The Dyna output transformers only have 4, 8, and 16 ohm outputs as we all know. The 16 ohm tap represents 100% of the secondary. The 8 ohm tap represents 70.7% of the secondary. The 4 ohm tap represents 50% of the secondary. Now if there were a 2 ohm tap, that would be 25% of the secondary. This is where one would want to connect a 4 ohm load (speaker) on a bridged amp, between the two 2 ohm taps in order to provide a 4 ohm load across the two transformers with respect to ground.
An alternative is to disconnect the C or 0 ohm tap from ground and instead, connect the 4 ohm tap to ground. You would then connect a 4 ohm speaker between the two 8 ohm taps. While the winding count isn't quite at 25% between the 8 ohm and 4 ohm (ground) it is 20.7% which is much closer to 4 ohms than connecting the speaker between the two 4 ohm taps.
Now this will change the feedback voltage by half since you are now supplying the feedback voltage from the center tap (4 ohm) and the 16 ohm tap. Changing the feedback resistor from 1 K ohm to a 499 ohm resistor will get the gain back where it was. You could just as easily add a second 1 k ohm resistor in parallel to achieve the same results.
I realize that this may be beyond most listeners abilities. Those who have built the Dynakits or have done Frank's Super 70i or Ultra 70 rebuilds should have no problem. As for the UltraValve, I'm guessing the same can be done. Since most users may wish to to use 4 ohm speakers and want an UltraValve, I'm sure Frank or one of his folks could perform the change, test on the bench and report back. He may be able to configure an UltraValve in this matter for 4 ohm speaker use. Seems a more economical solution than spending $450-$950 for matching transformers. Just move one wire and add/change one resistor per channel.
Thanks,
Ron
