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previous: Stephen Mike,50mA per tube should be wo... -- 12/27/1997 4:28 PM

Re: Cathode Bias Idling Current

12/30/1997 10:02 AM
Doc	Re: Cathode Bias Idling Current Stephen: Thanks for the extremely clear explanation regarding hookup & current measurement with the 1ohm sensing resistors. I like your idea of putting the resistor on the outside of the socket adapter. I guess you could mount turret pins to the bakelite skirt for resistor mounting & clip lead fastening. I have an additional suggestion for those who would like to be able to actually balance the idling currents, not just measure them, in a cathode biased amp. Back in the early days of HiFi, cathode biased output stages were fairly common. There was always a way to adjust the cathode currents using a simple network of fixed plus adjustable resistors. Look at the famous Williamson circuit for a typical arrangement. Currents were measured by actually inserting a meter in the cathode circuit. Now we have inexpensive electronic milliammeters, so we can get an indirect measurement across a small resistor rather than having to physically interrupt the circuit. Incidentally, the point of balance usually coincides with the point of lowest power supply hum in the speaker (for pairs of tubes not grossly out of spec), so a reasonably good balance could be obtained by careful listening and no electrical measurement. Some Mullard circuits use separate cathode bias resistors for each tube. Doc

12/30/1997 10:02 AM

Doc

Re: Cathode Bias Idling Current

Stephen:

Thanks for the extremely clear explanation regarding hookup & current measurement with the 1ohm sensing resistors. I like your idea of putting the resistor on the outside of the socket adapter. I guess you could mount turret pins to the bakelite skirt for resistor mounting & clip lead fastening.

I have an additional suggestion for those who would like to be able to actually balance the idling currents, not just measure them, in a cathode biased amp. Back in the early days of HiFi, cathode biased output stages were fairly common. There was always a way to adjust the cathode currents using a simple network of fixed plus adjustable resistors. Look at the famous Williamson circuit for a typical arrangement. Currents were measured by actually inserting a meter in the cathode circuit. Now we have inexpensive electronic milliammeters, so we can get an indirect measurement across a small resistor rather than having to physically interrupt the circuit.

Incidentally, the point of balance usually coincides with the point of lowest power supply hum in the speaker (for pairs of tubes not grossly out of spec), so a reasonably good balance could be obtained by careful listening and no electrical measurement.

Some Mullard circuits use separate cathode bias resistors for each tube.

Doc

Replies:

stephen Doc,You could fit turret pins, ... -- 1/2/1998 1:52 PM