[Amps] Bridging 120v loads across HV primary windings

[Carl]

Several commercial amps place the fan, filament. and relay/bias supply 
across one winding with no apparent problem.

It is amazing that millions of dryers had been built with the motor wired 
from one leg to ground before the nanny state got wind of it. Lots of 
commercial ham amps also tht are still in sevice using the same concept.

Carl
KM1H


----- Original Message ----- 
From: "Vic K2VCO" <vic at rakefet.com>
To: "Amps reflector" <amps at contesting.com>
Sent: Saturday, May 29, 2010 11:05 AM
Subject: [Amps] Bridging 120v loads across HV primary windings


> 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.
>
> -- 
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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