I'm Rebuilding a friends a stereo 70 with one of Shannons driver boards and he was wanting to use KT 120 in it , the new power transformer is 450 MA and has two 6.3 volt windings at 6A . Will be using the 5 volt winding rectifed to DC for the filaments on the driver board , so the power supply should be up to it , but was looking at the specs of the KT120 and see they sugest a 51 k ohm grid resistor which is 3 times lower than the 150 k ohm grid resistors on the ST70 driver board.
Is it realy nescary to go that low with the grid resistor , thermal run away is ithe only reason i can think of is why they would go that low of resistance . What would be the best idle current for the KT120's? 70 or 80 ma
I'm using two chokes in the power supply on for each set of output tubes and 50 uf cap on each pair of outputs , then running the PCB off the right channel .The first cap is made up of two 470 uf 450 volt caps inseries with 100k bleader resistors acros them to equilize the voltage acros them , the two diodes are MUR4100e's 4 amp 100 piv fast switching , using a 25 anp bridge from my junk box to rectify the 5 volts for the pcb filaments .