LM334 constant current sources [CCS] are temperature sensitive. Its sweet spot is 20µA to 1mA. The Tubes4Hi / VTA implementation is 10x ideal current which causes a LOT of self heating. It is not temperature compensated and oscillates between ≈45°C and ≈55°C in a 16°C garage. R28 is plugged for close enough @ ≈50°C, a stability the device never achieves. In addition to the not inconsiderable 14°C self-heating, another 20°C is supplied by the tubes after about 30 minutes. With amps side by side, Left amp reaches design value sooner due to heat from Right amp power tubes. In a cool room w airflow, Right amp may never reach design parameters. Space amps adequately to ensure similar values. In a warm room with restricted airflow, the LM334 may come perilously close to 70°C Tmax. ALL Tubes4HiFi / VTA designs using the LM334 for the Phase Inverter tail suffer thusly: long warm up time, temperature sensitivity and balance instability. This can be remedied very easily.
Due to the LM334 instability, the Phase Inverter gain constantly changes, up to ½dB, and as a consequence, the image wanders. The amps never reach equilibrium.
At a minimum, move the compensated LM334 to the underside of the driver board. Note the orientation of the compensated LM334. On the bottom, the LM334 rounded side is toward the transformers. Mounting under the board will prevent some heating by the tubes, helping to stabilize thecompensated LM334. The current may not achieve design values quickly as the internal chassis temperature rises slowly.
For the Temperature Compensated Full Monty:
CCS parts go on the PCB underside as shown. The amp now very stable. With compensation, the LM334 stabilizes rapidly and does not drift. Sonic image now remains rock solid. Phase splitter balance is surprisingly good, tracking perfectly from naught to clipping.
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