[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 1

[hbr@mailman.qth.net]

Jim quotes me

> >  The 1st mixer gain is low partly because of the circuit but also 
> >  because of the low injection voltage from the 1st oscillator.   And
> >  *that's* low because with the small crystals of the FT-101 my simple
> >  Pierce oscillator wouldn't deliver more without drift.   
 
 and adds:

> Two words: Buffer stage.

Maybe.   But it's not a power issue -- 1/2 12AT7 will deliver plenty of 
power.   The first thing I'm trying is a super-Q oscillator coil and after 
that, a transformer.   Only if there isn't another way will I get to a 
buffer stage.   

The overall goal is a bit different than some receiver designs you 
mention.   I'm interested in an outstanding all purpose HF ham 
receiver -- the one designers might have produced in 1970 had there 
been an effort to do it -- rather than a specialized machine with a lot 
of knobs. 

> The Type 7 is single conversion, using a 6EH7 RF amp and 7360 mixer.
> After the mixer is an 8 pole 500 Hz xtal filter which I think was
> originally meant for a RACAL receiver. 

Single conversion is definitely the way to go but it trades internally 
generated spurious signals for the extra mixers taken out of the 
signal path.   And I felt that at my level of experience there was a 
huge advantage to staying with an existing conversion scheme and 
bunch of recycle-able hardware.  You can only answer so many 
questions wrong at the same time and make any progress.   Call 
that Walt's (rusty) razor.

The 6EH7-7360 sounds like a good combination, although I wonder 
at the need for that much gain in front of a 7360.   And beam tubes 
have so many difficult design challenges, from the DC voltages to the 
very high LO drive requirements (and should be push-pull, at that); all 
that means a lot of heat which complicates the stability challenge.

All of which is another way of saying that one day I'd like to try a 
beam tube receiver.   Just not there yet!

> The basing of the 7044 is such that a single shield across the
> socket will ground both grids and shield input from output. Most dual
> triode pinouts won't allow that. The 2C51 and 5687 have similar
> bases. The RF amp has only enough gain to overcome the loss of the 4
> tuned circuits - it's not there to boost the sensitivity much. 

The trade off is peak noise performance for dynamic range, is it not?  
I'm betting that a cascode 6ES8 comes close on the noise (6.5 db in 
a TV tuner) and kicks anatomy dynamically.  1% crossmod at 0.5 
*volts* of signal as you near cutoff -- and not bad at higher gains.   
And since it's impossible for a conventional mixer to equal the large-
signal performance of an amplifier, a key function of an RF stage is 
to have a place to do AGC ahead of the mixer.   

I'm thinking in the direction of a 12AU7 1st mixer.  That would pretty 
well deal with the large signal issues but would require a whole 
bunch of LO drive so we'll see.   Added advantage -- you can put 
some AGC on that tube.

Jeff said:

> ...Yaesu Mark V, at least mine. It hears  SWB signals on 40m in the cw
> band  that do not tune and are wide and sound muddy, They are not
> there on the Tentec Omni lV+. 

While that could be a design flaw, there are also radio and other 
hardware failures that could produce the problem.  I'm not 
experienced enough with modern gear to have an opinion.   Have you 
compared notes with other owners of the same rig?

But it definitely could be a design flaw.  An interesting comment that 
I heard recently: "The basic RF performance of modern ham gear is 
going down steadily, even as computerization gives us more bells 
and whistles." 

In the 60's the ham marketplace accepted SSB sets that drifted a 
few kcs the first hour and a good fraction of that, forever.   Signal 
quality and reception was variable, at best.   By the end of the 
decade Yaesu gave us better performing gear but saved money on 
quality control.   (Anyone out there running an FR-100B/FL-100B 
combo?)   The radios with strong all-round performance cost much 
more and never sold comparable numbers -- Collins is the leading 
example.   Computerization vs. sound basic RF performance is 
another such issue.  

Another quote, possibly more controversial:  "The FT-1000D is 
today's Collins and has a similar future."

Let's face it:  the average ham set today is a transceiver used an 
hour a day for talking to buddies in a 1000 mile radius.    Every set in 
the market, from an eBay FT-101E upwards, will do that job -- at 
least when it isn't out for a few weeks for repair.   

The question of what could have been done in 1970 remains very 
interesting.   Thanks for the discussion and suggestions!

Walt Hutchens
KJ4KV