[HBR] Gain in IF amplifiers

[Hopperdhh at aol.com]

Every so often I read on this list of someone finding some IF transformers  
and building up a receiver with two or more IF stages.  I always want to  jump 
in and warn them what can happen, because it reminds me of something that  
happened to me years ago.  I decided to soup up my Hallicrafters S-38 by  adding 
an IF stage for more gain and selectivity.  Unfortunately, all I got  was 
squawks and squeals!  After trying many fixes like isolating the  screen and B+ 
supplies to each stage and shielding, etc., I finally had to  give up.  It 
wasn't until years later that I realized the basic cause of  the regeneration.
 
Here is the problem.  Low cost receivers with only one IF  stage get maximum 
gain by keeping the impedance as high as  possible.  This means using only 
about 100 or 125 pF capacitors and making  the inductance about 1 mH to resonate 
at 455 KHz.  Most IF transformers you  find in the junk box will be of this 
design.  If you use these transformers  for a 2 stage amplifier the gain will go 
through the roof, so to speak.
 
If you look at schematics with values for the capacitors in the IF  
transformers, you will notice that receivers with 2 IF stages usually have  
capacitances of 220 to 390 pF.  This is to lower the gain per stage to make  the receiver 
more stable.
 
One way to make use of these transformers is to add or replace the  
capacitors to get say 270 pF, and then remove turns from the coils until they  come 
back up to 455 KHz.  Also, it seems like one could use these 1 mH  transformers 
at a lower IF frequency by increasing the capacitance.  This  would at the same 
time lower the gain and let you use 2 or more stages.   For example, 370 pF 
and 1 mH resonate close to 262.5 KHz.
 
 
One place to find lower gain transformers to use in home brew receivers is  
in old tube type CB radios.  Many (but not all) of these radios have 2 IF  
stages.  If it has only one stage then the transformers will be just like  those 
in cheap broadcast receivers, with 100 pF capacitors and too much gain for  2 
or more stages.
 
 Hopefully this information will help someone  avoid the frustration that I 
experienced when I attempted to  improve the old S-38.
 
Dan K9WEK