The AM Forum

Good afternoon Gents.  My first post on this new form software.

I'm in the process of homebuilding a single 4-400 modulated by a pair of 4-250's (yeh, I know that's more modulator than I need, but you gotta use what you have available).  I plan to use a Ranger for RF exciter and P-P audio drive to the 4-250's.  Everything is going along pretty well so far, I'm about 40% complete with the packing, testing power supplys, etc.  

My question is driving the 4-400 grid from the Ranger.  If I look at the actual "Johnson Desk" schematic, I see they use an elaborate switched link coupled system to the 4-400 grids.  I think some sort of an adjustible  L newtork can be made to work for 160-40 meters (all I plan to run), but don't know how to "throw away" the additional drive power from the Ranger.  Can I use a resistive load (approx 100 - 150 ohms) across the Ranger output and make this work?  I don't want to overdrive the grid of the 4-400.  

Or is the answer to get inside the Ranger and control the 6146 screen voltage to limit drive???  I don't think that's the way it was originally done.

This is the last major hurdle I need to understand...

I know there are quite of few hams that have built similar transmitters and sure appreciate your sharing your thoughts.  thanks Jeff W9GY, Kokomo, Indiana

Controlling the screen in a Ranger with something like a 250K pot seems to work to get the drive down to 10 watts or so. Under that the tube getes flacky if you are modulating it but you are not so no sweat. The ranger is just a power VFO & audio driver for the grids of the modulator tubes. You will probably need to set up the balanced input circuit that is in the handbook and rewire the accessory plug to steal the modulated audio for the mod. grids. The Mod transformer winding is CT for jamming in the bias. This is very similiar to my 833 project that I used a DX-100 the same way. You can probably drive a 4-1000 with this. Go for it, you know you want it! Ha

Update after several months of on-air use   ...I decided I didn't like the audio quality from the ranger.  So fed the modulator grids with a solid state IC audio amplifier(now using 4-125's for mods.) through a backwards connected hi-fi output transformer.  Audio fidelity is much improved over the Ranger (which, by the way, had the standard audio "improvements').  Since the Ranger was no longer used for audio,  I decided to substitute it with an older solid state transceiver to provide RF drive.  It's a more stable RF source in terms of frequency stability and drive level than the Ranger.  ;D

Yup. Many have travelled the same road. Some of us don't want to hear about it and like to prove things out for ourselves. That's half the fun. Bet it works quite well now.