safe mv bias range for KF50 anyone

[aaronz28]

the last time I set bias, I did it measuring the miliamps across the output transformer - i'm pretty certain that measuring MV on the back of my Komet requires a different measurement.

anyone know what the safe M.V. reading should be when biasing EL34 in a Komet KF50 with 475b+ ?

thanks

Aaron

[M Fowler]

EL34 in a Komet KF50 with 475b

Plate voltage of 450 = min 28mV to max 39mA

Plate voltage of 500 = min 25mV to max 35mV

No more than 70% of maximum plate dissipation of the tube. The EL34 has plate dissipation of 25w so AB operation (0.7*25/475)

So 25/475=.05263 x 0.7 =.0368 or .037mA

You don't just automatically set to 70% you may like it lower.

Mark

[aaronz28]

what is the min mv to max Ma?

the amp has a mv measure jack on the back?


thanks

A

[M Fowler]

If the amp has 1 ohm resistor from pin 1-8 to ground then it will convert one mV to mA.

1mV = 1mA

[M Fowler]

Insert red lead from DMM to red jack on back of amp and black lead into black jack on back of amp and if no black jack then connect to chassis ground.

Unlock bias knob and turn the bias knob watching the mA reading on the DMM.

[aaronz28]

thanks - thats what i've done - but i get some glowing with them biased at 32.5mv
and that is with a pair of NOS mullards - i didn't think i was doing anything wrong, but surprised to see one of them glow with only 32.mv reading

thanks

[Richie]

swap the tubes and see if the tube glows in the other socket. If it does, then that tube is not matched to the other tube. If it doesn't glow,and the non glowing one starts to glow in the other socket then you know something in the circuit for that socket is not right. Either that or you could have some other issues. You could also adjust the bias a little cooler,and test one tube at a time in the same socket. Then test each tube in the other socket one at a time. Do you test with the volume control down,nothing plugged into the amp input?

[M Fowler]

What is the screen voltage?

[leadfootdriver]

Weber Bias Calculator anyone?

http://www.webervst.com/tubes1/calcbias.htm

[leadfootdriver]

Bias charts.

http://www.diyguitarist.com/GuitarAmps/ ... beBias.htm