Hi All,
FWIW, the original HF-89 used a rather convoluted global feedback network consisting of two 3k resistors in series coming off the 16Ω tap, going to the middle of the 1K/100Ω voltage divider on the cathode of the 1st AF amp. There was also a 225 pF cap across the sum of the 3K resistors, as well as another 500 pF cap going from the 4Ω tap to the midpoint of the 3K resistors.
When Eico came out with the second generation HF-89, which used a different OT (the one my 50 watt OT was cloned from), the NFB became more straightforward: 7.5K shunted with 225 pF from the 16Ω tap to the 1K/100Ω junction.
Since, for some reason, I can't pull up the eiclone schematic, I don't know what the resistance values are for the NFB or the 1st audio stage. It's a moot point, however, for the following d'oh reason:
Contrary to popular belief and Gertrude Stein, an OT is not an OT is not an OT. I'm not talking about comparing a transformer that weighs 12 pounds with one of the scrawny OTs used in the K502; they 're obviously in different leagues. What I am trying to emphasize is that the NFB resistance and capacitance values need to be tweaked to get optimum performance from the OT you are using. Most of you don't have the test equipment to verify how much NFB you've applied, so use the eqiuipment Mommy and Daddy gave you: your ears. A 50K pot in place of a fixed resistor will allow you to "dial in" the NFB value that sounds right to you. For safely's sake, put a 1K resistor in series with the pot so you don't accidentally take the output to (essentially) ground.