[AMRadio] Modulator design needed

John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Mar 26 17:50:30 EST 2006

Although the output tubes are 6146s the circuitry of the driver is what
is in question.  The older 6SN7 could be replaced with the 5697 9 pin
mini or at lower voltages the 12AU7s could work.  The idea of the pahse
splitter is to use a triode with exactly the same resistor for both
plate and cathode load resistors.  Use a small value such as 2 X 22K the
at the grid of the push pull long tail driver use a very high resistor
such as 470K each.  The common cathode resistor of the long tail makes
the circuit a semi differential amplifier which compensates of
discrepancies in the phase splitter.  These discrepancies include
different output capacitances for plate and cathode and load handling
capabilities.  The differential amplifier corrects a lot of the
differences making the outputs closer to a true phase 180 degree
difference at a single moment and across a wider frequency responds.  Of
course if your only want a few watts of audio you can chose modulator
tubes that will allow a larger grid resistor for AB1 operation with out
going into thermal grid leakage run away.  6V6s, 6L6s, etc. with the
smaller tubes and less heat you can use higher grid leak resistors and
come right off of the phase splitter to the grids of the output tube
with out using the differential amp in between.  

73, John,

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