quadaptor wrote:Well now I'm kinda confused. Is the bias setting for a stock ST-70 amp still 1.25 volts or is it a different value?

The setting is still 1.25 volts DC to give 80 milliamps per each tube PAIR. 80 milliamps per each tube PAIR gives 40 milliamps for each individual tube which is what you want. As has been said earlier in this thread, Dynaco gave a recommended bias setting of 1.56 volts DC for each tube pair in conjunction with a 15.6 bias resistor (again for each tube pair - one bias resistor for two tubes). This gave an idle bias current of 100 milliamps for each tube pair OR 50 milliamps per each individual tube. This will lead to shorter tube life in an EL34 than using 1.25 volts DC as your bias voltage.
By the way, Dynaco chose this seemingly odd bias voltage (1.56 volts DC) because that was (and maybe still is) the output of a NEW standard D cell battery in the 1950's and 1960's. Back then there were no digital meters like there are today and the accuracy of many analog meter was not that great either. What you were supposed to do is get a NEW D cell battery and note EXACTLY where on your meter the needle pointed when you "tested" your new D cell. You then set your bias on each channel to that exact point on your meter.

Bob

1.56 volts is what a fresh CARBON-ZINC dry cell puts out, and still does today; the voltage is defined by the relative electrical "activity" of the anode and cathode and the chemistry of the battery. Alkalines are different, rechargeables based on nickel (NiCd, NiMH) are even more different, lead-acid rechargeables are different still.

The original Dynaco tube amp, the Mark II, at first didn't even HAVE a cathode resistor to enable the bias-measurement method we're familiar with. The cathodes were grounded and the bias had to be set by measuring the negative voltage on the grids of the output tubes. This method is still used by some amplifier makers...Jolida is one that I can remember. But the Dynaco method is better under non-testbench conditions and for kit builders because it effectively permits measurement of the actual current flow through the tube under quiescent conditions.

The addition of the 1% precision resistor between the output tube cathodes and ground (of a specific resistance value especially made for each Dynaco amplifier model according to the desired idle current in the tubes) and the wire to the octal socket for easy measurement, didn't come until later on. By the time of the Mark ((( and Stereo 70 it was standard; and it was available as the Dynaco Biaset Kit for retrofit into the early Mark II amplifiers that didn't come with it.