about EL34 bias Voltage in class A

[kldguitar]

EL34 Class A amp, How many is it bias Voltage. ? if it is -14 votlage, the bias Resistor maybe only 100 ohm. same time it's outpower only 2-3 w.

[Jag]

There are many tube data sheets available on the net that will give you all of that information.

Take a look at this one for example http://www.drtube.com/datasheets/el34-sed2002.pdf.



ASSUMING CATHODE BIAS
===================
Using some ballpark numbers as an example, let's say the scenario is as follows;

Let's say your B+ is 275 V
Per the data sheet, you want Plate voltage of 250 V
This means Cathode voltage will be 25 V (275 - 250)
25W is max dissipation for an EL34 and that's about where you want to idle for Class A.

First figure out the idle current using the formula Power (Watts) = Current * Voltage or P = I * E
We know max dissipation is 25 W (P)
We know we want Plate Voltage of 250 V (E)
Using this information we can figure out the required idle current (I)
P = I * E
25 = I * 250
I = 25 / 250
I = 0.1 A (100ma)

Using this information you can calculate the required resistor using Ohms Law E = I * R.
We know we want Cathode Voltage of 25 V (E)
We know that idle current is 0.1 A (I).
E = I * R
25 = 0.1 * R
R = 25 / 0.1
R = 250

So, given the above scenario, we want a 250 ohm bias resistor.

NOTE: There will be a few ma of screen current included in that 100 ma so the plate current might be 95ma or so. We don't have to be too picky about that.


FOR FIXED BIAS
============
Using the same general scenario;

Let's say your B+ is 275 V
Max dissipation for EL34 is 25W
Calculate the idle current required using the formula listed above, P = I * E
25 = I * 275
I = 25 / 275
I = 0.091 A (91 ma)

Knowing that B+ is 275 V and I want 91 ma idle current, I would look at the performance curves on the data sheet for a ballpark bias voltage. I'd probably start with -19 V and adjust from there.

I'd probably design the bias supply to give me a range of around -12 to -24 volts.

It's not really the resistor value that matters so much as where does the bias voltage need to be to give you a plate voltage and idle current that does not exceed max dissipation.



In either of the above scenarios you need to be aware that idle current and Plate voltage will vary with change in cathode resistor (cathode biased) or gird bias voltage (fixed bias). There will be some "back and forth" adjustment of these elements, within their respective scenarios, until you get the idle dissipation where it should be for Class A (25W).

You also need to be careful to design a system that doesn't exceed the maximum design parameters specified in the data sheets.