Thanks for that!sluckey wrote: ↑Tue Jul 11, 2017 3:54 pm I suspect Merlin purposely avoided any calculations to determine the value of that resistor. Too involved and the result is often not perfect. Most circuit engineering is done through plagiarizing from reference books. The value of that resistor (often called the bias range resistor) is very quickly and easily determined experimentally. You already know that the higher AC voltage source will require a higher value resistor. It doesn't take much experience to know the resistor will typically be less than 1K when tha AC source is from a bias tap and it will typically be 100K or higher when the AC source is from the HT winding. You can either substitute different resistor values or even use a pot to dial in the proper value to give you the desired bias voltage range. The proper bias voltage depends on the output tube type. Typically for an EL34 a bias range of -30v to -50v will give you the exact bias voltage you need. Your R16 gets you in the ballpark and R20 will help set the amount of swing and the upper/lower limit. You don't even need the output tubes plugged in to get the bias circuit tweaked for proper operation.
See that's what I thought. However the brother amp to the one I'm building is spec'd with a PT that has HIGHER voltages on the secondary. Yet the bias supply for that circuit shows 180K and 47K resistors instead of 220K/56K. If the incoming AC voltage is larger, why are these resistors SMALLER in that amp compared to the circuit with lower voltages? Wouldn't it need to be the other way around? Both amps bias up EL34s just fine.
In any event, I already built the turret board with 220K/56K. So I guess I'll see what happens. Thanks for all the info!