Load line question

[dhuebert]

Just as an exercise, I calculated resistor values and frequency responses for 12AX7, 12AT7 and 12AU7 based on plate curves and the equations at aikenamps.com. One of the results that had me wondering was the rule of thumb that plate resistors (Rp) should be ~ 2 times plate resistance (rp). I calculated rp by taking two points on a given grid voltage line (Vg) @ 100 and 200 volts (Vp) and calculating the slope. For the 12AU7, for instance, this came out to 6.5K. I chose 20K for Rp (~3x) and proceeded to calculate a load line and gain etc.

In studying schematics I have seen a wide variety cathode and plate resistors chosen and would like some insight into why certain choices were made. For instance: Standard 12AX7 values are Rp=100k and cathode resistor (Rk)=1.5K and supply voltage (Vs)=250V. I was looking at a design recently where the Vs=300V, Rp=220K and Rk=2.7K.

I'm just wondering if some of our esteemed Gurus would like to comment on the differences that different choices can make to the operation of triodes in the above examples.

Don

[EWBrown]

Check in the back section of your RCA tube manual, under "resistance coupled amplifiers", there are a few pages of charts showing the idealized Rp and Rk values for all the common (small-signal) audio triodes and pentodes, for B+ values of 90, 180 and 300VDC, and the resulting voltage gains realized with these particular combinations.

An interesting experiment is to breadboard up a simple triode VA stage, and use trimpots for both the cathode and plate resistors, and then set them to various resistance values, and measure and record the results,
For example, a 5K trimpot for the cathode, and a 500K trimpot for the plate resistors, for a 12AX7. Then the plate current, and gain factors can be compared for various combinations.

Published charts and graphs are very useful, but nothing teaches like hands-on experimentation.

All the aeronautical angineering theories state that it should be impossible for a bumblebee to fly, but nature has proven them all to be wrong Yellow_Light_Colorz_PDT_05

/ed B in NH