04-21-2017, 07:28 AM
Hey Ron, how does the individual bias mod work? Are you connecting individual trimmer controls in series with each of the output tube cathodes?
As I recall, the cathode bias for all four tubes is developed across the filaments of the 12AX7 preamp tubes. Since there is no separate negative bias supply, it seems the only way to individually control each tubes bias is to insert a variable resistance in each output tube cathode.
Edit: I just looked at the schematic and it appears that the full voltage drop across the tube filaments (23.5V) is not used for bias. There is a voltage divider of R158-R159 which sets the bias on the output tube grids to about -12.5V. It then would be possible to have individual voltage divider pots for setting the bias for each output tube.
As I recall, the cathode bias for all four tubes is developed across the filaments of the 12AX7 preamp tubes. Since there is no separate negative bias supply, it seems the only way to individually control each tubes bias is to insert a variable resistance in each output tube cathode.
Edit: I just looked at the schematic and it appears that the full voltage drop across the tube filaments (23.5V) is not used for bias. There is a voltage divider of R158-R159 which sets the bias on the output tube grids to about -12.5V. It then would be possible to have individual voltage divider pots for setting the bias for each output tube.
