High Voltage Rectification
Tube amplifier uses high voltage rectification. That’s
something everybody knows, be it using silicon diode or tube rectifiers, all
will need a power transformer with high voltage output. Such power transformers
will have a lot of windings due to the step-up needed. It will be pretty
difficult to squeeze a lot of wires into the transformer.
Full wave rectification needs center tap and bridge or half
wave rectification will need only 2 wires. So, there comes the consideration of
whether to have a center tap or normal 2-wire output. Center tap will have more
wires and the wires used in the transformer will have to be thinner compared to
the 2-wire version. Things have just become harder for a center-tap type!
Well, it is not hard to explain this as usually we would use
a single rectifier tube to convert AC to DC. Using 2 rectifiers for bridge
rectification is possible but is 2x the price of a single tube. Using half way
is OK too but the efficiency is just too low. Therefore we resort to full wave
and that requires center tap winding.
It is OK to wind center tap windings. Usually experienced
winders will be able to use the right size of wire to use for satisfactory
results. The problem is for some lazy, inexperienced or simply ignorant ones
will wind from start to end, and draw out a center tap during midway for EI
core transformer that most of us use. If they did not calculate and lay
properly, the center tap will not be centered, as the diameter for inner
windings is smaller than outer windings.
So, the inner portion will have lower resistance and
therefore higher current, and vice versa. This will create imbalance in the
current delivered to the filament of the rectifier tube, and cause one side of
the filament to age faster and use up faster, where we are not really willing
to see. Unless special bobbins are used and windings are balanced, if not, the
above case shall happen.
Well, if the above method is not use, there’s another way of
doing it, where both taps of a center tap are wound at the same time, in
parallel. Sounds like a brilliant idea, right? Well, wrong again! The voltage
of the winding will be out of phase, so, at 1 point, the maximum voltage
present between the winding will be the maximum potential difference. This will
post as a great risk of breaking down the insulation and cause shorts! This is
worse, if the proper steps of making a good transformer are not followed.
Skimp on quality and you will pay for it eventually.
J&K Audio
19/12/2013
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