DC resistance of an OT Primary

[FunkyE9th]

So I just bought this new deluxe reverb OT from mojo. Haven't hooked it up yet, but wanted to measure DC resistance, just to get a baseline of the OT at DC. I'm measuring the resistance from the center tap to the tap that goes to the plates (note the OT is not connected or mounted to anything). I'm expecting roughly the same resistance, but I'm getting 68 Ohms and 92 Ohms. This is measured with respect to the center tap. Shouldn't this be pretty close? 68 and 92 seems like a big difference.

-FunkyE9th

[Firestorm]

I've seen worse. The beginning of the winding is always much shorter than the end of the winding. Some interleaved transformers do wind up with the secondaries being pretty close, though.

[FunkyE9th]

So even with a big difference in primary DC resistance AC properties are still OK? I just applied a 60Hz signal on each tap of the primary and it generated the same voltage at the 2ndary. So AC wise it seems impedance is even. I guess with the 24 Ohm difference at DC, bias will be slightly affected.

[Structo]

It's not really the DC resistance that affects the voltage but rather the number of turns around the core.

So as they wind the core it will take more and more wire to get the same amount of turns because of the diameter change between the center tap and start and finish.

Sound weird but that's the way it works.

[FunkyE9th]

I understand that it's not the DC resistance that determines the AC voltage. I was just surprised at the difference. I was expecting the DC resistance to be close, like my Express OT. Under DC, the plate of one tube will see more resistance. So if the tubes were to draw the same current there will be a some difference in plate voltage. Actually, now that I think about it, at 30ma the difference is very slight... 0.7V, so it's not even an issue. I was just surprised at the difference.