Does the Ikezilla KT88 quad amplifier take feedback off of the 8 ohm tap off of the 1650N or the 16?
In the previous Ikezilla PPP prototype, the binding posts were connected to the 4 ohm taps on the Hammond 1650T and the feedback was then taken from the 8 ohm tap. I had the binding posts on the 4 ohm taps because I was planning to use the amps with my Apogee Centaur speakers which present a nominal 4 ohm load.
In the current Son-of-Ikezilla PP project, I am using the same Hammond 1650T outputs with the binding posts connected to the 4 ohm taps. I am using these amplifiers with a pair of B&W 802 speakers which present a nominal 8 ohm load. Configured in this way, I am effectively doubling the primary impedance of the Hammonds from 1.9k to 3.8k. This is a better load for a pair of KT-88s than the 1.9k would be. The feedback is taken off the binding post (effective 8 ohm tap). So the feedback is taken off the same tap as the speaker connection.
Are the values you used for C7, R8, C8, and R49 the same as here for Ikezilla?
For the current Son-of-Ikezilla project, the final feedback values are, C7=680pF, R8=2.2k, C8=330pf, R49=3.32k.
In the PPP Ikezilla prototype, I used a RevA board that I modified to include the step circuit and LTP current source. I looked and did not find my notes from when Shannon and I tweaked the feedback values. Inspection of the prototype did not give a clear answer. If you want these values, I can take the time to look further. However, I am not sure we optimized these values at that time. I have learned a lot since then.
Should I go with a 12BH7 instead of the 6SN7?
I don't have any personal experience with the 12BH7 tube. I have used 6SN7s and 6CG7s in several projects with very good results. The 12BH7 looks like it would also be an excellent choice in the LTP. I think Shannon is planning to use this tube in a project.
What were the final THD readings?
I haven't done any further sweeping except to confirm the previous results after the feedback values were installed. (82 watts, 1 khz, 1%thd) I am currently using these amps and have heard nothing but very nice, transparent sound at all sound levels and music types over the past week. Today, I did the final component change replacing the Solen 0.22uF coupling caps with Multicap RTX 0.22s. ( Just arrived yesterday from Handmade Electronics.) These amps seem to be a near perfect match to the B&Ws.
Regards,
JT