Just finished building the K4040. A very good amp with excellent sound quality. I would suggest a couple of changes to protect the EL34’s.
-
replace all eight screen grid resistors (180R) with 510R 2W. This prevents any parasitic oscillations on the output stage which can be seen on a scope.
-
With the correct main voltage (230V) and the correct voltage selected the heater voltage is 7.1V out of spec of the valves. The winding ratio of the mains transformer appears to be a little incorrect (the HT voltage is OK). To fix add 0.1R 7W resistor in one the heater leads (two resistors required).
There is quite a loud hum (100Hz + harmonics) from the speakers which disappears completely when the first valve ECC81 is removed. This the coming from the heater supply which is coupling into the cathode as the resistor 12K is high to reduce gain of this stage down to 5 or so. Running the heater off a bench supply solves the problem. To fix you can either run the heater off DC (bridge rectifier + big cap + low dropout regulator), or replace with a TL072 amp as I did (I can provide circuit if anybody wants it and isn’t a valve purest).