[AMRadio] Modulator design needed

John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Mar 26 16:29:18 EST 2006

Try this on for size.  

I would recommend a balancing control in the cathode circuit of the
driver stage as the common cathode resistor compensates for
irregularities in the phase splitter it will also cause and improper DC
offset due to a possible imbalance in the two triodes.  A small 500 ohm
pot placed between the two cathodes and then the cathode resistor on the
wiper would be better.  This type of circuit is sometimes called a long
tail driver phase splitter.  I think?

If you need to go to AB2 and draw some grid current from the modulators,
then the transistors of the SS driver circuit, can be replaced with
small tetrode tubes.  The RCA 1KW BC rig uses two 807s as direct coupled
cathode followers to the grids of the 833 modulators.  There is no
reason a pair of 6AQ5s couldn't drive 811s or by the same token, a pair
of 6AU6s should drive 807s  or 6146s really nice.

The cool thing about driving with tubes instead of transistors is the
high Z input could come from a RC coupled phase splitter. 

The draw back is the need for a negative supply and possibly separate
filament windings for the drive tubes, since there cathodes will be hot
with DC and audio.  Don't forget to bypass the screens to the cathodes,
not to ground.  

None the less, it is a very nice project and would deliver very nice


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