Transformer questions

[manx82]

Hello,
I was hoping to gain some education concerning a transformer I have. It is an Edcor power transformer and has not been used. The specs are 500v, 300mA center tapped, 6.3v, 7A center tapped and a 5v, 5A. The project I have is based on the KEL84 design but is not a WD product. The original design used a valve rectifier that I have changed to a diode design based on the KEL84 schematic. I only need 250 volts so can I just use one 250V wire combined with the center tap and not use the other leg? My second question is when I power this and perform some voltage measurements, my 6.3V is actually 7.2V and I confirmed that with two meters.

Sorry, I am new to valve amps and was hoping for some help, Jim

[snowman_al]

Hi and welcome Jim,
As a newbie, please be careful when working on 'tube' amplifier voltages. They can be lethal...

The way to configure the rectifiers for a centre tapped transformer is to use both 250 volt 'arms' and 2 diodes. It is called Full Wave rectification.
The Centre tap goes to your -ve or ground connection and the two cathodes are connected together and go to the B+. Just like the valve rectifier would have done. If you use only one side as you describe, you halve the current rating, so not a good idea. (The WD transformer uses what is called a Full Wave Bridge rectifier. It uses the 4 diodes.)

Google or look here https://www.electronics-notes.com/ar...r-circuits.php

Remember also you will get 350 volts DC when connected to the first smoothing capacitor so make sure your capacitors are 400+ volt rated.

The higher heater voltage should drop when you put a load onto it. Try putting the 4 EL84s across it and measure the voltage then.

Good luck and take care ehy? Alan

[manx82]

Thank you so much for the information, it clears up hours of research I did before I asked.