[Amps] QUESTION?

[Manfred Mornhinweg]

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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