Fender AB764 Vibro Champ Output Transformer Measurement

Here's a chart showing the measurements I took on the output transformer in a Fender AB764 Vibro Champ that was in for repair. The measurement procedure is the same as in the Champion 600 output transformer post.

Look at the boxed impedance ratio 2,343 : 1
That's the impedance ratio at 1,000 Hz (or 1KHz).

It indicates a 2,343 ohm plate load with a 1 ohm speaker load. But obviously the speaker load isn't 1 ohm. For the Vibro Champ it's meant to be about 4 ohms.

To figure out what the transformer ratio is with a 4 ohm speaker load, just multiply both sides of the ratio by 4.

( 2,343 times 4 ) : ( 1 times 4)

is

9,370 : 4


9,370 is about 9.5K. That's a bit lower than the 11K I measured on the Champion 600 I modded back in December. The plate voltage in the Champion 600 is higher as well (366 volts as apposed to the 342 volts in the Vibrochamp). The AB764 serves as a template for the Champion 600. I'd assume the higher impedance ratio in the Champion 600 was made to compensate for that higher voltage.