biasing JJ E34L

[SteadyEddie]

In my 1987, I've got my E34L's biased at right around 75 percent dissipation, approximately 42 ma.
I was kind of wanting a little more clarity in the bottom end, will running it cooler clear it up a bit? Afraid of losing response and sustain in the trade.

[yladrd61]

I would back them off a bit, most knowledgeable techs recommend biasing between 60 to 70% of max plate dissipation based on B+ voltage and don't forget to account for screen current

[SteadyEddie]

Yeah, I bias based on plate voltage. How do you account for the screen current? That's a variable I haven't considered.

[OnTheFritz]

Yeah, I bias based on plate voltage. How do you account for the screen current? That's a variable I haven't considered.

The 1K screen resistor takes care of that. Best bet is to keep it at or under 70% max dissipation at idle. I like about 65% based on my ears personally. Of course it depends upon the tube. Subjective at this point. If you crank the bias, just watch for red-plating.

[yladrd61]

Measure the voltage drop across your 1K screen resistor, Ohms law Voltage divided by Resistance = Current example 5 Volts divided by 1,000 ohms = .005 amps {5 milli amps.} screen current. Max Plate Dissipation for EL34 25W / Plate Voltage (500 VDC} 25W/500Vdc = .050 X .70 = .035 Plus .005 = .040 The current (measured at the cathode pin 8 is Plate and Screen combined so with that plate voltage 40 ma would be 70%. that is the maximum you should use, I like 65% for EL34s. So they will still be in safe range if your wall voltage spikes !!!