6888 Transmitter

One of my many hobbies is working with ancient vacuum tube equipment, much of which are AM radios. However, as Ben Krasnow of Applied Science says: "after spending hours getting the radio working, you'll find that it sounds terrible." This has nothing to do with the radio, but is due to the fact that AM radio these days is mostly used for political talk shows and modern music, neither of which I particularly care for when listening to an old radio.

To play whatever I want on the radio, I need an AM transmitter, one that operates in the normal AM broadcast band of about 500-1500 kHz. Surprisingly, the FCC allows this for low power transmitters (there's a similar rule for FM, which is why you can buy those little adapters that make your phone play over your car's radio), though I kind of doubt anyone would mind as long as the signal can't be detected outside your property. Besides, these days the FCC is busy making sure the cable companies can keep your internet bill high.

Anyways, after completing a solid state version of a transmitter which turned out to only travel a few millimeters, I decided my next version would be tube based. After all, tubes operate at a much higher voltage, and while I am no expert in RF, it stands to reason that a higher voltage applied into the antenna would make for a better range.

The version I decided to build is the 6888 transmitter designed by Norm Leal and others on the Antique Radio Forums. Specifically, I based my design on a schematic drawn by Jon Stanley. The original version of the 6888 transmitter uses an audio transformer to match a low impedance audio output from a computer or audio player to the high impedance input of the 6888 tube, but I did't have a transformer that fit the bill, so I used this version which uses a 6AB4 tube as a preamp to do the same job.