The AM Forum

 ;) I'm an OT newbe. Ex K9ZSL licensed early 60's. Had a Conditional ticket. Let it lapse. Dumb. I'm going back on the air. Now that I'm retired am going after the retread and taking up where I left off. Built some dynomite gear especially audio because my other interest was recording. I have a 16-track digital studio in my basement with all that processing equipment. My audio will be awesome!

I'm building a 300 watt AM unit from scratch. Did I mention I have about 40 years in broadcasting and have a lot of the parts from a Gates BC-1T handy?
Driven.

Anyone willing to give advise on a simple exciter? I'm going to use the one from the 1949 Radio Handbook unless someone has something nifty with band switching and VFO provision for all the hot AM spots. 2 tubes (6AG7 to 6L6) feeding a 4-125A. The 1949 diagram uses plug-in coils. No problem. The Gates cabinet has provision for slide-in chassis. How vintage is that? Vintage is what I want. I already have power supplies.  73, Mike

In my experience those 2-stage exciters are a bit iffy, and prone to FMing, warbling, hum, noise, and just about anything else you can imagine unless you use a crapstal.

If you're gonna build a VFO (which isn't hard, if you are careful with mechanical construction) you need to buffer it really well.  IMHO a 6AG7 is a rotten tube for a VFO.  Runs WAY too much power, and is a legacy of the days where  FMing wasn't noticed much, and hams wanted to squeeze every last ounce of juice out of all the tubes (and use the minimum amount of tubes).

The standards for a FB signal have tightened over the years. 

I would use a miniature triode as an oscillator, followed by at least one, preferably 2 stages of buffering, then a 6AG7 or 6CL6 for the voltage munkey swang for the output.  If you're beholden to the old buzzard look, perhaps a 6SN7 dual triode (one section as osc, one as buffer) --> 6AG7 buffer --> 6L6 (or even better and cheaper, a 2E26).

The keys to successful VFO design are mechanical rigidity, LIGHT coupling to the tank and between stages, and single point ground each stage.  DC on the VFO fil won't hurt at all either!

Use a modern inexpensive transceiver as a VFO or exciter.    This month's Electric Radio has a schematic for this.