Rp = 2Ri

[ruffrecords]

Ian, would you like to explain more on this, how to go about when setting up an unbypassed stage?

The datasheet plots are for the tube, not for the circuit you put it in. So all the voltages are measured relative to the cathode. For an unbypassed preamp stage their only real use is to help you pick a bias point and work out the value of the cathode resistor needed.

im starting to think that my mind has stuck on this criteria for no use, and that it is just a ruff guide to the area where good symmetry is found..

I agree. There are a range of sometimes conflicting requirements when choosing an operating point. For low distortion you want Rp to be several time  rp. 5 times is a typical target. I am not sure where you got the Rp=2 * rp from.  As a rule a good starting point for a preamp stage is to aim for the plate voltage to be somewhere between one third and two thirds of the HT supply on the assumption that a huge voltage swing is not required. rp varies with plate current so you are tempted to raise plate current to reduce rp but this drops the plate voltage for a give Rp and so on. And this is just the dc operating point.

and when the bypass cap is removed, impedance and gain is different, and thats it..!

cheers!

Now we are talking about the ac characteristics. The cathode resistor provides negative feedback. At dc this stabilises the operating point. For ac it reduces gain and distortion and increases the output impedance.

Cheers

Ian

 

[5v333]

ian you are the best!!

some educational sites state this ratio. i think rdh4 says RP = 2Rp OR higher.

thank you and PRR for pointing out that the ratio can preferably be higher!

now its time to try something new!

 

[ruffrecords]

I have never designed a tube power amp but I seem to remember seeing something like Rp = 2 * rp for triode output stages as being a ;point where there is a good compromise between output  and distortion; but only for triode power stages.

Cheers

Ian