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[Amps] Bridging 120v loads across HV primary windings

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Subject: [Amps] Bridging 120v loads across HV primary windings
From: Vic K2VCO <>
Date: Sat, 29 May 2010 08:05:54 -0700
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My amp project power supply has several 120v loads as well as the 240v plate 
They include a fan that draws about 100 mA, a screen supply that will take 
about 350 mA, 
and a filament transformer that will probably draw about 1A.

The problem is how to connect the 120v loads. One solution is to just use a 
4-wire circuit 
which will provide a neutral in addition to the two hot wires and ground. Then 
the 120v 
loads can be connected between the hot wires and the neutral.

Another option is bridging the 120V loads across the two halves of the 240V 
transformer's primary. I've seen this done with fans, etc. The primary acts as 
autotransformer to provide half voltage. Then I could use a simple 3-wire 

But I wonder how far I can go with this. If I put the filament transformer 
across one half 
and the screen supply across the other, would the unbalanced currents cause the 
voltage on 
the lightly loaded side to be excessive? This is a smallish amplifier -- the 
transformer is about 1 kVA.

Has anybody tried this?

No, I am NOT thinking about using the ground as a neutral! If the bridging idea 
impractical, I'll bite the bullet and put in a 4-wire circuit.

Vic, K2VCO
Fresno CA
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