This was written by Casey4s, aka David McClain   -  Regis

Finding Unknown Values for Output Transformers

(A little Math is required)

If you have an Output Transformer with unknown values and want to find the intended speaker impedance for a given tube type there are several things to consider. An OT reflects the impedance load (the speaker) on the secondary back onto the primary winding.

There are a few abbreviations I am going to use for the purpose of this explanation:

VR =      Voltage Ratio
IR =        Impedance Ratio
TR =       Turns Ratio
IL =         Impedance Load (The speaker)
ELR =     Effective Load Resistance (from tube manual)

ELR Examples for two tubes in PP AB1 or AB2
6L6 ELR = 3,800 ohms
6V6 ELR = 8,000 ohms
EL84 ELR = 8,000 ohms
EL34 ELR = 6,500 ohms

The Turns Ratio (TR) can be found by applying a small AC voltage to the secondary and measuring the result on the primary. If you put 1v AC on the secondary and measure say, 31.6v on the primary you have a Voltage Ratio (VR) of 31.6 which means that the turns ratio is 31.6:1.

Finding a 1v AC source is difficult unless you have access to a Variac, small transformer or similar device. You can use a 5v rectifier winding from an existing PT or the 6.3v heater winding. You must then divide the answer by the number of volts you applied to the secondary.
For example the 31.6:1 TR would measure 158v with 5v AC applied to the secondary.

The Impedance Ratio (IR) is the square of the Turns Ratio (TR), in this case:
31.6 (sq) is 998.6

To determine the expected Impedance Load (IL) or speaker impedance you will divide the known Effective Load Resistance (ELR) by the Impedance Ratio (IR)
So: ELR / IR = IL

In this example we want to use a pair of 6V6 tubes that have an ELR of 8,000 ohms with this transformer and want to determine the proper speaker impedance (or IL)
8,000 / 998.6 = 8.0 ohms

So you would use an 8-ohm speaker with this transformer and a pair of 6V6 or EL84 tubes in AB1 or AB2.

What about a pair of 6L6 tubes? RCA says the 6L6s have an ELR of 3,800 so:
3,800 / 998.6 = 3.80 ohms

So you would use a 4-ohm speaker with this transformer and a pair of 6L6 tubes in AB1 or AB2.