The AM Forum

     These are dx60 mods I did years ago, incorporating ideas from Tim WA1HLR, Steve WA1QIX
Jamie KF2VM, Eric W2CAU and my own ideas garnered from William Orr's Radio Handbook.  A brief description as best I can remember.  From the schematic sketched below.  Microphone input is de-RF'd and fed into the base of a 2n3904, used as a emitter follower. This drives pin 8 the cathode of 1/2 12ax7 now running grounded grid. (provides a phase inversion and impedance transform from a hi-z mic to the lower z cathode). The output of that stage is coupled to the second 1/2 12ax7 voltage amplifier driving the grid (pin 7) 1/2 of the 6DE7.  The cathode of the 6DE7 stage (pin 5) is now passed through a "carrier set" pot, and connected to a -100v supply. 

     Reading the Orr book, much of tthe non-linearity of screen modulation is due to a near impedance square law change over the modulation waveform. I used a darlington coupled pair of
2SC1308 horizontal output transistors to provide current amplification of the 6DE7 output plate
circuit lowering its effective Z.  The Emitter of the darlington pair is connected to the screen of the 6146, and to 40k 20 watt resistor connected to the -100v supply.  For linearity and to
achieve 100% negative modulation, the screen must be brought to a negative voltage.

     The circuit marked "rf feedback" takes a sample of the RF output signal, rectifies, filters
and injects negative feedback into the cathode of the second 12ax7 amp. This helps linearity
but make sure you don't over modulate.

Dean Wa1knx