Hi,
I figure the capacitor between 6 and 7 of the ECC83 (anode and grid) was added for stability reasons, it may well have been added by test when the circuit exhibited HF instability – but I suspect it may well have been permanently added after the original ETR schematic (on Pauls disc) was produced so the schematic didn’t show the addition/revision. .......Ah the echoes of the recent Leak TL50 thread
I’ve attached a picture the two DP15’s I have in the workshop awaiting restoration showing the additional capacitors – one of the few projects I do have to hand!
As regards differences I do remember there was a slight difference in the HT supply with one less RC stage in the Dulci.
Regards
Terry