[HBR] modifying the 262kHz K-Tran IFs to 100kHz

[Hopperdhh at aol.com]

Tom and Walt,
 
According to my Radiotron Designers Handbook the maximum stage gain of an  IF 
amplifier with a double tuned, critically coupled transformer is:

Gain  = Gm x Q x 2 x pi x F x L / 2

All else being equal the gain is  proportional to the inductance of the 
windings.  I guess someone will have  to measure the L in both transformers to get 
the real answer.  Or else,  build an simple single stage amplifier and measure 
the gain with each  transformer.  That would be the practical method!

In general, for  the highest gain, IF transformers were designed with minimum 
C and maximum  L.  If we assume 150 pF for the minimum practical total 
capacitance, then L  for 262 Kc = 2.46 mH.  L for 100 Kc would be 16.9 mH.  This 
would be  16.7 dB less gain with the same capacitance.  I don't know if even this 
 value is that significant.
 
I agree.  Build it and see how it behaves.  If it seems low on  gain, maybe 
the transformers could be the reason.

Walt, given the same  capacitance assumption above, the difference in gain 
would be 1/(2.6^2), not  1/2.6, I think.

That doesn't mean that the "genuine" 100 Kc transformers  were designed for 
highest gain, but the 262 Kc transformers probably were.   The minimum 
practical capacitance is likely to be higher in the "genuine" 100 Kc  transformers.  
These factors are likely to be in you favor  Tom.

Critical coupling gives the highest possible gain.   Over-coupling causes 
double humps and tuning interactions.  Under-coupling  does cause a loss of gain. 
 It is my understanding that most IF  transformers were designed for about 90 
percent of critical coupling to be sure  they were not over-coupled to assure 
easier tuning.  So  coupling should not be that much of a factor in stage 
gain.

Dan  K9WEK



Dan wondered:
> I don't believe anyone has pointed out that  the  stage gain will be less 
with
> the modified 262 kc transformers than  with  "genuine" 100 kc units.
> 
> How significant will this  loss of gain  be?

Assuming the transformers are similar to the  'real' 100 kcs units,
then both the inductive and capacitive reactance will  be in proportion
to the frequency. So there'll be a loss of gain of 1/2.62 or  maybe 8-9
db per stage. However these are very high Q units to start  with
(probably significantly better than the original 100 kcs xfmrs)  and
the HBR designs generally have way more than the necessary  gain.

I'd bet it'll be fine.  Worst case you could upgrade the type  of tubes
used with them, as the original HBR's used very early (low  gain)
types.

Walt
Still fooling around with the BFO for an hour or  two a week ...



In a message dated 3/17/2009 8:59:39 A.M. Eastern  Daylight Time, 
bignick at execpc.com writes:

Dan,

Why would the stage  gain be less with the modified
coil?

According to to the hbr-11  receiver web page, the
unaltered coil
will be over coupled.  So won't  the moving apart
of the coils bring
the coupling and gain back to where it  should
be?

73

--KC9KEP (not the expert) Tom  Nickel


> 
> I don't believe anyone has pointed out that  the
stage gain will be less 
> with 
> the modified 262  kc
transformers than with "genuine" 100 kc units. 
> 
> How  significant will this loss of gain be? 
> 
>
Dan K9WEK 
> 
 
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