Math for Dummies (cathode biased power section...)

[Bruciep07]

Let me take this one step further with an example, and please someone weigh-in with with corrections...

Lets say we were using a JJ EL34 and used this data chart...

http://www.drtube.com/datasheets/el34-jj2003.pdf

Lets assume a plate voltage of 340V. According to the chart, maximum plate dissipation (is that the correct term?) occurs at approx 78mA(25W). We shoot for around 70% of that which would be 62.5mA. Based on the chart you are shooting for approx -17V bias at 340V and 62.5mA, is that correct? So in it's simplest form, assuming all 62.5mA of plate current flows out through the cathode, we would set our cathode resistor at 272 Ohms...
17V=62.5mA plate current * 272ohms
Am I on the right track?

[FunkyE9th]

Let me take this one step further with an example, and please someone weigh-in with with corrections...

Lets say we were using a JJ EL34 and used this data chart...

http://www.drtube.com/datasheets/el34-jj2003.pdf

Lets assume a plate voltage of 340V. According to the chart, maximum plate dissipation (is that the correct term?) occurs at approx 78mA(25W). We shoot for around 70% of that which would be 62.5mA. Based on the chart you are shooting for approx -17V bias at 340V and 62.5mA, is that correct? So in it's simplest form, assuming all 62.5mA of plate current flows out through the cathode, we would set our cathode resistor at 272 Ohms...
17V=62.5mA plate current * 272ohms
Am I on the right track?

You have to learn how to draw load lines and then you have to select quiescent conditions. That will tell you current and the bias voltage. That JJ chart is only valid if you have a screen voltage of 250V. You actually have to redraw the chart based on the screen voltage you are using, and then draw the load line.

[thepacifist]

Here's the excel.
Please note that most, if not all, characteristic values (Screen idle current, max power dissipation) are wrong. They need correction.
The good thing is the %age cathode resistor variance and the corresponding power dissipation.
So if the model tells you you need 250 ohms, but you put in 200, the chart will tell you how hot the tube would run.

The "Max power dissipation" column for each tube is not used in any form of calculation...yet !
Suggestions are welcome and I will apply corresponding changes and upload again.

EDIT : When you open the excel, please click on "Enable Macros". This excel has VBscript code for running GoalSeek etc.

EDIT2 : Updated the excel to include a chart as it shows the cathode-resistor/power relationship in a visual manner. Better than raw data, IMHO

[thepacifist]

Need your inputs on this. PLZ. do not use it for actual modelling. This comes from a guy who knows nothing about making amps, just some calculations that have been put in an excel.

[dehughes]

Need your inputs on this. PLZ. do not use it for actual modelling. This comes from a guy who knows nothing about making amps, just some calculations that have been put in an excel.

Well, I'm having some trouble with it, but I'm running a Mac (OSX). Keynote opens it up fine, and the calculations can be made, but I can't use the "button", and it won't re-set for re-calculation.

[thepacifist]

Will try out in keynote and let you know.
a) you dont need to reset it. Just change the values and recalculate.
b) This has vbscript code.....not sure if that goes well with Mac.

[dehughes]

Will try out in keynote and let you know.
a) you dont need to reset it. Just change the values and recalculate.
b) This has vbscript code.....not sure if that goes well with Mac.

Sweet dude. Thanks so much for working with this. It could be super handy. Lemme know what you find in Keynote...