Bias Calculations for Cathode bias??

[Jalmeida]

I have been playing with the Robinette Bias calculator and it doesn’t seem to show the whole picture. I am just wanting to know how to determine the cathode resistor based on operating bias point. Ohms law I am sure is the universal answer. But I am sure I not plugging the right #s into ohms law calculator. I don’t doubt that this may be 100% user error.

What bias calculators do you prefer??

[Lectroid]

I've made that same search and I think Rob Rob's calculator is one of the best around.  Re-read his explanations and then run though it a few more times.  It'll start to make more sense.  Yes Ohm's Law is all you need.

[HotBluePlates]

... I am just wanting to know how to determine the cathode resistor based on operating bias point. ...

For what amp?  What tubes?  What supply voltage?  Class A or "hot Class AB"?

I have been playing with the Robinette Bias calculator and it doesn’t seem to show the whole picture. ...

Yes, because it's up to you to supply the whole picture (or recognize it in the amp before you).

[Jalmeida]

... I am just wanting to know how to determine the cathode resistor based on operating bias point. ...

For what amp?  What tubes?  What supply voltage?  Class A or "hot Class AB"?

I have been playing with the Robinette Bias calculator and it doesn’t seem to show the whole picture. ...

Yes, because it's up to you to supply the whole picture (or recognize it in the amp before you).

It isn’t an amp made. It is a simple single ended amp with EF86 and EL34 i am playing with to try to learn the calculations and design aspects. So I am trying to figure how to come up with load lines, output transformer impedences and then quiescent operating points and then be able to calculate what size cathode bias resistor based on this. Versus copying a known good design.

EL34
Class A
Supply voltage 450v

[PRR]

Tubes are NOT resistors. You have to work on the tube curves. (Which, being curves, confirm that tubes are not simple Ohms Law resistors.)

For the usual tubes working typically at Vg2 near Vp, use the Triode curves. This is often easier than correcting the Vg2=250V curves to another Vg2.

You WILL need to specify a Dissipation.

Some parameters are not much use. An EL34 size tube for audio amplifier may not like 450V, because that implies <55mA current and >8K happy load.

Do NOT expect to hit that magic value first-cut. Plan a pile of 100r and 50r power resistors for cut-and-try. Yes, this may mean a second shipping charge. (Which is WHY the Dead Men published Suggested Operating Conditions: to make things easier for potential customers/designers.)

[tubeswell]

... learn the calculations and design aspects. So I am trying to figure how to come up with load lines, output transformer impedences and then quiescent operating points and then be able to calculate what size cathode bias resistor based on this. Versus copying a known good design.

EL34
Class A
Supply voltage 450v

So you saw this already?


https://www.valvewizard.co.uk/se.html

[Jalmeida]

... learn the calculations and design aspects. So I am trying to figure how to come up with load lines, output transformer impedences and then quiescent operating points and then be able to calculate what size cathode bias resistor based on this. Versus copying a known good design.

EL34
Class A
Supply voltage 450v

So you saw this already?


https://www.valvewizard.co.uk/se.html

I have. I have been reading it over and over trying to wrap my head around it. I grasp it until he goes into the Screen voltage determinations with the Mutual Characteristics chart. I was hoping that there was a calculator I can tweak and calculate while going along with that explanation and also play with mine as well. I just learn better and the concepts are more able to stick by having something I can see and manipulate to see correlative changes. I am also in the process of reading Merlin Blencowe’s preamp design book as well.

[Jalmeida]

Tubes are NOT resistors. You have to work on the tube curves. (Which, being curves, confirm that tubes are not simple Ohms Law resistors.)

For the usual tubes working typically at Vg2 near Vp, use the Triode curves. This is often easier than correcting the Vg2=250V curves to another Vg2.

You WILL need to specify a Dissipation.

Some parameters are not much use. An EL34 size tube for audio amplifier may not like 450V, because that implies <55mA current and >8K happy load.

Do NOT expect to hit that magic value first-cut. Plan a pile of 100r and 50r power resistors for cut-and-try. Yes, this may mean a second shipping charge. (Which is WHY the Dead Men published Suggested Operating Conditions: to make things easier for potential customers/designers.)

Thanks for the advise. I did buy a big assortment of resistors and varying wattages. My hope is 12w or more single ended. I am just picked a transformer voltage and calculated post rectifier for a starting calculation starting point. My hope is to play with numbers and change things around to understand how all of the factors change and then arrive at a plan based on that. I have been enjoying learning alot. But I wish there were another explanation than Valve Wizards. I have found that most of Valve Wizards explanations are informative and have been a huge help. But the bit about Screen voltage and mutual characteristic charts is not clicking with me.

[tubeswell]

the Screen voltage determinations with the Mutual Characteristics chart.

For simplicity, you need to find a datasheet with a plate characteristics chart and a mutual characteristics chart that have their y-axis at the same scale. e.g. on pages 3 and 4 respectively in this link https://frank.pocnet.net/sheets/019/e/EL34.pdf

Once you rotate the p3 chart clockwise by 90 degrees, you can put the two charts side by side, with the p4 chart on the left hand side and the p3 chart on the right hand side.

Then on the p4 chart, you estimate the screen voltage curve you want by sliding the closest Vg2 curve to the left or right (depending on whether you want to raise or lower Vg2), and plot this new curve.

Then plot some coordinate points on the new curve and slide these horizontally across to the p3 chart (which should be sitting right next door with the y-axis at the same scale). This will tell you where the y-axis plot is for the desired control grid curve at the desired plate voltage for the new screen voltage. You need to plot a range of control grid curves this way.

[Jalmeida]

the Screen voltage determinations with the Mutual Characteristics chart.

For simplicity, you need to find a datasheet with a plate characteristics chart and a mutual characteristics chart that have their y-axis at the same scale. e.g. on pages 3 and 4 respectively in this link https://frank.pocnet.net/sheets/019/e/EL34.pdf

Once you rotate the p3 chart clockwise by 90 degrees, you can put the two charts side by side, with the p4 chart on the left hand side and the p3 chart on the right hand side.

Then on the p4 chart, you estimate the screen voltage curve you want by sliding the closest Vg2 curve to the left or right (depending on whether you want to raise or lower Vg2), and plot this new curve.

Then plot some coordinate points on the new curve and slide these horizontally across to the p3 chart (which should be sitting right next door with the y-axis at the same scale). This will tell you where the y-axis plot is for the desired control grid curve at the desired plate voltage for the new screen voltage. You need to plot a range of control grid curves this way.

Oh wow!! That is the nicest data sheets I have seen. Well laid out and more data than the ones I have found. Thanks for sharing.

[Lectroid]

For simplicity, you need to find a datasheet with a plate characteristics chart and a mutual characteristics chart that have their y-axis at the same scale. e.g. on pages 3 and 4 respectively in this link https://frank.pocnet.net/sheets/019/e/EL34.pdf

Once you rotate the p3 chart clockwise by 90 degrees, you can put the two charts side by side, with the p4 chart on the left hand side and the p3 chart on the right hand side.

Then on the p4 chart, you estimate the screen voltage curve you want by sliding the closest Vg2 curve to the left or right (depending on whether you want to raise or lower Vg2), and plot this new curve.

What a great technique and learning aide!  Thanks for sharing that.