OPT considerations

[no gas]

I have several PP OPT from various sources but all of them have unequal primary impedences for center tap to plates. What do I need to do if anything in designing the drive circuit and output to take the imbalance into account? Is it as simple as adding a resistor to make both halves the same DC resistance?

[Yvesm]

Do you mean unequal impedances or resistances ?
Unequal resistances are very common and do not matter as long as that the difference is less than 10% or so, specially if you use tetrodes or pentodes that are not affected by small plate voltage errors.
However some hi grade OPT show near equal resistances.

Yves.

[DF96]

Significant differences in resistance could mean that the primary windings are not interleaved properly. This may indicate poorer coupling to secondaries or primary taps. Such a transformer might cause trouble with UL, but may be OK for less demanding use.

[LinuksGuru]

It is absolutely normal for EI and double C core transformers to have 10% or even more discrepancy of DC resistance between center tap and plate end #1 / #2. Winding wound on outer layer is more lengthy, and therefore, have higher R(DC). Compared to total impedance (2K - 8K) these 10 - 30 Ohm do not mean anything, so it is not source of concern. Only premium quality toroids allow this discrepancy to be very small.

[no gas]

Originally Posted by DF96

Significant differences in resistance could mean that the primary windings are not interleaved properly. This may indicate poorer coupling to secondaries or primary taps. Such a transformer might cause trouble with UL, but may be OK for less demanding use.

I doubt there is any interleaving on these old Zenith and Baldwin OPTs. None of them have taps for UL so that won't be an issue. Thanks for the help.

Mark