[HBR] Another Receiver Project -- HBR-4, Part 18

[waltah at earthlink.net]

Pat, K7YIR wrote:
> I am following your work on this project using the 6JH8 / 7360 series
> mixers with great interest. You are actually doing what I am always
> trying to find time for.  So for the time being I am vicariously
> enjoying the work of your hands and your willingness to also put it out
> here for the rest of us. 

The pleasure is of course mine.  The encouragement from the list 
helps keep me going and the suggestions help me stay on the 
right track.

> I am curious if you have actually measured 90 db with your same
> test setup on the R-390. 

No, and I was wrong.  I thought I was using a number from 
Sherwood Engineering's fine site at:

http://sherweng.com/table.html

But when I went there just now and checked, the R-390A IFDR at 
20 kcs spacing is 81 db.   They don't give a number for the R-390.  

I should have known better than to trust my memory.  Glad you 
asked ...

The highest number on the site is the Icom IC-7800 at 102 db, 
followed by the AOR 7030 and something called a KWZ-30, both at 
100.   There are a number of fairly recent sets in the mid to upper 
90's.  The Elecraft K2 (s/n 3170) is very impressive at 98 db ... gee, 
that's a *kit*, folks.

I believe the top vaccum tube numbers are those of the Collins 75 
S3B at 88 db and the Drake R4C at 85.   Neither is a remarkable 
design; it ought to be possible to beat them.   They haven't tested 
the SS-1R.

The G2DAF Mk II receiver certainly holds the top claim, at ~100 
db.   The design seems consistent with excellent performance.   
Unfortunately very few of them seem to have been built.

> I have the schematic of the Squires Sanders SS-1R but don't yet have
> it scanned or I would send it along also in case you don't have it.

I do have it, probably from the BAMA site -- but thank you.   They 
also put the antenna signal on G1 of the 7360.   Gee, I'd like to talk 
to one of those designers about the reason for that.  Either there's 
something I'm missing, or one guy made a mistake and everyone 
else followed.   I can't find a single major advantage to G1 antenna 
signal input, and the linearity seems markedly better doing it the 
other way.

(The minor advantage is you do get good rejection of feed-through 
on the IF if you balance the output from the mixer.   But most 
designs didn't do that and I don't see this as compelling, compared 
to dynamic range.   *Every* receiver has IF feed-through to contend 
with, for goshsakes.

I'm having a hard time getting started on the RF stage of the HBR-
4.   There's plenty of room for it (and I've punched the socket hole) 
but the trimmer and bandswitch layout there isn't going to be good 
for stability.   I'm sure that before I'm finished I'll have to completely 
redo that, to get more spacing and put in some shielding.  So I'm 
struggling with just biting the bullet and starting with a 6EH7 stage 
which (even at low gain) will likely force the whole job to be done 
before I have any results, or using a cathode-coupled 6ES8 as a 
starting point.   Because there's no phase reversal, this should be 
stable at reasonably low gain even in the existing configuration.  

An interconnected issue is exactly how to install the necessary 9 
mcs trap(s).   A parallel tuned trap can be put in the antenna lead 
to the antenna coil.   A twin-triode RF stage (either cathode 
coupled or cascode) allows a second similar trap between the 
sections; if using a 6EH7, there's no easy equivalent.   

With a single conversion design at least the trap need only be for a 
fixed frequency, meaning I can go for max Q and need not provide 
much of a tuning range.   I guess the thing to do is to install the 
antenna lead trap to start with and hope that that's all that is 
needed.  

Both ends of the trap are hot -- hafta build that on spacers or a 
terminal strip or something.   At least there are no strong signals at 
9 Mcs in this location -- nothing like 'World Harvest Radio' at 5820 
or whatever it was, with the HBR-2K's 5520-6020 kcs tunable first 
IF.   

Walt 
KJ4KV