[cheap-Jack]

Hi.

(1) The other kind of biasing is called cathode biasing. With cathode biasing, the grid has a leak resistor going to ground and no negative voltage is applied. The cathode is connected to a resistor which goes to ground. The potential across that resistor is what is considered the bias voltage.

(2) With DC biasing, a negative voltage is applied to the grid through something like a 100K resistor and the cathode is grounded.

(1) Yes, this old school type cathode bias using a resistor has been a historic classic after decades today.
     With HD audio equipment available todate, many find it sound not that pleasant.
     
     As posted above, battery bias to replace the cathode grounding resistor sounds so so much better, IME.

(2) Yes, this topology usually applies to O/P stage power tubes where high bias voltage, e.g. -32VDC is needed.

     For biasing of tubes used in phonostages & linestages, where the bias voltage is very low, e.g. 0.5V -2.5V, battery bias will be a much better alternative sonically.

     Needless to say, cathode biasing is far better than grid biasing under same conditions.

c-J

[loudobic]

Hi all,

I'am Chris from Athens Greece,

Ι recently got an RM200 from my father, with electro harmonix 6550EH.

Ι tried to biasing the output tubes,

First i set the  two bias control pot in zero position, right end position, in order to slowly increase up to 35mV per tube.

My problem is that the position (V5) LEFT TUBE, always run very high mV (from 80mV minimum to 105mV max) while the bias pot is at zero position , regardless of the tube, i true them all in this position.

I also check the 250mA fuses, it's all ok.

Do you have any idea please why this is happening?

Thanks in advance,
Chris