547: Difference between revisions

Bernie Schroder has found that the HV transformer degeneration can be slowed considerably by keeping the HV module cool and dry. The mod he was taught by Jim Willams involves replacing the tube rectifiers with NTE517 diodes. However when the tube HV rectifiers are changed with semiconductor types the regulator circuit has had about 1 Watt of load removed due to the lack of tube heater filaments. This forces the regulator past it's design limit and can result in lack of regulation until the transformer heats up and more power is required due to losses. After changing to semiconductor diodes It will be seen that grid pin 7 of V814 will be about 0 volts (fully on, with no swing to regulate) in attempt to force it's anode (pin 6) to about +55V which is the screen input of the 6AU5 (pin 8). One way to remedy this is to replace the anode resistor coming off pin 6 of V814 (R803)from 56kOHM to 82kOHM. This changes the grid voltage on pin 7 of V814 to a comfortable -0.8V at switch on. Again this becomes more negative as the losses inside the transformer increase but not by much if the transformer is healthy.