Converting a PP EL34 cathode biased amp to fixed bias, what changes?

[primosounds]

I have a PP el34 amp with Dynaco A420 output transformers. These are UL OPTs as it is installed in the amp. B+ currently is 420vdc, with 4 individually biased output tubes. I have installed, but not implemented, a negative bias supply which can supply -88vdc. The front end consists of a 6sn7 with 1 section per channel and a 6sn7 splitter tube per channel. I am going to keep the tube line up as is and wonder how much changes need to be made for the conversion from cathode to fix bias. There is no published schematic for the amp but I believe that it is a Williamson clone. The Dynaco A420 OPT is of 6600 primary impedance to 8 and 16 secondary impedance rated at 30 watts from 20 to 20KC. Well, I am not aware of circuit that uses this tube line up with fixed bias and UL. If anyone needs more info just let me know. thank you in advance, primo

 

[peterh]

How is the "individual" cathode bias done ? Is it one cathode resistor per tube ?
If so, replace these with 10ohm 1w , then connect the grounded end of the grid resistor to the viper
of each individual bias adjusting pots. Each pot is connected between ground and -88V , probably
with resistors on each end to reduce the sensitivity .( i guess that -10V on one end and -35V at the other
gives a decent sensitivity.
Then at start have all pots adjusted to max bias ( -35 or -88V ) , turn on the amp and measure
across the 10ohm resistors. Turn the bias until 0.4V is read , this corresponds to 40mA. Repeat for all
4 tubes and redo ( since changes of one tube will affect the other 3 )

And don't forget to create a schematics and save for future use,.