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How to select replacement Power Xformer?

11/13/1997 5:49 PM
Erik M. Wood
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How to select replacement Power Xformer?
Hi guys.  
I am wondering if the selection of a replacement power transformer is a simple process of picking out the required voltages/amps for the circuit, or if there is some calculation necessary to determine this. I have a power transformer which reads 380-0-380 on the B+ secondary with the rectifier tube pulled out, yet the schematic specifies 360-0-360. One of my professors told me that the fact that the tube was pulled out made a difference. He said that the voltage would be a little higher when the secondary was unloaded. So, assuming this is true, how does one go about selecting a replacement transformer for the amp? I want to use a solid state rectifier for the amp instead of the 5AR4 tube it currently has, and I want a transformer with a higher current capacity than the old one. That isn't a problem, but figuring out what secondary voltage to choose to compensate for the lack of the voltage drop across the rectifier tube (which won't be there anymore), is a conundrum. I had originally assumed that it was a simple process of calculating the required rectified DC voltage at the input to the filter stage (in this case 435VDC), and account for the .7V drop across each of the diodes. is that it? Or is there more?  
I guess my question is, are transformer specs listed as loaded or unloaded by a rectifier, and if so, is that load always relatively constant regardless of whether it is a tube or solid state rectifier? If not, how do I calculate what the voltage will be before/after the rectifer, with and without a load?  
I'm sure I'm making a mess of a simple problem...  
-Erik.