06-07-2020, 03:14 PM
(This post was last modified: 06-07-2020, 03:21 PM by Mike Watterson.)
Why the "infinite impedance" detector is pointless for a two valve TRF:
The so-called "infinite impedance" detector is just a cathode follower, i.e. Anode is common. It has no gain at all. The valve needs to be biased just to cut-off. It isn't necessarily lower distortion than a diode, except for larger signals. Because the gain =1, it's a bad application for weak signals or low level signals. An 1N60 or OA81 etc is better.
You do not need a floating cell. A filamentary (direct cathode) valve can be "common grid" (Cathode is input, Anode is output) or "common anode" (grid is input and cathode is output).
Historically a transformer wired as a common mode choke is used for RF (the DC + and - currents cancel, but the RF "sees" the two coils in parallel, which doesn't 1/2 the inductance if they are closely coupled but does double the Q). Two coils in series ON THE SAME core gives x4 inductance. A typical transformer would be a 2V valve interstage transformer or a 2V valve TRF audio choke for capacitive coupling. You need three valves minimum if using the "infinite impedance detector". The input impedance is load resistance x gain in series with space charge impedance, not infinite.
Because it's a low level signal and the gain is needed, I can't see the point. A simple way to limit the gain on strong stations is a pot on G2 on the 1st valve. Manual RF/IF gain on the Hallictrafters S72 is done this way, and on some Russian sets with Rod Tubes.
For a direct filamentary cathode valve audio "infinite detector" you can either use a common mode audio choke or a transformer that won't saturate with the filament current. If you have a "floating cell", you are placing what was the anode load in series with cell to -HT and the gain = 1. Also you need SOME current to bias the 1j42a to "almost off". This is best arranged by a resistor to a stack of button cells, though you can try using just the load current. The grid has to be connected to -HT or 0V by SOME sort of DC path unless it's an electrometer, like 10M Ohm.
Actually the very first "button cells" were mercury type and invented by the Chemist that founded Duracell, then called Mallory. They were to bias grids of out put valves without cathode resistors and decoupling capacitors, a cheaper solution to last 5 to 10 years in 1930s. It was a commercial failure. His success was selling them to Military in WWII for shell and bomb 'proximity fuze' electronics due to the years of shelf life, even in tropics where Zinc cells might self discharge in a few months.
The so-called "infinite impedance" detector is just a cathode follower, i.e. Anode is common. It has no gain at all. The valve needs to be biased just to cut-off. It isn't necessarily lower distortion than a diode, except for larger signals. Because the gain =1, it's a bad application for weak signals or low level signals. An 1N60 or OA81 etc is better.
You do not need a floating cell. A filamentary (direct cathode) valve can be "common grid" (Cathode is input, Anode is output) or "common anode" (grid is input and cathode is output).
Historically a transformer wired as a common mode choke is used for RF (the DC + and - currents cancel, but the RF "sees" the two coils in parallel, which doesn't 1/2 the inductance if they are closely coupled but does double the Q). Two coils in series ON THE SAME core gives x4 inductance. A typical transformer would be a 2V valve interstage transformer or a 2V valve TRF audio choke for capacitive coupling. You need three valves minimum if using the "infinite impedance detector". The input impedance is load resistance x gain in series with space charge impedance, not infinite.
Because it's a low level signal and the gain is needed, I can't see the point. A simple way to limit the gain on strong stations is a pot on G2 on the 1st valve. Manual RF/IF gain on the Hallictrafters S72 is done this way, and on some Russian sets with Rod Tubes.
For a direct filamentary cathode valve audio "infinite detector" you can either use a common mode audio choke or a transformer that won't saturate with the filament current. If you have a "floating cell", you are placing what was the anode load in series with cell to -HT and the gain = 1. Also you need SOME current to bias the 1j42a to "almost off". This is best arranged by a resistor to a stack of button cells, though you can try using just the load current. The grid has to be connected to -HT or 0V by SOME sort of DC path unless it's an electrometer, like 10M Ohm.
Actually the very first "button cells" were mercury type and invented by the Chemist that founded Duracell, then called Mallory. They were to bias grids of out put valves without cathode resistors and decoupling capacitors, a cheaper solution to last 5 to 10 years in 1930s. It was a commercial failure. His success was selling them to Military in WWII for shell and bomb 'proximity fuze' electronics due to the years of shelf life, even in tropics where Zinc cells might self discharge in a few months.
