by EWBrown » Tue Mar 27, 2007 6:08 am
The 1.56 volts is derived from it's being the voltage of a brand new carbon zinc D cell battery. In those days, digital voltmeters for the masses didn't exist, and even the most humble cheap VOM still can be very accurate as an analog comparator, so the process was, measure a brand new D cell, take note of the relative reading, and then use that value to set the bias (100 mA for a pair of tubes, hopefully pretty well matched).
For uppgrades or new builds, it's easier to use a 10 ohm ersistor (and set for a 1.00 volt reading) , and if you set it up for individual 50 mA for each tube biasing, so much the better. IIRC the MK III was set for 140 mA trotal, 70 mA for each of the 6550s or KT88s.
/ed B in NH
Real Radios Glow in the Dark