EM De-Embedding Magic – Part 4: Internal Ports

By James C. Rautio

When I was just old enough to find out that one should not try to catch a falling soldering iron, I was having a lot of fun building amplifiers and oscillators for amateur radio, a hobby introduced to me by my father. Back then, nearly everything was tubes. I remember working on the final amplifier of our “homebrew” transmitter, a pair of 807’s in push-pull. To make the amplifier stable, I first turned off the input power by pulling the driver tube, a 6L6, and the oscillator, a 6AG7. Being very careful of the 600 V on the plates (at the plate caps, on the top of the tubes, Figure 1), I would adjust a wire connected to the input (connected to the tube pins underneath the chassis, passing through a small hole in the steel chassis) until there was zero RF output, as measured by a light bulb acting as a dummy load. The negative feedback at high frequency neutralized the amplifier.