• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OPT considerations

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?
 
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.
 
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