https://crunchytubeamps.com/product-category/preamp-triodes/ https://crunchytubeamps.com/product/psvane-tube-6sn7-be-replace-6n8p-6sn7gt-6sn7c-6sn7-cv181-6h8c-matching-pair-for-vacuum-tube-amplifier-hifi-amplifier-audio/
Want to design a tube amp using the preamp tube of your choice? One place to start is with a load line. You could easily find an online calculator to do all the work for you, but what’s the fun in that? Plus you will have much more control of your design if you gather your own data. Using a load line can get you an entire text book full of information; however, we’re going to look at the cathode bias resistor and gain.
All we do is divide a plate supply voltage of our choice by a plate load resistor also of our choice. BAM! We got a load line:) For example, if we use the curves above of the 6F5 tube. Say we choose 300VDC (max volts for 6F5) for our supply volts, and apply a 250K ohm plate load resistor. We just divide (300VDC/250000 Ohms=0.0012), and we get 1.2mA. Using our curve data we draw a line from 1.2mA to 300 plate volts to expose our load line below. Experiment with the numbers until you get a load line that cuts across the curve line as even as possible. Now we can figure out the value of our cathode bias resistor.
Now we have to choose a bias point, and there’s going to be some guess work. Our load line crosses the 0 curve line at about 1mA, so half of that value would be 0.5mA. If we make another horizontal line across the 0.5mA line we will intersect our load line between the -1.5v and -2v curve. I would eyeball the intersection at about -1.6v, so that is my Bias Point. Ignore the negative and divide 1.6v by our current in Amps and we get (1.6/0.0005=3200). 3.3K Ohm is a close common value for our cathode bias resistor.
We can now find approximated gain. Draw a vertical line from the intersection of our load line and -1v curve to plate volts. Now do the same for the intersection of -2 curve and our load line to give us 1volt change between our two curves. Now just count the amount of plate volts between our 2 lines (each box=10v) and we get an approximate gain of 78 per 1 volt change in the grid curves. Our gain is 78:)
You can now draw a load line and figure out the bias of any preamp tube of your choice! You can also figure out gain which will come in handy for things like adding inverse feedback which you can read about here: https://crunchytubeamps.com/improving-your-budget-amp-build/
Figure out what input resistor to use here: https://crunchytubeamps.com/bright-or-murky-sounding-amps/
Here is a link to a video I posted covering this same topic.
