Load Line question

[surfsup]

If I have this for a preamp gain stage:

[img:916:501]http://chicagocadcam.com/ChrisHahn/LoadLine.jpg[/img]

I calculate a Rk of around 1.4k so Rk=1.5k

If I increase Rk to say double Rk=3k, would the operating point shift to the right ALONG the Voltage curve, or along the load line (because the Ra and Va didn't change)...I think I just answered my question.

[vibratoking]

You answered your own question.

Here is a link to an xls file that someone here put together. Sorry, I don't remember who at the moment. You pointed me to this file a while ago, so I am returning the the favor.


https://tubeamparchive.com/files/ll_plot_v701_635.xls

[surfsup]

Yea V is I*R so as Rk goes up, current should go down.

...And as I think about this more, my sleep goes down

[David Root]

If you want to keep the operating point in the middle of the 12AX7 load line when you change cathode resistor you should follow the "Rule of 66" and change the plate load resistor accordingly.

Rule of 66 is divide the plate resistor value by 66, gives you the cathode resistor, so 100K/1K5, 120K/1K8, 150K/2K2, 180K/2K7, 220K/3K3.

If you are looking to skew the operating point to alter the distortion characteristics you can of course do anything you want!

For $40 you can get a copy of TubeCad which will do this for you in a flash for lots of different dual triodes in 13 different major stage designs, of which grounded cathode is just one. I have been using it for years and highly recommend it.

[tubeswell]

Another load line plotter spreadsheet.

http://www.freewebs.com/valvewizard2/Lo ... lotter.xls

What David said - as a general RoT for a clean gain signal in an inverting triode gain stage, I'd keep the plate voltage about 1/2 to 2/3 of the HT voltage, and use bias resistor values that get me there accordingly.

[surfsup]

I am just running through some schematics of amps I like. I want to see what the builder was thinking when designing the preamp (not an easy task but I'm learning). For example, I plotted the Vox Night Train here:

[img:916:501]http://chicagocadcam.com/ChrisHahn/LoadLine2.jpg[/img]

The lowest/shallowest load line is both stages of preamp tube 1 (they are almost biased identically off the same B node with same Ra), then the red is next, then the steepest is the fourth gain stage.

In the schematic, there is a NFB loop from the grid of stage 3 (red) back to the input jack. So I assume that would tame some gain.

Looking at the load lines and trying to understand cold vs hot (I am still not sure if left is cold or hot btw), I assumed for most "headroom", you would bias to the center of the graph. So looking at stages 3 and 4 (red and steep black):

For 3rd, the red, is at ~1.65V and the max swing is 0 to 3.5V (where the load line meets the X axis is approximately at the 3.5V curve intersection). The 1.65V biaspoint represents almost the midpoint so "headroom" is maxxed.

For 4th, the steep black, we're at 2V and the LL intersects X axis at around 3.7V so again around middle for max headroom.

For 1st and 2nd, the load line intersects at around 3.6 and the bias is at around 1.2, so we're at the 1/3 mark here.

I didn't run the math because I'm just trying to "see" based on the graphs. The load line slope being more shallow returns a larger V swing based on a 1Vp-p signal so the gain is greater, I think. So it looks to me that most of the gain is up front then tapers off as each stage is hit.

It looks like there are two NFB: The NFB from after stage two to the input should reduce the gain a bit. There's also a NFB from the grid of stage three to the cathode of stage 2. I need to learn more about how the NFB affects the circuit.