How do you calculate the cathode current in a push pull class AB tetrode?

[demonstratedspatulas]

Assuming you have 4x6L6s in a 120 watt amp, that is 30 watts per tube.

A "dumb" calculation by me would say ok the B+ is 500V, that means 30/500 = 60 mA. This obviously isn't correct, as the B+ will sag at full power, and of course the screens draw some current also and they are at a lower voltage, say 450V. It also doesn't take the bias into account, which obviously is crucial to the difference between the amp producing any output at all.

What is the correct formula for calculating this, assuming you know the bias point, the idle b+ voltage, the idle screen voltage, and the tube type?

I'm fairly certain it involves taking a datasheet of a "close" idle screen voltage, and then scaling the datasheet somehow (how is this accomplished?) to match the correct idle screen voltage, then drawing a sine wave on the datasheet from the bias point to 0V, then somehow calculating the current from that, but I'm not sure, and I can't remember where I read all this years ago.

Is there a simple formula for 6L6s? Or is there a tool somewhere that makes this process easier?

Thank you!

[xtian]

Why memorize shit? I rely on the internet...which means I'm done when it quits.

I use this bias calc: https://www.amplifiedparts.com/tech-art ... calculator

So, given 500v and 6L6gc, 100% plate dissipation is, as you guessed, 60mA per tube. But you certainly don't want to run that hot in an class AB amp--aim closer to 50% and then use your ears, or scope to see if crossover distortion is an issue.

[wpaulvogel]

Are you asking for the cathode current per tube at full 120 watt output or when idle at bias? At full power you will need to know the plate voltage and screen voltage.

[pdf64]

Firstly forget the 30W per tube thing, that’s completely irrelevant to the query.
Merlin explains the process http://www.valvewizard.co.uk/pp.html
So you need the OT primary impedance and the HT voltage at full load. As it’s a guitar amp it has to accommodate being heavily overdriven, ie putting out a squarewave. So there’s probably going to be a lot of HT sag.
The current each side of the OT primary will be the loaded VHT - the tube’s saturation voltage, divided by 0.25 of the regular OT primary impedance.
Then the current per tube will be half of that.

This tool is handy http://bmamps.com/ivds.html

...I use this bias calc: https://www.a mplifiedparts.com/tech-articles/tube-amplifier-bias-calculator...

I’ve got a bee in my bonnet regarding the use of the phrase ‘bias current’ to mean idle plate / cathode current. Bias is negative voltage at idle between control grid and cathode, the current in that circuit is negligible

[demonstratedspatulas]

Firstly forget the 30W per tube thing, that’s completely irrelevant to the query.
Merlin explains the process http://www.valvewizard.co.uk/pp.html
So you need the OT primary impedance and the HT voltage at full load. As it’s a guitar amp it has to accommodate being heavily overdriven, ie putting out a squarewave. So there’s probably going to be a lot of HT sag.
The current each side of the OT primary will be the loaded VHT - the tube’s saturation voltage, divided by 0.25 of the regular OT primary impedance.
Then the current per tube will be half of that.

This tool is handy http://bmamps.com/ivds.html

...I use this bias calc: https://www.a mplifiedparts.com/tech-articles/tube-amplifier-bias-calculator...

I’ve got a bee in my bonnet regarding the use of the phrase ‘bias current’ to mean idle plate / cathode current. Bias is negative voltage at idle between control grid and cathode, the current in that circuit is negligible

Thank you! This calculator is awesome!!