by Ty_Bower » Wed May 18, 2011 9:59 am
I think the assembly manual indicates a 4.7K and an 18K resistor in the bias voltage divider network. This should give you a range of roughly -67V to -42V, depending on your line voltage, etc. Actual bias voltages aren't really important - it's the idle current that matters.
I would recommend you substitute a 1K resistor instead of the 4.7K. I've seen enough people who have had difficulty getting the idle current under 70mA, and making this substitution seems to help them out. For what it's worth, I stayed with the original 4.7K part but my bias adjust pot is pretty much turned all the way counterclockwise. If I had needed it, I wouldn't have had much room to turn the idle current down any lower. If you want, you can also sub out the 18K part for a 21K just to keep the total load of the bias network approximately the same, but I don't really think this is necessary.
You should consider adding a 100K resistor from the top of the pot to the middle leg for safety purposes. It won't affect the bias during normal operation but it will save your tubes and likely your output transformers in the unfortunate case where the wiper lifts.
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