[Boatanchors] Diode Detector in the Mate for the Mighty Midget Receiver

[Bill Meara]

Mike W6MAB recently posted a nice compilation of mods and comments on the Mate for the Mighty Midget Receiver (1966 by Lew McCoy).  I was especially interested in the comments on the diode detector circuit (see below).  
I built the diode circuit in LTSpice and was able to easily experiment with different values of capacitors.   I found that changing the output cap (the one going to the audio amp) from 100pF to .1uF had a big impact on the amount of audio making it to the AF amp.  Changing the input cap seemed to  have no effect. 
I then tried this mod in my actual MMM RX.  I just put a .1uF cap across the 100pf cap that I originally had in there.  I noticed a big increase in gain in the receiver.  Before the change, I had trouble hearing the band noise when trying to peak the front end on 75. After this mod, the noise was readily apparent.  
So, another typo?   
73   Bill  N2CQR http://soldersmoke.blogspot.com 
----- Original Message ----- From: "Michael Bittner" <mmab at cox.net>

Re. the "Mate For The Mighty Midget". QST, April 1966, by W1ICP, once again we see the floating diode detector about which there has been some controversy. Apparently these things work using the back resistance of the 1N34 diodes. However, the coupling capacitor to the audio amp grid seems small. Shouldn't it be more like 1000 pf rather than 100 pF? Mike, W6MAB

The coupling capacitor to the audio amp grid seems small. Shouldn't it be more like 1000 pf rather than 100 pF?

Yes, 100 pF does look crazy, and when I have built this circuit myself in the past, voltage doubler crystal radio, with hi-Z piezo headphones of course, I have always used 1000 pF. Possibly with the very high impedance load, in fact the highest Z load, here 100 pF suffices? It still seems small. If I was building this, I would have a couple 1000s on hand and try paralleling them just from curiosity. Here's something that will surprise you: in a crystal radio circuit, if you do use piezo earphone and absolutely not a magnetic phone, you can use a floating diode circuit with no lack of performance. -Hue

The general consensus seems to be that the 100 pF is way too low for the audio coupling capacitor. Probably a drafting error. Maybe the draftsman left out the load resistor (another 1 Meg) too? Mike, W6MAB

It's quite possible that the value is a typo. Has anyone checked for feedback?