[Amps] HV transformer and Variac

[Colin Lamb]

 >Hello all.
 >Question for the experts.
 >How does using a Variac to control the output of a HV transformer 
affect things like efficiency?

Since the auto-transformer is a transformer, it will introduce some 
losses and therefore reduce efficiency.  Losses depend upon the core, 
the turns per volt and various other factors.  If the input and output 
voltage is the same, losses would be fairly low, but as the difference 
between input and output voltage increases, losses will increase.

With an auto-transformer, you would change the plate current when the 
voltage is changed, so the plate load remains constant.  This is the 
nice thing about an auto-transformer because you can switch from low to 
high power without retuning, just as you can with a tapped and switched 
secondary.

Efficiency will change when you depart from the design voltage range of 
the final amplifier tube - but in your case you are using the 
auto-transformer to run the tube at the rated voltage, so the main loss 
will be with the auto-transformer.  So, to answer your question 
specifically, it will depend upon the auto-transformer.

73,  Colin  K7FM