Project 3 – Superheterodyne octal tubes
This is a work in progress. The radio will probably be completed in the first half of 2010.
It will have 5 section; rf, lo-mixer+if, if-amp2, det+first-af and af-amp.
January 8 2010
Well, I had time to complete my 5 tube radio project.
I have opted for a 1940’s classic, an all American 5 superheterodyne AM receiver.
I had a few 12SA7, …, 50L6 series lying around the shack and two old 455kc ‘IF’ transformer.
The schematic is quite straight forward, the usual RF/MIXER, IF amp, DET/AF amp and power amp.
The strength of the signal – from the detector section – is use to AGC.
Tube heaters are connected in series and a 35Z5 is use to provide the B+ from an isolation transformer connected to 117AC.
It was quite fun to revisit an old part of radio history.
Oh yes, tuning was a bit tricky using a scope. When the scope was disconnected, the circuit was detuned! Measuring the AGC negative voltage and using my ears was much more effective.
A frequency counter was use to calibrate the L.O.
By the way, Max Robinson has a lot of valuable information on his site ‘FUN WITH TUBES’ about tube radios and AA5 theory at http://www.angelfire.com/electronic/funwithtubes/.
And Dave N2DS has build very nice radios http://makearadio.com.