> Patrick, > > For a "legal" AM plate modulated ham rig, running the tubes you > mentioned as you mentioned is way over kill. Heck, ~ 300 watts or so of > audio is all you will ever need to modulate the a rig designed for 375 watts > carrier output. If I had 4-400's I'd go AB1 and run it as a tetrode. Sure, > that requires a bias supply, and a screen supply. The drive will be simpler, > and a lot less swing, enough simpler to justify the extra complexity of the > extra power supplies. A 12AX7 driver would do fine. > > As for your question, yes it's workable. I would use negative feedback > however to linearize those tubes a bit since distortion will be fairly high. > The hardest part of your plan is getting undistorted drive from the driver. > Maybe use one of your 2 X 8005 PA amplifiers driving a quality audio output > transformer in reverse. > > Regards, > Jim
I just don't see a pair of ANYTHING as needing 1000 grid volts of drive in Amatuer service... or am I reading that wrong? ------------------------------- Patrick said: Ok, in the name of versatility, suppose there's this simple transformer coupled push pull modulator chassis, set up for 4-400A's in class B. (yes but read on) In the chassis, the socket terminals which would go to the 4-400 control grids have a 20K resistor between them and the screen grid terminals. The idea is to limit the control grid current of the 4-400, and let most of the audio drive the screen grids. Kind of like the old 120W class B zero bias 807 modulator from the ARRL and RSGB handbooks. I won't nit pick about exactly what class it is, probably more like real-world AB2. Ok. so I drop in a pair of 3-500Z's. Of course the 20K resistors would not matter when the 3-500Z was inserted, as all 3 non-filament pins are connected to the triode's grid. I throw a DPDT switch, to change taps on the driver transformer from ones supplying 1000V peak grid to grid (to drive the screens of the 4-400's as triodes), to those taps supplying a lesser voltage appropriate for the 3-500Z's. ------------------------------ Man, my 250TH's (triode), in Class B, only require around 400vpk g-g drive, for full output at 3kV. And 400vpk is a bunch for driving modulating tubes! 73 = Best Regards, -Geoff/W5OMR
