Is it absolutely necessary to bias new power tubes??

If I recall correctly, the SB12 is a fixed bias design. Would it be best if your head was checked by a tech and rebiased accordingly (yes, you can bias fixed bias heads with resistors)? Sure. But if it works to your satisfaction, it should be OK.

 

New tubes in any fixed bias amp should be biased to suit them.

Paul

 

According to the schematic in my files the SB12 is indeed a fixed bias design running 6L6GCs.

Paul

 

Doubler, don't misinterpret my post...Paul is right. However, I've replaced power tubes in my B-15 without biasing them and they've been fine. OTOH, it's about to go to the shop because it's not working right

 

BTW guys Fixed Bias can have a pot or not.

There is Cathode Biased and Fixed Bias.

Fixed Bias either uses a resistor change or has a Pot.

Fixed Bias does not mean the Bias can not be changed, it means the Bias does not fluctuate like Cathode Bias does.

 

A simplified explanation: Cathode AKA self bias is how the vast majority of pre-amp tubes operate. The cathode has a resistor to ground and the cathode floats at a positive voltage. This makes the "grounded" grid negative with respect to its cathode. In the case of an output stage that voltage is in the order of say 50V so the resistor has to be a high powered wire wound part. As the tube swings what it can of the voltage across it that 50V is subtracted from the B+, let's say 450V leaving 400V to play with. Fixed bias supplies a fixed negative voltage for the output tubes and thus dispenses with the cathode resistor and lets the tube swing the full B+ voltage. Therefore more output.

Amps running cathode bias do not need to be biased after a tube change. Fixed bias amps should even though the existing bias level may be fine. There are so many different qualities of tubes around today.

Paul