• 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.

Silly transformer question

sparkydave

Member
2013-02-01 6:32 pm
Thanks! I was pretty sure it meant plate-to-plate. Interestingly, I stumped one of the electrical engineers where I worked when I wondered if one leg of this 8000 ohm transformer would look like 2000 ohms or 4000 ohms, but when I did the math it would be 2000 ohms as John pointed out. Glad I remember something about transformers from school.
 
Thanks! I was pretty sure it meant plate-to-plate. Interestingly, I stumped one of the electrical engineers where I worked when I wondered if one leg of this 8000 ohm transformer would look like 2000 ohms or 4000 ohms, but when I did the math it would be 2000 ohms as John pointed out. Glad I remember something about transformers from school.

Depends on the conduction angle.
Class A: 4000 Ohms
Class B: 2000 Ohms
 
Sorry for being away from this thread. N4BBQ's link describes the technique better than I could! One thing I might add is that the choice of RL may have to be an educated guess. For example, if you have a mystery output transformer that has two secondary taps, you have to decide whether they are 4 and 8 ohms or 8 and 16 ohms. Vintage amps often had only 8 and 16, whereas modern tube amps more often opt for 4 and 8. If there are three taps, it is almost always 4, 8, and 16.

- John