I ran across a schematic for an amp using an 813 tube. I had pretty much all the components needed in a box of junk. So, I decided to give it a go. Got it all assembled, and fired it up. No majik smoke. burned in the tubes, and shot the HV to it. Still no majik smoke, and the diodes giving a nice purple glow. HV tested to 1kv. Shut it down. So far all seems well. I don't know why, but I decided to hook up my antenna analyzer to the RF input to make sure that there would be a 50 ohm impedance, prior to hooking up the driver. The analyzer shows 66 ohms, and swr of 1.5:1. Hmmm, sez I. I fire up the filaments, and key it, impedance jumps to about 75 ohms, swr off the scale. I know that the analyzer doesn't have enough power to drive the amp, (something in the mw range) But I decided to give it a go anyway, and shot the HV to the plates. Of course there was no change. But my concern is, that with the amp off, and dummy load connected, the analyzer is showing me a 66+/- ohm impedance. And an swr of 1.5:1. Which is what my transmitter will see when hooked to the amp. There's something between the RFin, and the RFout, causing this, but I can't find it.