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RE: SUN SV 2A3 stereo SE amp .... MEASUREMENTS

First, you figure out what 62% is, in terms of Watts of plate dissipation. Maximum dissipation of a 2A3 plate is 15 Watts. Sixty two percent is 9.3 Watts of continuous dissipation.

I envision tubes as "voltage swing" devices, so I maintain the usual voltage, typically 250 VDC P-K, and REDUCE THE CURRENT draw such that voltage times current equals "about" 9.3 Watts. 9.3 Watts divided by 250 VDC is 0.0372 A., or 37.2 mA.

In actual practice, audio people I trust the most ( Dennis Fraker and Jac of Jac Music) privately say that their favorite 2A3 op point is 42 or 43 mA. at 250 P-K , or 10.5 to 10.75 Watts plate dissipation, so THAT is what "I" will shoot for in my amps.

The 2A3 WILL sound more relaxed, less stressed sounding, BETTER, at this "loaf mode" op point, unless your amp is so poorly designed with a typical crummy power supply , and insufficient wiring ( transfer efficiency in both cases) such that you need to HOTROD the tube to get the amp to play effortlessly and "dynamically".

The difference in lifespan of a 2A3 run at maximum dissipation ( 250 VDC P-K at 60 mA.) and at 42 mA. might be 10,000 hours at 60 mA. versus PERHAPS 50,000 hours useful life at 42 mA.

So, the wonderful thing is, IF the amp is well executed, the 2A3 lasts five times longer and SOUNDS BETTER at 42 mA. all the while !! That is smart audio design, conservative. You can easily justify to yourself to buy the better sounding and better made 2A3s, like the current EMLs, and be ahead in TWO ways, quality of musical performance and far longer lifespan.

For a Type 45, I'd use ABOUT 250 VDC P-K and about 27 mA., where 25 mA. equals 62.5 percent of 10 Watts maximum rated dissipation.

The Golden Ratio is just a CONCEPT that applies to design, and many many other things in our lives. Hope this helps. Have fun. I sure do !!

Jeff Medwin


Edits: 04/01/15
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