I've been planning to build a pair of Ikes for a long time, but the project kept getting put on hold. Probably because I installed a pair of Shannon's prototype ST70 boards in my MKIII's and was so happy with the result I didn't need another pair of 60 watt PP UL KT88 amps. However, more power would get me excited.
So, I approached Shannon to see if he was interested in playing around with PPP UL KT88 designs. We pooled resources and threw together a prototype amp. It is based on the stock Rev A Ike circuit. In fact, we used an Ike board for the power supply, grounded cathode, and LTP. (12ax7 and 6SN7) I then wired up some chassis mount octals with flying wires to the octal footprint on the board. Also threw in a simple bias board to allow individual bias adjustment on the power tubes. The PT was from Mayland Toroid, http://www.toroid.com/standard_transfor ... s_tube.htm and the OT was a Hammond 1650T (1.9K 120 watt). The tubes were a well used quad of Sovtek KT88s.
This was a good learning experience. The first time we powered up, we had blue arcing in one of the tubes due to parastitic oscillations. We had to add plate and screen grid resistors to stabilize things.
Yesterday I took the prototype over to Shannon's and we tweaked the feedback and swept it to see how it looked. Open loop gain was just over 40 db. Here are 50 watt results with a B+ around 450 volts and a lean bias of 68 ma per tube.
20Hz 14.66V +21.34dB .746%THD+N 51.84W @ 4ohm 81.88dB S/N
25Hz 14.38V +21.19dB .464%THD+N 51.62W @ 4ohm 81.76dB S/N
30Hz 14.39V +21.19dB .406%THD+N 51.77W @ 4ohm 82.1dB S/N
50Hz 14.37V +21.19dB .355%THD+N 51.48W @ 4ohm 81.62dB S/N
100Hz 14.37V +21.20dB .292%THD+N 51.48W @ 4ohm 81.97dB S/N
200Hz 14.37V +21.20dB .292%THD+N 51.55W @ 4ohm 80.85dB S/N
500Hz 14.36V +21.18dB .304%THD+N 51.48W @ 4ohm 82.16dB S/N
1kHz 14.37V +21.20dB .297%THD+N 51.55W @ 4ohm 82.dB S/N
2kHz 14.37V +21.20dB .291%THD+N 51.55W @ 4ohm 81.43dB S/N
5kHz 14.32V +21.16dB .311%THD+N 51.19W @ 4ohm 81.89dB S/N
10kHz 14.18V +21.07dB .389%THD+N 50.2W @ 4ohm 81.18dB S/N
20kHz 13.72V +20.78dB .661%THD+N 46.92W @ 4ohm 81.08dB S/N
30kHz 13.03V +20.32dB .988%THD+N 42.32W @ 4ohm 80.99dB S/N
Our goal was to get 120 watts so we cranked up the B+ to 475 volts and biased the tubes to 81 ma. Here are the results.
20Hz 20.85V +20.77dB 18.316%THD+N 108.3W @ 4ohm 80.01dB S/N
25Hz 22.02V +21.25dB 1.882%THD+N 121.2W @ 4ohm 83.23dB S/N
30Hz 22.04V +21.26dB 1.142%THD+N 121.3W @ 4ohm 83.94dB S/N
50Hz 22.07V +21.28dB 1.312%THD+N 121.W @ 4ohm 84.2dB S/N
100Hz 22.07V +21.28dB 1.179%THD+N 121.1W @ 4ohm 83.98dB S/N
200Hz 22.08V +21.28dB 1.056%THD+N 121.2W @ 4ohm 83.94dB S/N
500Hz 22.05V +21.27dB 1.055%THD+N 121.1W @ 4ohm 84.06dB S/N
1kHz 22.07V +21.28dB 1.011%THD+N 121.3W @ 4ohm 83.83dB S/N
2kHz 22.07V +21.28dB 1.006%THD+N 121.3W @ 4ohm 83.89dB S/N
5kHz 21.99V +21.24dB 1.002%THD+N 120.5W @ 4ohm 83.81dB S/N
10kHz 21.79V +21.16dB .994%THD+N 118.5W @ 4ohm 83.91dB S/N
20kHz 21.06V +20.87dB 1.161%THD+N 110.5W @ 4ohm 83.48dB S/N
30kHz 19.94V +20.37dB 2.27%THD+N 99.1W @ 4ohm 83.02dB S/N
1% distortion at 1k at 121 watts!!! Yellow_Light_Colorz_PDT_10
Now that's something I can get excited about.
JT