Dragon Slayer? PT 2
These transformers are very interesting in that the things they excel at they do very well, while the things they are lacking in are glaring.
Static measurements revealed that when properly loaded, an 8Ω load on the 8Ω tap reflects 11,654Ω back to the output tubes, while a 4Ω load on the 4Ω tap reflects 12,913Ω to the tubes. This increase in primary impedance results in about a 0.5 watt loss in power output on the 8Ω tap, and just over a 2 watt loss on the 4Ω tap, all reference to 10 watts. On the primary side, it was quite pleasantly surprising to find that the UL taps on this transformer are appropriately located at 25.0% of the winding, which is ideal for tubes of the 6V6GTA class. Referencing my comments above, this is very different from the oh-so-standard 40% that most manufacturers offer. While the taps are well positioned, they do none the less produce a loss of power output, producing just over 7 watts of power on the 8Ω tap. This is getting close enough to the stock power output capability that the efforts using the taps were set aside for potentially a different project where more B+ is available to counter the power loss produced.
Dynamically, in pentode mode, the transformer produces very fine results.......... on the 8Ω tap. Power, as previously mentioned, is about 9.5 watts, with distortion never exceeding 1% at that power level from 20 Hz to 20 kHz. Frequency response throughout the audio band is very flat, being down just 0.15db at 20 kHz, and down 1.0 db at 46 kHz. Stability, is in fact utterly amazing. This is the most unflappable transformer I've ever worked with in this regard. Not only is it absolutely stable with any amount of capacitance only load applied, the unloaded square wave presentation changes very little when the capacitance is applied! Such stability always translates into a very high level of detail reproduction in my experience. HF transient response is plenty good as well -- not the best I've seen, but still quite good overall. In typical use, the elevated primary impedance will likely work in favor of performance, since many speakers tend to average somewhat lower in impedance in operation than that of the rated nominal impedance.
In the listening room, the transformer made itself immediately apparent, with a very balanced presentation, solid, deep bass, precise high frequency detail, and balanced mid-range presentation, that Ch 2 (modified as presented using the stock OPT) simply could not match. By comparison, Ch 2 -- against the Raphaelite transformer in Ch 1 -- sounded weak on the low end, and loud in the mid-range -- and Ch 2, already sounded
much better than the original stock design. The performance with the Raphaelite transformer was really very, very good. But there is a catch.
The disappointment came with performance on the 4Ω tap. Mid and LF power output is reduced to the 7-8 watt range as the reflected primary impedance is even greater on this tap, and most importantly, HF performance simply falls apart. To produce a good square wave on the 4 Ohm tap, it takes a completely different type of NFB and HF stability scheme that is connected to that tap (the 8Ω tap requires its own NFB and HF stability network as well), and can only produce a 2 kHz (!) square wave with any accuracy. What's more, power output at 20 kHz on the 4Ω tap is a measly 3.1 watts RMS! Granted, little power is needed at this frequency in actual use, but for the cost of these transformers (about $80 each, delivered from the most inexpensive vendor), this is very poor performance indeed. The need for different FB arrangements for different taps is hardly uncommon with aftermarket hifi transformers. But for all practical purposes, this transformer should
only be considered as having an 8Ω secondary winding, as performance at the 4Ω tap is so poor.
So the Raphaelite transformer comes off as a Dr Jekyll and Mr Hyde piece. It would be hard for me to recommend it unless you absolutely knew that you would never be using the 4Ω tap. As a result, I do not plan to even draw up a schematic for its inclusion in the modified design. Once again then, this really only leaves the new manufacture Z565 Dynaco transformers as (to my knowledge) the only truly high quality transformers that could properly work in this application on BOTH the 4Ω and 8Ω taps -- and that would take some significant design changes to accomplish at that. Still, I am committed to seek out the best performance of which the basic design is capable, so a pair of the Dynaco transformers have been ordered. That puts this project on hold for a while until the transformers come in, and significant design changes can be made. But as before, I will get to it in time and report back. For now then, a few pics to offer.....
BELOW: A fine looking piece indeed. It even includes an internal shield with it's own dedicated lead for grounding.
BELOW: All mounted up for testing in Channel 1. It definitely is intimidating to the original stock OPT in Channel 2.
BELOW: 10 kHz square waves. Top is Channel 1 with the Raphaelite transformer loaded on the 8Ω tap, while the bottom is Channel 2 with the stock OPT in the modified design loaded with 4Ω. Note the significant improvement in rise time with the Raphaelite transformer.
BELOW: Oops. Nothing has changed except that now the scope is displaying the Raphaelite transformer's 4Ω tap with a 4Ω load connected to it. As stated, it can be made to show an excellent square wave on the 4Ω tap, but only to 2 kHz, and the load stability is at best average.
BELOW: Underside of the amplifier showing Channel 1 with the Raphaelite transformer installed.
So that's about that. If the Raphaelite transformer proved anything, it's that the stock OPTs can definitely be improved upon, but that the Raphaelite transformer only gets you part way there. Quite a bit of work to do to ready the 8800 for the Z565s, but as the quality of that transformer is an known value, it should be well worth it -- unless, anyone has any other suggestions.......
Happy Listening!
Dave