It is way too small and would saturate almost instantly.
becomming about as ferromagnetic as a piece of wood.. In fact, I could prove
to you with some additional numbers that you couldn't even build the required
choke out of this type of ferrite no matter how big a piece you used- mu is
way too low.
In retrospect, I think this was a good case where a well-intended but
incorrect conjecture led to a good electronics lesson for us all.
73
Eric von Valtier
P.s. You may have heard of one of my sources on commercial transformer design
and mfg - his name is Peter Dahl.
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