[HBR] Gain in IF amplifiers

[Byron Tatum]

Thank you for the good info Dan!
----- Original Message ----- 
From: <Hopperdhh at aol.com>
To: <hbr at mailman.qth.net>
Sent: Sunday, July 23, 2006 4:03 PM
Subject: [HBR] Gain in IF amplifiers


>
> 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
>
>
>
> ************************************
> Visit the HBR Receiver Web Site with over 100 pictures of receivers and
> construction notes...... via http://www.qsl.net/k5bcq/
> there is also a mirror (faster response)at http://k5bcq.edebris.com/
>
>
> Retrieve reflector archived data via 
> http://mailman.qth.net/mailman/listinfo/hbr
>
>
>