The House Finch Direct Conversion Tube Receiver

Published:2013/3/7 3:14:00 Author:Ecco | Keyword: House Finch, Direct Conversion Tube , Receiver | From:SeekIC

After building the Little Chickadee 6U8A vacuum tube 80 meter CW transmitter project, your author decided that it would be a good idea to build a companion 80 meter tube CW receiver. A direct-conversion design seemed like a good approach since it is fairly simple and is effective for receiving morse code. Not wanting to reinvent the wheel, the Internet was searched for vacuum tube direct conversion designs. Surprisingly, no other projects were found. The online projects that were found were either solid state designs or regenerative tube circuits. Nature abhors a vacuum, but not necessarily a vacuum tube so this project was born. Of course, designing something new with vacuum tubes is not an entirely rational pursuit. Rationalizations aside, tube projects are fun, nostalgic and they glow in the dark. This project was a lot more difficult to complete than originally anticipated, but solving the many problems was certainly educational. Those who copy this design should have a much easier time getting the circuit running. It should be farily straightforward to adapt this design to other HF ham bands by changing the resonant sections in the front end filter and oscillator. The 6U8A audio amp project was already functioning nicely and was perfect for the audio amplifier stage in a communications receiver. The 6U8A seemed like a good choice for the mixer and oscillator circuits. In fact, that's how the 6U8A is usually used. One can think of the 6U8A as a vacuum tube equivalent (sort of) of the popular NE602 IC. The first version of this receiver used just two 6U8A tubes, it worked (barely), but there was a need for additional audio gain and isolation between the antenna and the mixer. The 6C4 preamp tube and the 6BA6 RF amplifier tubes were added to solve these problems. The receiver is divided into three physical sections, 1: the RF filter and amplifier, 2: the mixer, oscillator and audio filters and 3: the audio amplifier. A few extra grid bias resistors and filament hum cancelation resistors were added so that the individual sections can be disconnected and operated in a stand-alone mode. This also allows the sections to be used as stand-alone modules for other projects. An external power supply provides 6.3VAC for the filaments and 250VDC for the B+ supply, the supply can be shared between this receiver and the companion Little Chickadee transmitter.



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