Determining Output Transformer Reflected Loads
By Paul Marossy
Last updated 7/7/04
Any transformer which is operating within its expected range will have a primary impedance directly proportional to the load connected across the secondary. This is often referred to as the transformer impedance ratio.
For a tube output transformer with a rating of 5K/8 ohms you can also expect a secondary load of 16 ohms to reflect as a primary/output tube load of 10K. Conversely, a load of 4 ohms on the secondary will reflect as a primary/output tube load of 2.5K, and this can be significant when you calculate the currents involved. The output transformer, power tube(s) and other parts of the amps can be damaged with too low of a load on the secondary. That is why it is not usually a recommended practice to use a speaker with a lower impedance than your amp was designed for. In practice, it is true that some amplifiers like old Fenders don't seem to mind, but in my book it is better to be safe than sorry.
You can calculate a change from the specified reflected load on an output transformer. It's fairly simple to get a rough estimate, not accounting for frequency response that will be shifted because of the change in impedance.
Suppose you have an amp with a 5K/8 ohm transformer and you want to place a 16 ohm load on it. First, you need to find the transformer impedance ratio. Assuming that your output transformer is 100% efficient (yeah, right!), that can be calculated by dividing the primary impedance by the secondary impedance, in this case 5000/8=625. To find the new impedance ratio, we multiply this number by the new 16 ohm load, 16x625=10,000. This will be the new load presented to the output transformer.
One obvious thing here is that the current capacity of the output transformer will be reduced (not factoring in bias or muEg). To find what the reduction in current would be, we need to know what the current is for the 5K/8 ohm condition. We'll assume that the plate voltage is 375. Using ohm's law, we can calculate this: I = 375V/5000 = 75mA. Compare this to the new load, I = 375/10,000 = 38mA. This is equivalent to approx. to 50% less current. Less current means less output power. If you do the same the calculation, but substitute a 4 ohm load on the secondary, you will see a dramatic increase in current. This can literally burn up an output transformer!
So that was an easy example. But, what if you have a mystery output transformer? Well, there is an easy way to determine that using an DMM (Digital MultiMeter) and a 115V VAC power source. With the transformer disconnected from the amp, connect the primary/plate winding to the 115VAC power source, using proper fuse and safety precautions. Set the DMM to measure VAC and connect it to the secondary/voice coil winding. The reading on the DMM may be found on the chart below under several speaker impedances or close to them. For example, a reading of 2.9V could be used for 3K/2 ohms, 6K/4 ohms, 10K/6 ohms or 12K/8 ohms. Just make sure that your transformer is large enough for your application so it will not overheat or reach magnetic saturation. For impedances not listed below, the following formula can be used:
E=SQRT(Zs)/SQRT(Zp)*EI , E = DMM reading on secondary, Zs = secondary impedance, Zp = primary impedance and EI = voltage applied to primary.

Speaker Impedance in Ohms 
Zp 
2 
4 
6 
8 
16 
25K 
1.03 
1.45 
1.78 
2.06 
2.91 
15K 
1.33 
1.88 
2.30 
2.66 
3.76 
12K 
1.48 
2.10 
2.57 
2.97 
4.20 
10K 
1.63 
2.30 
2.82 
3.25 
4.60 
8K 
1.82 
2.57 
3.15 
3.64 
5.14 
7K 
1.94 
2.75 
3.37 
3.89 
5.50 
6K 
2.10 
2.97 
3.64 
4.20 
5.94 
5K 
2.30 
3.25 
3.96 
4.60 
6.51 
4K 
2.57 
3.64 
4.45 
5.14 
7.27 
3K 
2.97 
4.20 
5.14 
5.94 
8.40 
2K 
3.64 
5.14 
6.30 
7.27 
10.29 
1K 
5.14 
7.27 
8.91 
10.29 
14.55 
0.5K 
7.27 
10.29 
12.60 
12.53 
20.57 
