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I am building my first 18 watter, an sIII, and am almost finished. As part of the start up check, I calculated the idle bias on the EL84s.

I have about 345 volts on the plates, 12.5 volts on the cathode and a 150 ohm cathode resistor. If I did my math correctly, I'm running about 13.8 watts on the bias.

Here are my calculations. Net plate voltage = 345-12.5 = 332.5 v
Cathode current = 12.5/150 = .083 ma. Divide that by 2 tubes and you have .0416 ma. .0416 * 332.5 = 13.8 watts.

An EL84 is rated for 12 watts max plate dissipation and I would think somewhere around 8 watts would be correct for the idle bias.

Did I screw up somewhere?

Confirm the bias current by measuring the voltage across the cathode resistor and dividing by the resistance value to obtain the cathode current. For an 18W with two tubes you must divide the current reading by the number of tubes sharing the resistor. Then calculate the power from Power = Current squared X Resistance.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

That's what I did, Stephen. I measured 12.5 volts across the cathode resistor, which is within your spec.

12.5/150 ohms = .083ma. Half of that (for one tube) = ..0416ma

.0416ma X (345-12.5) = .0416 X 332.5 = 13.8 watts. That's over the max rating of 12 watts for an EL84.

Isn't that too high? What are we shooting for? Randall Aiken recommends 70% of rated, which is .7 X 12 = 8.4 watts.

Thanks!

He also says
"Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive."

You're OK with the values you have recorded. I have used 120 ohms there. 150 will give you longer life.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Ok. Thanks. Time for another post.