Hi Lou,
> Hi, does anyone have any idea how much current/power can be drawn from the
> dual primary of a plate transformer from one leg of the 120V to the (center
> tap) 0 volt connect when both windings are in series for 240v?
If the transformer was of the same size but without a secondary, with
all the space devoted to the dual primary, then you could draw twice as
much power from the center tap as you can from your present secondary.
With your transformer as it is, and without any load on the secondary,
you can draw its nominal power from the center tap of the primary, and
slightly more if the thermal coupling across the winding is good (20% more).
When you are loading the secondary to its nominal capacity, then you can
only draw a small amount of power from the center tap. Any current drawn
from there will cause an overcurrent in the primary half not connected
to the 120V load, and reduced current in the other half. Strictly
speaking, if the transformer is designed to work at the limit at its
nominal load, then NO current can be drawn from the center tap, because
any current taken from there will increase the heat generation in the
transformer beyond the normal one. But in practice, you can make use of
the headroom that's usually designed in, and draw a small current, but
with emphasis on "small".
> With most
> amplifiers you will see the blowers/fans run off in this way.
Yes, it's indeed the case with mine. But there the high power plate
transformer is used to get the 120V for the fan. The transformer is
rated at two kilowatts, and the fan at 20 watts. That certainly
qualifies as a "small" load!
> The reason being, I
> have a 120v filament transformer for a single 3CX800 tube. The total
> primary current would probably be less than 200 mills. Since the 3CX800 is
> 13.5v
> at 1.5 amps, that is about 21 watts, I would assume the primary wattage
> would
> be near the same, about 21 watts.
A little more because of the losses. Small transformers like that are
rather lossy, so you can assume about 30 watts on its primary.
> Anyone see any problem with running the filament transformer using the
> plate transformer 120v tap and the center tap on the dual primary?
I'm not sure I understand this. But if what you mean is powering the
primary of the filament transformer from the center tap and one of teh
ends of the plate transformer's primary, then no, I see no problem. The
plate transformer should be large enough to handle that added load
without even noticing it.
Of course, you must check other issues. For example, is it acceptable
for that tube to get high voltage before the filament has warmed up? For
some tubes that's not allowed! I don't know about the 3CX800. Check this
before proceeding.
Manfred.
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