I did simulation using the SPICE models of your D3a and
Orange Color LED of LNJ812R. The schematic is shown in the
attached photo. My simulator cannot calculate KOREN's 6SN7
due to "Missing of OpeLand". Right side of the schematic
shows Ayumi's 6SN7 model. The results are indicated in the
photo. THD=0.95% @20.62Vrms Output is not so bad figure.