previous: Gil Ayan q{I think if you look at the chart<... -- 3/1/2000 7:12 AM

Re: Varible 12ax7 cathode resistor in preamp?

3/1/2000 7:52 AM

Randall Aiken

Re: Varible 12ax7 cathode resistor in preamp? Gil,     Not having seen the Torres article in question, I suspect what he is talking about is having to change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing.   If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs. The two go hand-in-hand, if you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point. At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand.     People tend to view preamp tubes and circuitry differently from power tubes, but in reality, they are the same animal, just on a different scale. Preamp tubes need to be biased, just as power amp tubes do. There are ranges that are more optimum than others. This is why it is not a fair test to just swap different tube types and compare tones. A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone. A straight swap without regard for circuit operating conditions will lead to incorrect conclusions regarding the tone.     Randall Aiken

 

Replies:

Gil Ayan q{Gil, Not having s... -- 3/1/2000 9:14 AM ken gilbert q{having to change the plate resist... -- 3/1/2000 3:28 PM