028d74fe   
   From: apogosso@tpg.com.au   
      
   "Engineer"  wrote in message   
   news:917123db-2bf9-490c-84c6-65e82eeff949@d1g2000yqm.googlegroups.com...   
      
   Alex, this interests me as I'm an avid AM receiver "improver", too,   
   but it's not clear to me just what you are referring to.   
   Yes, the common 0.05 uF and 10 meg grid leak (or even the more common   
   4.7 megs) gives the triode audio stage "near DC amp" behavior (to as   
   low as 0.3 Hz -3dB) but the DC comes from the detector diode,   
   proportional to RF signal level, and feeds back via the AVC line (to   
   the IF and converter tubes) to alleviate this.  Also, I don't see this   
   VLF getting to the o/p tube unless someone has used a huge coupling   
   cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.   
   Perhaps there's a good reason for this little 0.005 uF... it also   
   stops the tiny speakers from rattling too much!   
   Could you please describe the cct fix you allude to?  Sorry, I can't   
   access binaries so it has to be words or .jpg pics to Flickr, or   
   somewhere!   
   Thanks and cheers,   
   Roger   
      
   Alex:   
   Nice to meet another radio man on the tube amp list. Of course the easiest   
   improvement of a tube radio would be getting rid of the tubes and replacing   
   evething, in the first place, an audio amp with an IC, source follower after   
   the detector, op-amp based tone control, etc. A simple op-amp based audio   
   amp powered from ~6.3V winding would work far better than a boring 6AV6 +   
   6AQ5. However, it is not the way. The trick is to improve a radio keeping   
   the tubes and approximately the same topology, not adding much.   
      
   There are several issues, e.g.:   
   1. How to make AM detector to handle 95% of modulation, not 60...70% as in   
   most of the boring radios;   
   2. Reduce distortion and booming in audio.   
      
   Part 1 would cover optimisation of the AM detector, including unveiling of   
   the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which   
   detector is the best? Vacuum diode? Silicon? Germanium? And why.   
      
   Part 2 would cover adding NFB to the audio amp for distortion reduction and   
   speaker damping, at the same time turning the amp in a high-pass filter (not   
   to overload a lousy OPT and speaker with bass and DC level fluctuations).   
      
   I will try to write up something on these issues from both theoretical and   
   practical experience.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)