peterh wrote:As this amp uses a built-in indicator circuit you will have changed bias if anything else but
a 15.6 ohm resistor is used.
Using a 15 ohm will increase bias from 50 to 51mA.
Using 2 parallelled 33 ohm resistors would be a better choice, resulting in 47.5mA / tube
Correct me if I am wrong.
Two 33 ohm resistors in parallel = 16.5 ohm. So my 15.6 ohm resistors that have drifted to 16.2 will be just fine and in fact will lower the current slightly, and that is not a bad thing.
For the sake of learning. Is there simple formula for calculating the bias setting for a stock Stereo 70 with a resistor other than the 15.6?
Yes, but a resistor that has drifted means it is old or has been subject to currents approaching it's maximum, it's weakened, I wouldn't count on it. I don't know if you've had that bias resistor blow on you but it is not fun, it makes a big mess and usually takes a tube or two with it and possibly other resistors in the way, not to mention what it might do to your speakers, it makes a horrible loud noise as it's going. It's a good idea to replace that resistor every several hundred hours, and for sure if you put in a new set of tubes. You can see the burn spots on my amp from R8 and R9.
Bob Latino says running the EL-34 on an ST-70 at 0.40mA is good, no loss in sound quality and extends tube life, plus less strain on that bias resistor. Because of the circuit design, the bias resistor can be subject to flashes of high current if a tube arcs.
RI=V, I=V/R, 100mA=1.56V/15.6ohms. R=V/I, 1.56V/80mA=19.5 ohm (80mA is for both tubes, 40mA each). I would run 2 x 39 ohm resistors, closely matched, you'll probably even notice an improvement in sound quality with the paralleled resistors. 2 x 39 ohm = 19.5 ohm.
My other recommendation to make your amp next to worry free is screen stability resistors. Tubes will no longer arc, no more big stress on bias resistor, tubes will last even longer, 10,000 hours some say. And lastly, tube arcing can mean coupling caps are wearing out, skinning dc current. (I learned most of this stuff from the guys here on this forum) http://tronola.com/html/maximize_tube_life.html
(read the article pdf)