7977f22d   
   From: apogosso@tpg.com.au   
      
   "Patrick Turner"  wrote in message   
   news:f8060a23-5198-42fa-87cf-9a3241471282@17g2000prr.googlegroups.com...   
   ........................   
   In most old radios with DC flow across the volume control pot track   
   the adjustment of the volume is often very noisy after the pot has   
   aged a few years.   
      
   So instead of the conventional arrangements put forward by RDH and   
   most others to avoid parts costs I will have the last IFT coil biased   
   at say +50V at one end, and the live end goes to a triode grid of 1/2   
   12AU7 CF buffer to remove any loading effect of diode detection on the   
   last IFT LC. Then I use Ge diode feeding RC circuit, and this can   
   directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to   
   any a volume control and while employing time passive poles to give   
   say -3dB at 30Hz before any power amp which has NFB. I often add in   
   another 12AU7 gain stage for tone control to boost/cut treble; bass in   
   AM is usually OK.   
      
   Alex:   
   Is adding two extra tubes a reasonable "mod"? It is the radio redesign. If   
   one wants to go that far, op-amps would be far better. Take TLE2074. With   
   10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase   
   sensitivity, tone control, buffer, AGC integrator, etc.   
      
   And after that why do you need to retain a lousy tube amp? Why not to use an   
   IC? All the radio can be powered from a 6.3V winding with a bipolar voltage   
   doubler (four caps, four 1N4004 diodes)   
      
   ...................   
   I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the   
   audio amp and with 12dB global NFB.   
   This works far better than anything with 6V6 or EL84 etc.   
      
   Alex:   
   Works better in which respect? EL34 takes as many amps for its heater as the   
   whole original radio circuit. What about the poor lousy power transformer?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)