After starting a new HiFi amp project, basically rescuing a Dyancor ST70 from the trash heap, I have had to learn a bit more regarding impendence matching OPTs. This new knowledge raises a question about running KT66 or KT88 / 6550 tubes and speaker impedance matching.
The trip top OPT is 5500 ohms at the primary, and I haven't actually tested the transformer, but I assume since the amp is based on the 6L6GC that the 4, 8 and 16 output taps are a correct as labeled for the corresponding speaker. The 6L6GC in AB push pull has a plate resistance 5600 Ohms at the typical 450V plate voltage. Should be perfect as is.
The KT66, KT88 and 6550 has something more like 3500 ohms plate resistance under the same conditions. Again, I have not done the measurements / mapped out the values for the OPT in question. For those running alternate tubes with the Trip Top, are you using the 4, 8 and 16 ohm taps as labeled for a given speaker impendence, or are you doing something different??? Say using the 8 ohm tap for a 4 ohm speaker, or 16 ohm tap for an 8 ohm speaker?
Am I over thinking this???
By the way, nothing stays the same! I have been running Tung Sol 6550s, biased at about 55mA for a few weeks and they are wonderful, but now I am concerned I should be using the 4 or 16 ohm tap for my 8 ohm speaker.
Thoughts?
EDIT... I used the data for the OPT in the builders guide to determine impedance ratios (ZR). With 5500 ohm primary and the 4, 8 and 16 ohm secondary taps, I come up with ZR of 344 for the 16 ohm, 688 for the 8 ohm and 1375 for the 4 ohm taps. If using alternate tubes, I've learned you can find the new secondary impedance with some simple math by using the plate resistance (from the tube data sheet) of the tubes in use. With 3500 ohm plate resistance of the KT88 / 6550, the secondary taps come out to 2.5 ohm, 5.2 ohm and 10 ohm if my math and understanding of this process is correct.
Hmmm, which tap to use and how critical is this? Kind of want to try the tap labeled 16 ohms with the 6550s as it's closer to my 8 ohm speaker load, but not sure...
|