Tweaking a Boatanchor Receiver?

Bob And Bettina Groh rgroh at SWBELL.NET
Mon Jan 16 11:50:19 EST 2006

Wow, that is a series of somewhat open ended questions. I don't have 
specifics but I can offer my opinion based on 40+ years of designing 
communications equipment (all solid state) plus 50+ years of mucking 
about with ham gear.

1.   The term 'hot' is perhaps a bit of hyperbole on the part of the 
seller. Or just an indication that it is not 'dead' or 'not really 

Now the technical stuff:
2.    Most tube type rigs have more sensitivity than they can use (at 
least on most bands 20M and lower).

3.   A receiver's sensitivity is a measure of it's overall noise figure. 
The overall noise figure is a product of the individual gains and noise 
figures of each of the stages up to the final detector (although the 
noise figure is usually pretty well determined in the first few stages, 
e.g. RF amplifier and 1st mixer).

4.   Usually increasing the gain in the front end is the biggest factor 
in improving the noise figure.  Also decreasing the noise figure of the 
front end usually helps.  What this means is a tube substitution might 
be of advantage.

5.   Unfortunately everything has a cost!  In this case, increasing gain 
may (probably) lead to poorer strong signal performance (a bigger 
problem today than back in the 'old days). A different tube may have 
different gain control characteristics so that the AGC may be adversely 
affected.  Etc, etc.

6.   Also the inherent external noise levels on most frequencies below 
20 MHz or so are greater than any reasonable receiver's internal noise. 
So you may not see much improvement (if any).

Well, that is just a rough outline. The other folks on the list will be 
able to add to this as well as add specific information relative to 
specific receivers. There was an article or series of articles some time 
ago where someone measured a wide variety of representative radios (the 
Drake radio's were one of them, I think) relative to sensitivity, noise 
figure, intermod, etc.  I probably have that around here someplace or 
maybe someone else can contribute some web references.

73 de WA2CKY
Bob Groh

kd4e wrote:

> I just read this post and it got me thinking.
> "Drake TR-3 ... with a very hot receiver."
> What are the specific features in certain BA receivers
> that makes them "hot"?
> Is it mostly the tube selection?
> Are there other specific things done in the front
> end?
> Which of these may be duplicated via minor mods to
> existing BA receivers to enhance their performance?
> Just wondering ...

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