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I decided to try changing the output transformers on my Ella to those used on the latest Signature version. These are Japanese "perfect lay" OPTs with the Z11 cores. They are much heavier than the originals and also a bit bigger.
Fitting them was a bit tricky. First of all the mounting holes weren't in identical places. I worked out that by using the existing front holes I would only have to drill two additional holes on each side. Getting the covers over them was a tight fit as the L-shaped brackets that hold the covers have to slide down the side next to the transformer wires before rotating them underneath the "bulge" of the transformer.
Even then the covers wouldn't go down to the case, but I couldn't work out which part was stopping it. I noticed on each transformer one side had the metal core almost exactly flat with the edge of the bracket, while on the other side there was a gap of one or two millimeters. In case I needed to file the brackets down completely I decided to mount them with the flat edge at the bottom next to the case. This meant on the transformer where the choke screws are exposed on the case I had to file down the metal plates to give room for the screw heads. I found out later this hadn't been necessary and I could have mounted the transformers up the other way, in which case the screw heads would have fitted in the gap between the metal core and the case.
I could see that the back corners of the mounting brackets were rubbing the transformer covers to I started filing there. After taking off a few millimeters all the way down and a bit more on the top corners I found the covers would now sit on the case properly. This took me a long time as I only had a grinding stone attached to my power drill. I think using a proper file and holding the transformers in a vice would have made the job much easier and much faster.
After all of that actually wiring them in was really easy. I decided not to bother cutting the wires short and using silver wire to the tubes, and just used the original transformer wires. So you can see from my photos which wires go where.
The end result? The sound is clearer across all the frequency ranges and there seems to be a bit more bass. Softer passages are more delicate and there is more impact when the loud music starts. They also seem to play slightly louder than the old OPTs. Note that my speakers are B&W 802s so use quite a lot of current and might benefit more from the larger, better quality OPTs than something more efficient.
There might be a bit too much bass when the amp is cold but that seems to wear off after about 10-15 minutes as it warms up. Since it goes so quickly I suspect it might be the tubes rather than the transformers as I recently changed to the Gold Lion KT88 reissues and I think it takes a while for the bias to settle down when they are cold.
Also I think maybe the input transformer is running slightly hotter. The resistance between the P1 and P2 wires fell from about 200R on the old transformers to 130R on the new one so maybe that explains it. But there hasn't been any smoke so I think it’s OK!
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