• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

300B bias of a JE Labs?

Ok, I'm starting a new project to build my first tube amp, to see what it is all about.

Joseph E. write on his site about the new Hammond 125ESE, certainly not the best iron but worth a look from, the price and his comments. The one thing to note is a spec current rating of 80mA. Now from my reaserch I have found a number of people who bias their 300B's at ~80mA. I wanted to find out how to calculate the bias of the JE circuit and any suggestions on bias adjustments to around 60-70 mA.

Your advice is certaily welcome.

Thanks
Greg
 
350V -74V 60mA (B+ about 430V)
right off the WE300 data sheet

74V/ .06A = 1.233K (Two 2.5K's in Parallel should be close enough)

Keep in mind that the B+, Cathode Volatage and current all work together.

Check out:
http://www.westernelectric.com/products/300b/300B.pdf

Also on the WE web site is a table that shows opperating points
based on the Voltages, current and OPT

I have been thinking about the same topic for a 2A3
No one correct answer... so I'm sticking with one right off the
"Typical Operating Conditions" from the mfg's data sheet
 
You really have to play around with the operating points and find one that you'll like.

My friend runs his 300B at high voltage, low current and uses Tamura F-475. It sounds very nice.

Now, the classic and deluxe JE Labs 300B has 880R cathode and 70V on the cathode. That gives 70 / 880 = 0.7954 mA

If you want to run one at 70mA, you need 420V B+, 1K cathode resistor. That will give you ~70V on the cathode.

For 60mA increase the cathode resistor to 1k2, that will give you 72V on the cathode and 60mA current draw.

You won't get 8W from this. YMMV.