I have a couple of push pull EL84 transfos with single unknown secondary taps. In order to measure turns ratio I put a 16 volt AC source on the primary between the CT and one of the outside leads (0.73 volt). The voltage ratio was 22 and I take that to be equivalent to turns ratio. That squared should be the impedance ratio of 484. With an 8 ohm speaker load that should be 3872 ohms.
Now, assuming that the above methodology is sound (?), I'm wondering how this relates to EL84 application. Most reference data shows a required "Plate to Plate Load Resistance" of 8K ohms. Plate to plate suggests to me that we are measuring load impedance from one plate to the other, whereas I have measured from the CT to one end of the winding (CT to plate rather than plate to plate). That suggests to me that I need to double the measured value in order to compare it with the plate to plate load resistance numbers. IS THAT CORRECT?
If so, I would think these transformers applied with EL84s would be a good match with an 8 ohm load. This is a little puzzling because I'm pretty sure they were used with a pair of 8 ohm speakers in parallel (4 ohm load). It's kind of a head scratcher so I suspect that I might not thinking about it correctly.😕 What can you tell me?
Now, assuming that the above methodology is sound (?), I'm wondering how this relates to EL84 application. Most reference data shows a required "Plate to Plate Load Resistance" of 8K ohms. Plate to plate suggests to me that we are measuring load impedance from one plate to the other, whereas I have measured from the CT to one end of the winding (CT to plate rather than plate to plate). That suggests to me that I need to double the measured value in order to compare it with the plate to plate load resistance numbers. IS THAT CORRECT?
If so, I would think these transformers applied with EL84s would be a good match with an 8 ohm load. This is a little puzzling because I'm pretty sure they were used with a pair of 8 ohm speakers in parallel (4 ohm load). It's kind of a head scratcher so I suspect that I might not thinking about it correctly.😕 What can you tell me?
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Hello
Your reasoning is sound up to the point where you duoble the impedance instead of the turns ratio.
CT to anode lead have a turns ratio of 22 then anode to anode has 44. If you square that and assume 4 ohms load you end up at approx the usual 8k a-a for an EL84 output stage.
Regards
/olof
Your reasoning is sound up to the point where you duoble the impedance instead of the turns ratio.
CT to anode lead have a turns ratio of 22 then anode to anode has 44. If you square that and assume 4 ohms load you end up at approx the usual 8k a-a for an EL84 output stage.
Regards
/olof
D'oh. I should have figured that out! Thanks for the help.
I suppose one should always measure across the full winding on a push pull transformer as the data is stated for that convention; ie. plate to plate - both tubes / both windings.
It had not occured to me before, and I suppose it's obvious, but I would imagine that is why the SET numbers are always about half the PP numbers. SET numbers are just for a single tube tube.
Isn't it great when the lights come on!!!
Thanks for the help, Olof. I do appreciate it.
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