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EL84 Bias Questions

6/21/2000 5:50 PM
Dean Hazelwanter	EL84 Bias Questions I converted a single-ended console stereo with 2 EL84s to a mono guitar amp power amp, and built a single 12AX7 preamp. It sounds extremely good. After I had it working, I started thinking about the bias and static power dissipation of the remaining power tube - since both power tubes shared the same cathode resistor and bypass cap. All I did to change the power amp section from stereo to mono, was to remove the 2nd channel's power tube. I later also removed that channel's OPT. I recalled reading that 12W is about right for an EL84. I calculated that at the moment, that mine was running at about (6.11/64.3 * (307-6.11)) = 28.6W!!! I changed the 68R (measured at 64.3R) cathode resistor to what I had on hand that was about twice the original - 150R. This brought the static dissipation to ~19.5W. Questions: - What is the optimum static power dissipation for an EL84, to get the best tone with reasonable life expectancy? - What is the maximum that one should be run at, before it: - Drastically reduces life expectancy - Starts on fire 8^) I also realized that another factor as to why it's running hotter is due to the fact that the load on the power supply was cut in half, which raised B+.

6/21/2000 5:50 PM

Dean Hazelwanter

EL84 Bias Questions

I converted a single-ended console stereo with 2 EL84s to a mono guitar amp power amp, and built a single 12AX7 preamp. It sounds *extremely* good. After I had it working, I started thinking about the bias and static power dissipation of the remaining power tube - since both power tubes shared the same cathode resistor and bypass cap. All I did to change the power amp section from stereo to mono, was to remove the 2nd channel's power tube. I later also removed that channel's OPT.

I recalled reading that 12W is about right for an EL84. I calculated that at the moment, that mine was running at about (6.11/64.3 * (307-6.11)) = 28.6W!!! I changed the 68R (measured at 64.3R) cathode resistor to what I had on hand that was about twice the original - 150R. This brought the static dissipation to ~19.5W.

Questions:
- What is the optimum static power dissipation for an EL84, to get the best tone with reasonable life expectancy?
- What is the maximum that one should be run at, before it:
- *Drastically* reduces life expectancy
- Starts on fire 8^)

I also realized that another factor as to why it's running hotter is due to the fact that the load on the power supply was cut in half, which raised B+.

Replies:

MBSetzer Hi Dean:I can't act... -- 6/21/2000 7:22 PM