Tranz4mr’s ZW Page


ZW 250 versus ZW 275 Type R - Power Output


I always wondered what the difference in power output was between a ZW 250 and a ZW 275 Type R.
From ZW Hum Fix we can see the difference in the cores. The 275 Type R core is more solid with no voids. The 275 Type R core layers are riveted together while the ZW 250 layers are loose. The result of this is that a ZW 275 Type R that is properly assembled is quieter than a ZW 250. The 250 will be less efficient and generate more heat. Otherwise they are pretty much the same. The question remains which one is more powerful?

In order to figure this out I borrowed a few 5.5 ohm rather large 250 watt industrial resisters and a few calibrated volt/amp meters from the lab where I work and set them up in series as shown in the 1st photo below. I hooked them up to a recently cleaned and refurbished ZW 275 using solid 12 gauge copper wire and slowly brought up one of handles while monitoring AC voltage and current. The handle was raised until just before the breaker tripped. The ZW 275 put out 178 Watts RMS at 18 Vrms  just as Greenbergs repair manual said it should. It didn’t seem to matter how long I ran it. It was consistent and didn’t get very hot.

The same test on a recently cleaned and refurbished ZW 250 had similar results except that it put out 200 plus watts when cold but then settled down to 180 watts after it reached its peak temperature. The voltage dropped as it heated up from a peak of 21 to about 17.8 Vrms.
The  ZW 250 got pretty hot! Keep in mind that the ZW’s put out less voltage than some might measure due to the lower than average voltage in my home.

The bottom line is that they both generally put out the same power but the ZW275 ran much cooler and was much more stable, voltage and power wise. The ZW 250 is much cheaper though. It makes the choice a little more difficult doesn’t it?