[Amps] Bridging 120v loads across HV primary windings

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sat, 29 May 2010 08:05:54 -0700, Vic K2VCO <vic at rakefet.com> wrote:

>
>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.

REPLY:

Interesting question. Your first option would work, of course. That is
basically how a house is wired normally.

The second option is more problematical. The transformer will function
as an autotransformer like you suggest, but the question is how good
the regulation would be as the loads are varied. You will have two
loads which vary: The screen supply on the primary side, and the plate
load on the secondary side. Both will affect the other and both will
be affected by resistance of the transformer windings and the
tightness of coupling between the primary and secondary. If the
regulation is not good, that will have a bad effect on your heater
voltage as well. 

I doubt if there is any way to predict the behavior in advance. I
would just try it and find out. I don't see any safety hazard, and if
it doesn't work, you can always add the fourth wire. 

Please let us know how it works out. Good luck.

73, Bill W6WRT