Volt Ampere (VA) Story

Volt-Ampere (VA) Story

A volt-ampere (VA) is a unit used for apparent power in an electronic circuit. The VA rating is use to indicate the limited maximum permissible current in a device.

Defining the transformer current rating depend a lot on your amplifier circuit, classes and topology design. There is no one transformer suitable for all amplifier but they can tolerance well with each other.

The rules is, the primary must be able to handle all current handled by secondary. Let say we have 2 units of EL84 drive by one ECC81 in single ended configuration.

High tension (B+) voltage is 300v. Each EL84 draw about 0.05A and each triode from ECC81 may need 0.003A.
0.05+0.05+0.003+0.003 = 0.106A.

Filament 6.3v need 0.3+0.76+0.76 = 1.82A.
If you are using tube rectifier 5U4G with 5v filament may need 3A.
If we just sum up 0.106A+1.82A+3A = 4.926A, this is not correct, as you can see here 3A only needed at 5v.

To total it up, we need to convert it to VoltAmpere(VA). 300v x 0.106A = 318VA;
6.3v x 1.82A = 11.466VA and 5v x 3A = 15VA.
So 318+11.466+15 = 344.466VA @ 344VA.
Okey, now, If the primary is 240v then 344VA / 240V = 1.43A. and
If 120v then 344VA / 120v = 2.86A,
so the current rate is doubled for 120v.