ST 70 Power out versus EL34 Bias point

Since the original st70 recommended a bias point of 50 mA and Bob recommends a bias of 40mA on the EL34 tubes, I was wondering what the effect of the two different bias points has on output power and harmonic distortion?

I do understand the benefits to tube life with the lower bias setting.

Has anyone made these measurement?

Bob, I'm sure you have some useful data concerning this.

Thanks!

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Thanks Peter!
That was an excellent read. My takeaway was that running the EL34s at 50mA is optimal as far as distortion in concerned. Also, running them at the currently recommended 40mA increases distortion and doesn't radically increase tube life. The 50mA bias point gives the lowest distortion values.

So, if you want the best possible sound run the tubes at 50mA.

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I have a VTA st70 from Bob that is about 3 years old. With the bias set at 50mA and the input voltage set at exactly 120V using a variac, I get 410V B+ which is exactly what the original 1950s st70 schematic says I should get. This confirms that the new PT is wired for 120V as Bob has stated. I am using the 5AR4 rectifier. Also, the EL34 heater voltage is very close to 6.3V.

I agree with Peter in that we need to know what amps like Jolida and Rogue are running their B+ at.
If they are running the same B+ as the ST70, distortion is going to be much higher at 35mA idle current. There's a reason that a bias point is chosen other than seeing how long we can make the tubes last.