I have an old power transformer that I'm retrofitting into a new valve amplifier. Having measured the voltages on the secondaries when 230V is applied to the primary and got the following results:
P: 230 V
S1: 356.2 V
S2: 34.56 V
S3: 6.453 V
The turns ratio of each winding is therefore:
S1: 356.2 / 230 = 1.549
S2: 34.56 / 230 = 0.150
S3: 6.453 / 230 = 0.028
The current flowing in primary at 230V was 0.07A. Using the turns ratio I we can calculate the current flow in each secondary:
S1: 0.07 * 1.549 = 45 mA
S2: 0.07 * 0.150 = 460 mA
S3: 0.07 * 0.028 = 2.495 A
This seems correct at first, but using these figures the combined power output of the secondaries would be three times that of the primary which is obviously incorrect as Vp * Vp = Vs * Is.
Two of the questions to this answer suggest that we can calculate the VA value as V^2 / R, sum the three VA values and then us the result to calculate how the VA of the primary is distributed between the secondaries. In this case the resistance would be DC resistance of the primary multiplied by the square of the turns ratio, plus the DC resistance of the secondary.
The measured DC resistance of each winding is:
P: 7.5 R
S1: 28.60 R
S2: 3.10 R
S3: 0.10 R
Therefore the total resistance on each secondary is:
S1: ( 7.5 * (1.549 ^ 2) ) + 28.60 = 46.59 R
S2: ( 7.5 * (1.150 ^ 2) ) + 3.10 = 3.27 R
S3: ( 7.5 * (0.028 ^ 2) ) + 0.10 = 0.11 R
Therefore the VA rating of each winding is
S1: ( 356.2 ^ 2 ) / 46.69 = 2723
S2: ( 34.56 ^ 2 ) / 3.27 = 365
S3: ( 6.453 ^ 2 ) / 0.11 = 393
Therefore the total VA rating is 3482 and the power distribution between each secondary is:
2723 / 3482 = 78.2%
365 / 3482 = 10.5%
393 / 3482 = 11.3%
So now we can revise the current values we calculated earlier:
S1 = ( 0.07 * 0.782) * 1.549 = 0.035 A
S2 = ( 0.07 * 0.105) * 0.150 = 0.049 A
S3 = ( 0.07 * 0.113) * 0.028 = 0.282 A
This balances the power on each side of the transistor however it doesn't really correlate to reality. The third winding is for the valve heaters and given that his amp contains 4x 12AX7 and 4x EL34 the current required is going to be around 5.4 A however that's clearly not the case. The first secondary should also be much higher, in the range of 500 mA.
Where have I gone wrong? Is my original current measurement on the primary wrong? Do I need to be considering the load into which each secondary will be delivering the current?
Thanks for any help.