Build of the EL34/KT88/6550 SE UL amplifier with some test using the E-Linear connection.
Final schematics
Measurements
Experiments with choke-loaded paralleld 6N7 and 6J5
The low frequencies were not impressive. No feedback at all. The experimental measurements are below. The low frequency behavior is due to loading of anode with Hammond 156C 150H (3500 Ohm) choke. The 90-100kHz resonance is typical for UL connections. Blue horizontal likes are 0 and -3dB lines.
Experiments with E-Linear 6SJ7 pentode
THD wend down but 6SJ7 has pretty low gm (~ 1.1) so is not perfect for e-linear connection, there is not enough clean drive for KT88 (the input sensitivity is about 2.8V which is not practical). The model of the transformer is 5.2k:8 Ohm. The above is the improved configuration, and below is that one which was implemented. In the end, the current through 6SJ is only 1-2mA which is a bit too low to drive KT88.
Experiments with E-Linear D3a pentode
Better parameters, the sensitivity is good, the current through D3a is 16-18mA. Lots of heat dissipation, especially via R10 (about 3-4W). The measured distortion was lower compared to the classical schemes, but not impressive.
UL connections, C5 0.1uf (blue) and 0.22uf(red) 
Triode connections, C5 0.1uf (blue) and 0.22uf(red) 
Photos
Power supply filter tests
Two Hammond chokes 158M and 156M: 10H (195 Ohm) and 3H (85 Ohm).
