[HBR] Progess report

Mon Apr 24 15:22:57 EDT 2006

 There is a freeaware called tubepad.  i'll have to
look up garys site for the link. ifyour interested.

 keith.

--- "Walter A. Hutchens" <waltah at earthlink.net> wrote:

> The little 7-tube (6 + rectifier) receiver which I
> am calling the R8040A 
> since it is basically a major makeover of one I
> build in 1993 is pretty much 
> done; the diagram needs only the BFO and how hard
> can that be?
> 
> I do need to study the software around here to see
> what I've got to do a 
> computer drawn diagram.   If anyone has a cheap or
> free suggestion, I'd be 
> glad to hear it.
> 
> I measured drift: starting a minute after turn-on it
> went down about 100 
> cps then back up about 70 at about half an hour,
> then very slow wandering.  
> This is excellent for such a casually laid out
> oscillator.   But it's very high-C 
> and tunes 1350-1950 kcs so the tube is all but out
> of the circuit.   A bit 
> more tinkering to get the heat to the compensating
> capacitor just a bit more 
> quickly will finish that up.   
> 
> Intermodulation distortion measurements are in order
> and I should check 
> the sensitivity.   The latter is good enough though
> -- you can copy stronger 
> signals with no antenna at all in the evening.   The
> S-meter doesn't move and 
> you have to turn the audio way up, but the 4" or so
> of wiring to the front 
> panel terminal for a short antenna is enough.  
> 
> The basic plan for a general coverage receiver using
> the R8040A design as 
> an IF seems to be complete.   The plan is to cover
> 1.75-31.75 Mcs in 250kcs 
> bands.  250 kcs wide bands (rather than 500 kcs or 1
> Mcs) make 
> management of spurs easier as well as allowing a
> lower tuning rate without 
> using the most expensive gearing.  
> 
> They key ideas of the G2DAF Mk 2 design are (1) use
> of push-pull front end 
> to minimize intermodulation distortion, and (2) a
> below-unity gain RF stage, 
> included only to allow AGC ahead of the first mixer.
>   The push-pull front 
> end is incorporated in the GC-HBR; with AGC on the
> mixer stages (it worked 
> fine in the R8040A) an RF stage isn't essential.
> 
> The fixed (2nd) IF will be 4.096 Mcs (instead of
> 5.4, I have some surplus 
> 4.3 Mcs IFTs that will work), the LO will tune about
> 1.454-1.704 mcs, 
> making the variable (1st) IF 5.550-5.800 Mcs.   I
> have a four section AM-
> FM BC tuning cap; two sections are 32-370 mmf which
> will be used to tune 
> the push-pull oscillator; the other two sections are
> ~30-50 mmf and I'll use 
> them to tune the variable IF.   A big toroid will
> have a Q of > 200, meaning 
> quite a bit of rejection of unwanted signals into
> the 2nd mixer.   
> 
> Up to that point the circuits are identical to the
> R8040A.
> 
> The first mixer will be a 6922 -- a dual triode very
> similar to the 12AT7 but 
> with a Gm of 12,500 which is about 3x higher.   Like
> the R8040A the grid 
> circuit will have a single tuned circuit feeding the
> grids in push-pull.   A 
> double section BC cap will cover the range ~1.75 Mcs
> - 31.75 Mcs in three 
> ranges, something like 1.75-6-18-32.
> 
> With only one tuned circuit ahead of the first
> mixer, there will be some 
> break through at the first IF but this is entirely
> tolerable in the R8040A -- 
> one can either use an antenna that doesn't have high
> gain on the unwanted 
> frequency or use a trap in the antenna lead.   
> 
> The first conversion (to the 5.550-5.800 Mcs IF)
> will be crystal controlled.  
> For a ham band only receiver you'd switch in a
> different crystal for each 
> ham band.   What I plan is to synthesize a crystal
> controlled frequency from 
> 7.550 to 37.300 in 250 kcs steps by multiplying a
> selected crystal from the 
> 120 crystal set for the BC-1335 (5.675-8.650 Mcs in
> 25 kcs steps) by 10 
> and subtracting a fixed frequency of 49.2 Mcs.  
> Overtone crystal for that 
> one -- no need for a whole 'nuther chain of
> harmonics.
> 
> Compared to the usual overtone oscillator circuit
> this very simpleminded 
> synthesizer adds considerably to the complexity (two
> more tubes) but it's 
> worth it to be able to get continuous coverage with
> crystals that are on 
> hand.   It's impossible to calculate all the
> spurious signals in a set of this 
> kind but there aren't a heck of a lot and none of
> the obvious ones 
> (harmonics of LO passing through an IF, fundamental
> crystal frequencies) 
> are in a ham band.
> 
> The plan is to provide internal sockets for six
> crystals covering the five real 
> ham bands 160-10 with two on 10.  That will be two
> ham bands per front end 
> range, plus a socket on the panel for any desired
> other crystal.  
> 
> It will be necessary to gang tune three circuits in
> the synthesizer according 
> to the selected crystal.   These will also have to
> be bandswitched.   
> 
> The layout is something of a mess because of
> shielding and connection 
> requirements but I think the issues are manageable;
> the toughest looks like 
> finding room for all the stuff that has to be hooked
> to the bandswitch.   A 
> fallback position is to just provide the one crystal
> socket on the panel as 
> that would eliminate the switching of crystals,
> mounting of the sockets in 
> the scarcest part of the chassis real estate, and
> the need for an 
> INTERNAL/EXTERNAL switch.   
> 
> I think there'll be room for the internal sockets.  
> That's a more polished 
> scheme.
> 
> I have a Miller MD-7 dial which would work (and
> takes very little space), 
> but the two-speed tuning is annoying and I'd rather
> have a more positive 
> drive for this.   I also have an Eddystone 898 but
> 100:1 (=5 kcs/revolution) 
> is a rather slow rate.  I want easy tuning of SSB
> signals, no backlash, etc.   
> I've looked at assorted military junk, thought about
> string drives -- 50:1 
> doesn't  seem wise with a string and the 1-1/2" drum
> I have on hand.    Still 
> thinking about the dial -- probably the MD-7 will
> get the nod.
> 
> Does anyone have experience with string drives at
> ~50:1?   What about a 5:1 
> ball drive and a 10:1 string drive?   Not too
> complicated and with a 1-1/2" 
> drum the string drive shaft would be a bit over 1/8"
> -- not unreasonably 
> small.
> 
> (I did consider 500 kcs bands but ruled them out
> because I wasn't able to 
> get rid of the spurs without using a higher LO
> frequency.   You just can't 
> get the stability at 8 or 9 Mcs that you can at 1.5
> Mcs.)
>  
> There are always complications.   We'll see how it
> goes.
> 
> Walt 
> KJ4KV
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>  
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