[Amps] Bridging 120v loads across HV primary windings

[Martin Flynn]

Vic,
Run the 4 wire circuit and call it a day.    I ran this very issue by a 
licensed master electrician on Friday.  

Direct quote: "If any portion of the load is 120 volt, you need a 
neutral.  If you are hell-bent on a 2 wire + ground supply, get a 208 
volt fan and appropriately tapped transformer"

Martin
w2rwj

Vic K2VCO wrote:
> My amp project power supply has several 120v loads as well as the 240v plate transformer. 
> 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 plate 
> transformer's primary. I've seen this done with fans, etc. The primary acts as an 
> autotransformer to provide half voltage. Then I could use a simple 3-wire hookup.
>
> 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 plate 
> 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 is 
> impractical, I'll bite the bullet and put in a 4-wire circuit.