Can I convert self bias to Fixed?

[sser2]

CCS ... is the perfect solution...
GoatGuy

There is no perfect solution. CCS has its advantages, but also its disadvantages. It wastes a lot of power and, by limiting the stage to Class A, limits the output.

For a 6L6 stage biased to max power in Class A, a well-implemented CCS would be a serious undertaking. A solution as elegant as flying pig.

 

[Zero Cool]

Fixed vs. cathode bias decision depends on one's goals.

If I want maximum power with minimum open loop distortion, it is fixed bias.

this is my goal!

 

[DAK808]

Fixed vs. cathode bias decision depends on one's goals.

If I want simple driver, trouble-free operation, long tube life, and no customer returns, it is cathode bias.

If I want maximum power with minimum open loop distortion, and tubes are cheap (6L6 types are cheap), it is fixed bias.

Changing cathode to fix bias may not be straightforward. In addition to driver problems already mentioned, fixed bias will need different output transformer for optimal performance.

I echo SSERs approach. If one is building his own amp then you are already familiar with where to look if a problem appears. Therefore you strive for the advantages of fixed bias, which is NO capacitor in the signal circuit, easily adjusting operating point for different power tubes ( el34 to 6l6 to 6550 if your ps can provide the extra current, making your amp more flexible ). OPT matching may not be ideal if going from cathode bias to fixed bias but there is "leeway" and also if necessary you can switch the secondary connection to get a better match. Finally, you can add a resistor from the bias pot output to the bias source voltage which will always give some negative voltage to the power tube grids if the pot fails.