OK, lets look at this in steps. You can take the primary of a push pull transformer and hook up two P-P driven audio tubes to it, B+ goes to the center tap. Then take either plate and tap into the wire and make this modulated B+ to plate modulate a class C RF amplifier. This is called push pull Heising modulation. If the mod transformer has a secondary winding, it is not used.
The beauty with this concept is that a zorched mod transformer can be replaced with a P-P hi fi output tranny (or maybe a HV power tranny) so long as the primary current imbalance does not saturate the core. If it does, then the E&I laminations might need to be re-arranged to add an air gap.
So with modified Heising as described we have a limited peak swing available (70-80% of B+) due to the modulator tube saturation characteristic at maximum signal level (plate V min cannot get to zero volts). This limits the maximum modulation percentage to ~ 70-80%.
So what Tim did with the article mentioned was to take the 500 ohm winding and properly phased and put in series with the mod B+, it increases the peak AC level to overcome the limitation of Heising modulation such that a higher modulation percentage can be obtained. Those trying this with a power transformer, can similarly use the 115v winding like Tim did with the 500 ohm winding.
To know further how Tim's mod changes the modulator P-P load impedance to get more audio power requires knowing the turns ratio of the transformer between all the windings. Does anyone know the published impedances, or turns ratios?
The mod transformer secondary now does nothing since one end is disconnected.
I hope my explanation is clearer than mud..
Jim
WD5JKO