The AM Forum

   Been pine-boarding stuff lately, just for fun, and thinking about putting a 75 meter pine-board transmitter on the air for the 2022 AM Rally. Pine-board transmitters are fun - just keep wandering idle fingers under control while you're operating (ask me how I know).
   I worked up a nice little 6AG7 VFO - very stable and generates gobs of swing. My question is, given that I've got enough swing to drive the final PA directly, is that a good idea? Conventional wisdom is that coupling between the final PA and the VFO can result in the PA pulling the oscillator around a bit, but has anyone actually had problems with that, or is it largely theoretical? And, if this pulling does occur, does it occur under modulation?
   Not even sure what my final PA will be at this point, but probably something like a 6146, 807, or 6DQ5. Looking for around 25 watts out.
   I'm thinking, I know people who run a single-tube AM transmitter - a modulated oscillator - and if you can get away with that without FM'ing the thing all over heck, then how bad could a VFO direct-to-final configuration be?
   One advantage of using an intermediate tube as a driver/buffer would be that I could use it as a doubler on 40 meters.

I started messing around with AM a few years ago playing with the Heil pineboard circuit, then after a while, pineboarded and then built on chassis a 6AG7/807 transmitter, with 6SN7/6CA7pp modulation. This was a straight plate modulated setup with a mod transformer. On those rare days when the local noise floor permitted me to receive anyone, I got decent signal reports. The unit put out about 38-42 watts carrier, depending on operating frequency. I ran it on 80m and 40m. You can see details and my trials and tribulations in my post history....

My only comment on your endeavors is that 'enough swing' may not actually be enough drive for the grid of your amplifier. I know many hams built 6AG7/807 transmitters  over the last 80-90 years, but I still fought with grid drive for a while. I'm a few milliamps over the limit for the 6AG7 plate, and yet the grid current is still a little weak on the 807. If I were redoing the design, I would at least experiment with a buffer during the pineboard stage.

Ed

Yeah, I definitely plan to experiment with a driver/buffer; that's the beauty of the pine-board. I'm just wondering what my chances are of getting away with a minimalist two-tube RF deck. I know that getting a good modulation percentage is highly contingent on getting enough grid drive to the final. Just enough for the carrier is most often not going to be enough for full mod.

So, aside from potential difficulties in getting enough drive to the final, has anyone experienced problems with the VFO's frequency stability when driving a final PA directly with a VFO, with no driver or multiplier in between?