Calculating cathode bias resistor

[dwinstonwood]

Hi all,
I've hit a math roadblock, because I'm lousy at math.

So, let's say I have a single-ended OPT with a primary impedance of 5,482Ω with an 8Ω secondary load: https://www.hammfg.com/files/parts/pdf/1750B.pdf

And, I want to use a single EL34 with this OPT. Using the Valve Wizard's formula I get: ZOut = 370^2/25 = 5,476Ω. (plate at 370VDC, 25W max dissipation)
That's really close to the OPT's spec.

Using those online bias calculators, it looks like I want about 61mA for ~90% plate dissipation at 370VDC.

The question is, how do I calculate the cathode resistor value for this operating point? I'm stumped. Am I overlooking something obvious?
Thanks!

[HotBluePlates]

... I want about 61mA for ~90% plate dissipation at 370VDC.

The question is, how do I calculate the cathode resistor value for this operating point? ...

You need to look at an EL34 data sheet to find out what bias voltage is required to get 61mA of plate current when the screen voltage is about 370v (assume plate voltage will be 370v, too).

Then find Resistance using Ohm's Law:

   Resistance = Bias Volts / Cathode Current

You could find the expected screen current and add it to the 61mA of plate current to get the expected cathode current, but omitting it will be an error in a safe direction.

_________________________________

Work through the process.  You're very lucky to have a data sheet with a graph using almost exactly your desired screen voltage.  When you calculate the cathode resistor, round it up to the next nearest standard value.

You should arrive at an answer around 470Ω.

[dwinstonwood]

Thank you HotBluePlates!

I hope I used the correct graph...

I got 442.6 Ohms. Like you said, rounded up to the nearest standard value = 470 Ohms!

Much appreciated.

[HotBluePlates]

I hope I used the correct graph...

Nice!!  I used a different graph in the same data sheet, but it gave the same values.  I also got a bias of -27v, and 27v / 0.061A = 442Ω, which rounds up to 470Ω.

Good job!