Filament Transformer for 2A3 tube
Lets take a look at 2A3 tube. And how to make good quality filament transformer for AC power supply. On the old RCA 2A3 data sheet manual I found that the 2A3 can be supplied form AC and DC source. The important is the filament voltage and current. At the image below you can see capture of the original datasheet of RCA 2A3. With red color are marked the important parameters for the filament. The criteria for making a high quality filament transformer are listed too.
one transformer for all filaments
secondary winding with center tap
AC supply bifiliar wound winding
high quality transformer core
optimal magnetic induction
working without core saturation
low working temperature
low external (stray) magnetic field
low vibration and acoustic noise
2a3 has a filament of 2.5V and 2.5A. So first I want to try to wind a secondary winding with center tap . That will be something like 1.25V_CT_1.25V (2.5A). In the bifilar configuration there will be two copper wires coiled together in parallel. The idea of AC power source for the filament, from my point is that the sound of the DHT is better when the valve is powered by AC. But the AC filament mode is not so easy to make it silent. The hum induced by the power source goes directly through the filament to the signal path. Different designs can be applied to reduce noise. The most classic is put center tapped winding. Look at the image below, pls.
At the image we have the schematic of the transformer. It is a two-wire(bifilar) winding. Take two enamelled copper wires together and wind them together. I want the two wires to have the same length. I want between the two windings to connect a variable resistor. This resistor will give me the possibility to vary the voltage of the filament in a range of 30%. In my case I have 2.5V. The 30% of 2.5V is 0.75V. I have two 1.25V windings . I divide approximately 0.75V between the two coils and will have 0.75 / 2 = 0.375V. We add to each arm 1.25 + 0.375 = 1.6V. The result is two secondary windings each one 1.6V. If the voltage range of adjustment of the filament supply is 0.7V for the resistance is: 0.7V/2.5A= 0.280Ω. The adjustable voltage drop of 0.7V with filament current 2.5A can be generated by resistive element with value of 0.280Ω . This value is realy low for standard potentiometer. That’s why it’s my way to make myself with resistive wire from Kanthal or Nichrome.